7.12                              Oates Land, Victoria Land, the Transantarctic Mountains, the Ross Ice Shelf, and the South Pole area  

This section covers Victoria Land, the Ross Ice Shelf, the Transantarctic Mountains, and the South Pole area: the total region covered by these areas spans approximately the longitudes 150º E to 150º W (see Figure 7.12.1). From west to east, key features or stations referred to in this section include:

·                         Leningradskaya                                               (69º 30´ S, 159º 23´ E, 304 m AMSL);

·                         Terra Nova Bay Station                       (74^o 41´ 42″ S, 164^o 7´ 23″ E, 17 m AMSL);

·                         McMurdo Station                                 (77° 51´ 36″ S, 166° 40´ 12″ E, 24 m AMSL);

·                         The Transantarctic Mountains;

·                         The Ross Ice Shelf and the Ross Sea;

·                         Amundsen–Scott (South Pole) Station                                          (90º S, 2,800 m AMSL).

The New Zealand operated Scott Base (77.86 º S, 166.75 º E, 14 m AMSL) is only a few kilometres distant from McMurdo Station. As Scott Base experiences similar conditions to McMurdo Station and the base operations rely on forecasting from McMurdo, Scott Base is not explicitly covered.

7.12.1                            Leningradskaya Station  

7.12.1.1                      Orography and the local environment

Leningradskaya (69º 30´ S, 159º 23´ E, 304 m AMSL) opened in February 1971 with operations ceasing in 1991. The station is situated on the coast of Oates Land. The station buildings are located in the ice–free area near the top of the Leningradsky nunatak.

Leningradsky nunatak is a rocky feature. Its ridge comprises alternating leucocratic granites and grey biotite gneiss extending from east to west for over 1 km with a width of 100–150 m. Snow covers two thirds of the nunatak area. The station is located on the western part of the nunatak at a distance of 600 m from its top (330 m AMSL). The nunatak height is 100–230 m above the surrounding glaciers. The ice edge in this area has a height of 15–20 m.

The station has not been extensively studied with respect to synoptic influences. However, the local weather is characterized by a close proximity to the area of cyclone activity that is usually located over the Ross Sea, as well as by unique local conditions influencing the weather processes. The station's weather is characterised by its persistent and frequent storms that occur due to its considerable elevation above sea level.

7.12.1.2                      Operational requirements and activities relevant to the forecasting process

The station structures consist of several houses with living space, a radio–station, a power station, a meteorological station, an upper–air sounding complex, a garage and a warehouse. The station territory is restricted being only 200–250 m in length and not more than 50 m in width. The station was re–supplied by vessels that stop far from the station in drifting ice or near‑land fast ice. The personnel and cargo were delivered to the station by helicopters.

7.12.1.3                      Data sources and services provided

The station closed in 1991.

7.12.1.4                      Important weather phenomena and forecasting techniques used at the location

General overview

It is known that at high latitudes of the Southern Hemisphere, cyclonic pressure features prevail. However, large high–pressure ridges are quite frequent, developing from north to south and sometimes combining with the Antarctic high creating a blocking effect and causing the displacement of cyclones in the meridional direction. One of the areas where the southerly displacement of cyclones is most frequently observed is the Australian–New Zealand sector. In the area between 140º and 170º E, there is a change in the direction of displacement of cyclones that persist then in the Ross Sea. Note that the cyclones of the Australian–New Zealand sector have large energy supplies as the warm air outflow from the north and cold air sinking from Antarctica contribute to their regeneration. As to the Oates Coast and the George V Coast, the cyclones mainly pass over the oceanic regions in the zonal direction. However, from May to August up to two to four cyclones a month move along meridional trajectories from temperate and sub–tropical latitudes in the southeastern Indian Ocean.

Surface wind and air pressure field

The main feature of the pressure fields of the Australian–New Zealand sector of the Antarctic is the presence in all seasons of climatic cyclonic centres over the Ross and Mawson Seas. The average monthly pressure fields suggest the existence of either an increased pressure ridge or an isthmus between two cyclones in the Oates Coast area during much of the year. In winter and spring, the climatic centre from the east is located close to the Leningradskaya area whereas in summer and autumn, the direct influence of cyclones from the Mawson and d’Urville Seas increases.

Leningradskaya Station is located in the Antarctic maritime zone with rapidly changing of weather conditions. The main wind types in the coastal regions – cyclonic, katabatic and transient, are quite pronounced here, but they have their peculiarities, primarily in respect of the prevailing directions. Southeasterly winds in all months of the year are most common. Their frequency of occurrence is around 50% decreasing only in May and June to 40–43%.

The southerly winds are the second most common, occurring on 13 to 18 % of occasions, increasing to 20–22% in autumn from March to April. Westerlies are the third most frequent wind direction (8–12% in all months of the year) and together with northwesterly winds are close to the frequency of occurrence to the southerly winds. The pronounced easterly winds at the coastal stations of East Antarctica (for example, 34% on average for a year at Mirny and 30% at Molodezhnaya) only occur 6% of the time at Leningradskaya. The wind rose for Leningradskaya (not shown) reflects the features more typical of stations at much lower latitudes, for example, Bellingshausen.

The increased frequency of occurrence of westerly winds at Leningradskaya is related to three factors:

·                         Firstly, cyclones developing over the Ross Sea in certain instances, especially in summer, attain an easterly component, cross the Scott Coast and in rare cases reach Victoria Land, an extensive plateau at 1,500–2,200 m height.

·                         Secondly, some cyclones move from the east Indian Ocean–Western Australian area to the George V Coast, at a sufficiently large angle to the coastline to also sometimes appear above Victoria Land. In both cases, the Leningradskaya Station area becomes for some time at the northern periphery of the cyclone, i.e. in the zone of westerly winds.

·                         Finally, westerly winds can be observed in the event of a high–pressure ridge moving to the Oates Coast from the north.

Strong winds, storms and snowstorms come to the station from the southeast. Average wind speeds in the colder time of the year are 8–9 m s^–1 (~17 kt). In summer, this decreases insignificantly, only by tenths. Table 7.12.1.4.1 (in Appendix 2) shows mean–monthly wind speeds at Leningradskaya for a three–year period and does not necessarily reflect the longer‑term trends described here. On the other hand Table 7.12.1.4.2 (in Appendix 2) shows mean–monthly MSLP at Leningradskaya for a 21–year period and should be more representative of the longer–term mean MSLP.) The number of days of strong winds (wind speeds of 15 m s^–1 (~ 30 kt) or more) in autumn and winter months is 18–20 and in summer up to 14–16 days a month. Strong wind events are especially frequent and long from June to September. The maximum ten–minute average wind speed for the 1971–79 period was 48 m s^–1 (~93 kt) with a maximum gust of 60 m s^–1 (~117 kt). The largest wind gust was recorded on July 9, 1989 when under conditions of persistent (several hours) storm force winds; the instrument twice recorded the maximum gust of 78.3 m s^–1 (~152 kt). Between these readings, there was an instrument failure and there is no certainty that the wind gusts were not greater.

At Leningradskaya there is no strict difference between the katabatic and cyclonic wind. It is difficult to determine the end of one type of wind and the beginning of the other. This comment does not obviously refer to the case where the cyclonic wind is expressed in the form of westerly flows.

Mesoscale vortices deserve a special comment with respect to the Leningradskaya area. In most cases, cloud meso–vortices in East Antarctica occur to the rear (in the southwest) of extensive polar front depressions when a cold polar front is displaced far to the north and the cold Antarctic air moves with the southern flows onto a comparatively warm water surface. In some cases, the conditions that are sufficient for the development of a mesoscale cyclogenesis are created above the ocean areas. The clockwise turning of the wind with height to the rear of depressions contribute to increased cyclonic circulation in the lower atmosphere that probably also influences the formation of meso–vortices. The frequency of occurrence of meso–vortices is probably lower near the Oates Coast, than in the Ross or Mawson Seas, but obviously they are generated with the development of the most active cyclonic features that move here along the meridional trajectories from the northern oceanic areas.

During the warm period of the year, the sub–synoptic cloud vortices can in some cases develop near the coast and influence the weather in the vicinity of Leningradskaya Station. Typically, this is manifested in a wind increase to 10–15 m s^–1 (~20–30 kt), low stratus forming and solid precipitation with deterioration of visibility. With satellite observations received on a daily basis, a forecaster does not always notice such perturbations. As a rule, these phenomena are not long lived, but due to their sudden occurrence they can adversely influence aircraft operations. The dimensions of these meso–vortices formed typically of stratiform clouds comprise 200–300 km with duration of not greater than two days.

During the colder period, the area of mesoscale cyclogenesis is displaced northward and cloud meso–vortices more frequently form near the northern boundary of drifting ice. At this time of the year, they appear more active as viewed on satellite imagery, being in some cases similar to polar lows typical of Arctic ocean areas. These vortices do not usually reach the coast, quickly decaying when moving above sea ice.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained.

Clouds

A high frequency of occurrence of overcast sky (7–8 oktas) is observed at all stations of East Antarctica, comprising 63% on average for a year for Leningradskaya. In the summer months, more than 70% of all observations report a complete sky coverage while in the winter months this percentage decreases to 55–50%. However, the low cloud frequency is much less being not greater than 20%.

Clear sky frequency in winter and in early spring (including October) is 30% decreasing in the summer months and reaching only 15% in January and February.

Middle and high clouds of undulated forms typically predominate in summer. In winter, the fraction of stratus cloud forms increases. With deep cyclones reaching the coast along meridional trajectories, the passage of fronts is accompanied by continuous multi–layer clouds, which often sink to the level of Leningradskaya and is recorded as fog in some cases.

The number of stratus–rain and high stratus clouds in the observation results is probably slightly underestimated since it is impossible to accurately assess the cloud form and height during precipitation and in snowstorms.

Visibility: blowing snow and fog

There are good conditions for observations of visibility at Leningradskaya as there is a sufficient number of orientation marks both at the station, near the station (for example, a island within 2 km) and at some distance (for example, the mountain tops in the southwestern direction are located between 8 and 19 km). The farthest object that is seen at very good visibility (Governor Mountain) is located at a distance of 38 km. The frequency of occurrence of visibility of more than 10 km is 86–93% of all observation times in the spring and autumn months decreasing in winter and summer. The worst visibility over the period 1971 to 1979 was observed in December when the frequency of occurrence of good visibility was 75% and that of poor (less than about 2 km (~ 1 nm) visibility was 22%. The main factor restricting visibility is snowstorms. The number of days per season, with snowstorms obviously increases in winter reaching 15 whereas in summer it is about 10.

Note a peculiarity of determining the meteorological visibility range at Leningradskaya relates to the high elevation (304 m) of the station. In the presence of clouds below the station and in the absence of other meteorological phenomena, visibility is reduced to 2–4 km whereas with blowing snow (slight and even moderate), visibility of up to 15–20 km is often preserved. The average number of days with snowstorms is more than 140 a year. In 1989, there were 149 days with snowstorms, 41 days of them with general snowstorms. An example of a quiet year was 1974 when the number of days with snowstorms was only 62.

Frost fogs are rare at the station (on average 1–2 cases for a winter month). They occur under low wind conditions when the amount of cloud is small and significant air temperature decreases occur. In summer, the number of fogs increases due to advective sea fogs as well as low stratus clouds covering the station area. Typically in December and January up to 10 cases a month are recorded. In January 1972 17 days with fog and in January 1974 14 days were recorded.

Surface contrast including white–out

No specific information on forecasting has been obtained.

Horizontal definition

No specific information on forecasting has been obtained.

Precipitation

Around 150 days with precipitation are recorded on average at the station during a year. This number includes all days with snowfall, tapioca snow and granular snow regardless of their amount. Total precipitation amount is about 600 mm per year. The 1989–year was the most snowy year when precipitation comprised 830 mm. The maximum precipitation is observed in the summer and autumn months (specifically January to April) when 70–80 mm of precipitation falls on average for a month. The fact that precipitation in July 1989 comprised 154 mm against a 50 mm norm for the month indicates a large inter–annual variability. This is due to the anomalous high frequency of occurrence and intensity of the meridional flow in the Australian–New–Zealand sector in the indicated month. On the other hand, it is necessary to remember that with blowing snow and during snowstorms, snow is both blown in and out of the snow gauge bucket.

Note that during storm wind events and in snowstorms generally, abundant deposition of granular rime is observed, which is probably connected with a cloud passing across the station. The number of days with rime is about 100 a year reaching in some months especially in summer and autumn 10–15 days and more.

Temperature and chill factor

Table 7.12.1.4.4 (in Appendix 2) shows mean–monthly maximum and minimum temperatures at Leningradskaya Station. The temperature regime of Leningradskaya is influenced significantly by local conditions, its elevation above sea level, as well as the synoptic processes that contribute cloud amount, particularly overcast or clear sky conditions. The main factor is a large frequency of occurrence of southerly and southeasterly winds from the cold mainland. The mean annual temperature is –14.6ºC. By way of comparison, this is much lower than the temperature at Molodezhnaya and Novolazarevska Stations, for example, but slightly higher than the mean temperature at Mirny.

The mean–monthly temperatures for the period of observation (~1971–91) are characterized by minimum value of –21.7ºC in August and maximum value of –3.9ºC in January. The extreme values have a large spread between –0.5ºC to –37.4ºC in August and 8.9ºC to –13.0ºC in January. A successful temperature forecast depends on correctly taking into account the aforementioned features and factors.

Low air temperature and wind speed are the main factors influencing the time people can work outside. A complex assessment of weather severity is typically expressed in points and is calculated by Bodman’s formula given by Equation 7.12.4.1.1.

S=(I + 0.272 V) (I – 0.04 T)                                                                  Equation 7.12.4.1.1

where V and T are the wind speed (m s^–1 ) and air temperature (ºC), respectively. The calculation of the severity coefficient (S) was performed using data at the meteorological observation times for Leningradskaya and for other Antarctic and Arctic stations over a number of years. The most significant S values are observed from May to September. The average severity values for the indicated period comprised 6.5 points for Leningradskaya, 7.4 for Mirny and 9.3 for the Vostok Station. Note that the S coefficient at the North Pole in winter (5.3 points) is almost the same as at the Vostok Station in summer (5.5 points).

Icing

No specific information on forecasting has been obtained.

Turbulence

No specific information on forecasting has been obtained.

Hydraulic jumps

No specific information on forecasting has been obtained.

Sea ice

The sea ice regime of the coastal area of the Somov Sea within a radius of about 60 km from the Leningradskaya is unique. One of its typical features is a very early onset of ice formation. In most years, it occurs as early as mid–February and in some years even during the second half of January. This is primarily attributed to the limited summer heating of the surface sea layer due to a constant preservation here of the Balleny ice massif.

Thus, during the entire 20–year operation of the station (1971–91) no complete ice clearance of the area in question was recorded. Moreover, on average up to 60% of its area was typically occupied by residual drifting ice off the southern periphery of the massif and often by land–fast ice that was preserved over 20% of the area east of the station near the barrier of the Gillett Ice Shelf.

However, in January–February, a mostly continuous polynya with a total area of up to 1500 km² exists directly along the glacial–land–fast ice coast between 158 and 160º E. The polynya has a wedge–like form with its width being 30 km on average in the west behind the outlet Tomilin glacier, around 10 km opposite the station and decreasing to zero near the land–fast ice edge in the vicinity of 160º E.

An unusually early (even for the Antarctic) intense ice formation develops in the polynya, occurring at first only at night hours due to radiation cooling. It can also be suggested that strong local freshening of the shelf zone contributes to this due to the discharge of a large volume of sub–glacial melt water along the channels of numerous outlet glaciers in this region.

In contrast, the formation of new land–fast ice begins quite late in mid–April, approximately two months after the onset of ice formation. The land–fast ice expansion is interrupted with frequent breaks at its marginal area. As a result, the land–fast ice often reaches its maximum dimensions only in late July, when the last area of the aforementioned summer recurring polynya west of the Tomilin glacier finally freezes. The width of the stabilized continuous land–fast ice band varies during August within 50–60 km. Continuous drifting ice with 100% concentrations is located behind the land–fast ice.

However, sometimes irreversible land–fast ice edge break–up begins as early as September and in late October–early November a complete decay of its westernmost segment behind the Tomilin glacier usually occurs. Here, a recurring near–glacier polynya with average dimensions of 20 by 15 km appears again at the place of broken and exported land–fast ice. Offshore katabatic winds keep its surface ice–free.

The break–up  of the main land–fast ice belt east of the station combined with expansion of the polynya typically extends up to late March of the next year.

During the 1971–79 period, the break–up  process never ended with the final land–fast ice break–up. A land–fast ice segment directly under the station and east of it with a length up to 70 km and a width of 10–20 km was preserved by a concentration of icebergs grounded in coastal shallow water. Multiyear land–fast ice was predominantly comprised of ice more than 10 m thick whose age in 1971 was estimated as 10–12 years old.

In 1980–87, the land–fast ice was completely destroyed every year. From 1988, a tendency for formation of multiyear land–fast ice in this region was again observed.

Although there are little data, first–year ice seemed to form in the vicinity of Leningradskaya in April (see above), grew by early December on average up to 1.5 m, and had about 50 cm of snow on the surface.

The period of its melting is obviously restricted to only one month – approximately from mid–December to mid–January. The decrease of land–fast ice thickness for this time comprises not more than 10 cm.

The growth of unbroken land–fast ice is already observed in mid–January. Probably, it grows both from the top due to freezing of the lower wet snow layer at night hours and from the bottom due to frazil ice formation.

In the case of land–fast ice preservation, it reaches a thickness of not less than 2 m at the second year of its existence and is probably not subjected to melting at all.

Wind waves and swell

No specific information on forecasting has been obtained.

7.12.2                            Terra Nova Bay Station  

7.12.2.1                      Orography and the local environment

Terra Nova Bay (TNB) Station is located at 74° 41' 42" S, 164° 07' 23" E, 17 m AMSL, on a promontory extending out into the Gerlache Inlet sector of Terra Nova Bay (Figure 7.12.2.1.1), on the western side of the Ross Sea. The Northern Foothills shield the base on the west against the katabatic flow sloping down along the Priestley and Reeves Glacier (Figure 7.12.2.1.2). A very extensive and persistent polynya is sustained by such a flow at the confluence of these two glacial valleys in the area known as Nansen Ice Sheet that is only a few miles south apart from the station. The Priestley Glacier flows from the Antarctic Plateau into a narrow canyon 8 km wide and 90 km long aligned along a northwest–southeast direction before joining with the steeper and wider Reeves Glacier that falls in and east–west direction from the same elevation to the sea level in a distance of only 45 km.

Shelter for the Gerlache Inlet is offered by the Campbell Ice Tongue to the east and by the huge Drygalsky Ice tongue to the south (Figure 7.12.2.1.2). that prevents the sea swell driven by the easterlies and the southerlies reaching the station in January and February when the sea ice has drifted away from Terra Nova Bay itself.

7.12.2.2                      Operational requirements and activities relevant to the forecasting process

TNB Station was established in 1985 by the Italian Antarctic Research Programme (PNRA) and is currently open from late October to mid–February only. However, during the summer season the base is very active and both fixed–wing and rotating–wing aircraft are used. From October to December a sea ice runway in the Tethys Bay, the inner part of the Gerlache Inlet, is operational for Italian wheeled C–130 aircraft operating the intercontinental flights to and from New Zealand and is an alternate airstrip for the US Air National Guard ski–equipped LC–130 and Royal New Zealand Air Force (RNZAF) C–130 aircraft heading to McMurdo. Due to the local orography such a runway offers a single direction of approach (Figures 7.12.2.2.1 and 7.12.2.2.2) and the great incidence of katabatic winds (for which this area is famous), makes the glide/take off path very sensitive to turbulence, vertical and horizontal wind shear. For these reasons the wind behaviour is continuously monitored by a set of two anemometers on the runway and other two complete automatic weather stations located at different heights on the surrounding hills. The data gathered are processed to provide real time turbulence and vertical and horizontal wind–shear factors.

The same airstrip is used by Twin Otter aircraft involved in the continental flights to Dôme C and Dumont d’Urville and when in January the air–strip breaks up due to temperature and to mechanical stress induced by wind and sea swell, these operations are diverted to a “white ice” ski–way in Browning Pass (Figure 7.12.2.1.1). a glacial valley adjacent to the Northern Foothills.

In addition every season two to four single–engine Squirrel helicopters are used for the operations in the Transantarctic Mountains between Cape Adare and the McMurdo Sound and are also tasked for Search and Rescue during the take–off and landing of fixed–wing aircraft or in case of emergency along the coastal region of the Victoria Land.

There is frequent HF and telephone communications with the Weather Office at McMurdo Station and within the framework of this cooperation the exchange of TAF, METAR and SPECI messages is included. A similar agreement has been reached with the Dumont d’Urville and Dôme C weather stations for the provision of METAR and SPECI messages before and during the Twin Otter flights heading to Concordia Station and to Adelie Land of which TNB Weather Office is responsible for weather support.

Moreover. the sea operations in which the Research Vessel Italica is involved are of significant importance from the scientific and logistic point of view. Every season such an “ice–class” ship carries on an oceanographic campaign all around the Ross Sea and provides the valuable service of resupplying fuel and goods to the station. Detailed weather assistance is required both when the ship plies the Ross Sea, providing real time weather and sea ice‑concentration maps, and when it is moored on the ice for the load/unload operations and a meticulous monitoring of wind essential.

7.12.2.3                      Data sources and services provided

Two forecasters work at the Terra Nova Bay Weather Office and their duty time is normally from 1700 UTC until 1200 UTC but it can be extended to cover the 24 hours when required. A full six hourly surface observing programme is in operation at the station and there are radiosondes launched every 12 hours at 0000 and 1200 UTC. When air operations are in progress, hourly and special aeronautical observations are carried out and coded in METAR and SPECI codes. Nine and twenty–four hours aeronautical forecasts (TAF) are issued as well. Surface and upper–air data are put in real time onto the GTS through the Italian Air Force Weather Service.

The network (Figure 7.12.2.3.1) of AWSs deployed along the Victoria Land between Cape Phillips and Cape Ross and in the interior of the Antarctic Plateau consists of 13 installations (see Table 7.12.2.3.1 in Appendix 1) aimed on one hand to collect data to enrich the climatological archive and on the other hand to provide valuable observations for aeronautical use and in forecasting on the mesoscale.

All the “climatological” AWSs deliver the information they gather through to the Argos system, while the AWSs installed in crucial sites for air operations can also transmit back to the base via HF radio–modem their measurements to reduce the time between observations down to the proper scale. At Dôme C in the period of camp activity, from November to February, a very advanced AWS has been operational since 1997. Beyond the conventional sensors, it includes also a ceilometer and an RVR sensor and all the data are set up for the issue of METAR and SPECI messages on a Personal Computer controlled by a weather observer. Any aircraft flying within the range of 75 km (~40 nm) from the station can receive on demand such observations by triple–clicking on the VHF radio and get the messages through a vocal synthesizer.

Figure 7.12.2.3.1     Satellite image showing coloured wind roses from part of the       AWS network in Victoria Land. (This is an expanded view of Figure 7.12.2.1.2.)

From 1985 to 1996 a NOAA satellite receiver has been in operation at the base providing high resolution imagery for the forecaster. In 1997 the old receiver was replaced in favour of a dual system able to receive both NOAA and DMSP satellite data. This new implementation has thus increased the imagery availability up to 35–40 passages per day not only for the augmented number of transmitting platforms, but also for a better noise/signal ratio of the receiver. The AVHRR and OLS sensors are extensively used and also the 85 GHz microwave data from SSM/I are processed almost in real time to produce colour sea ice‑concentration maps (Figure 7.12.2.3.2) according to the Svendsen, Mätzler and Grenfell algorithm (Svendsen et al., (1987).

The fields produced by the ECMWF models [1] for atmosphere and sea swell are used for forecasting and are routinely received in GRIB code once a day via INMARSAT. The production of weather maps is then carried on “in situ” and the fields refer to two different frames; a wide coarse grained area (2.0° grid spacing) (see for example Figure 7.12.2.3.3 in which the Southern Ocean is included to emphasize the synoptic signals and a fine resolution area (0.5° grid spacing) (not shown) focussed on the Victoria Land and on the sites of operational interest in the Antarctic Plateau, Adelie Land and Ross Ice Shelf. The atmospheric cross section examined in the forecasting process ranges from the surface up to 100 hPa to monitor the evolution of warm air intrusions in the upper levels over the Antarctic Plateau.

The same products are used to prepare the documentation for continental and intercontinental flights or sea–cruises and are available on demand for any aircraft or ship also in transit.

7.12.2.4                      Important weather phenomena and forecasting techniques used at the location 

General overview

The monthly mean temperature for TNB (see Figure 7.12.2.4.1 in Appendix 2) shows the typical behaviour of the Antarctic coastal regions with a short summer from late November to January and a coreless winter and very short transition seasons (spring and autumn) interposed. In July a reversal cooling trend is evident for all the sites in which the AWSs are installed and the simultaneous relative humidity increase (see Figure 7.12.2.4.2 in Appendix 2) and pressure fall (see Figure 7.12.2.4.3 in Appendix 2) is coherent with the assumption that moister air masses are more frequent in July than in other winter months. An increase of cyclonic activity in this period is also confirmed by the occurrence of stronger katabatic winds (see Figure 7.12.2.4.4 in Appendix 2).

The climate of TNB is heavily influenced by the circulation of the Ross Sea and the strong orographic influence of the Transantarctic Mountains to the west of the station. The Ross Sea is often occupied by decaying depressions that have moved south from the Southern Ocean, with there being climatological easterlies over the southern Ross Sea and a southerly barrier wind along the coast of Victoria land. The ECMWF model shows a good skill for large–scale synoptic weather systems and large–scale fronts and even if it does not provide a fully detailed description for the smaller mesoscale weather systems (<50–60 km), in several cases some useful hints about where and when they form and develop are given to some extent. The partial failure of the model on the mesoscale can be traced back to the following two elements:

·                         The complex orography of the several glacial valleys cutting the Transantarctic Mountains is not highly resolved and such a smoothing leads to a systematic underestimate of the katabatic outflow from the high Plateau. The same phenomenon in the gentler–sloping area nearby Dumont d'Urville is in fact fairly better represented by the model.

·                         The sea ice cover dynamics included in the model is incomplete in that no freezing of the water or melting of the ice is allowed and the sea ice distribution is kept constant during the forecast neglecting the energy involved in these processes that is crucial especially during the austral summer.

Satellite imagery has shown that TNB has many cyclogenesis events on the mesoscale at the boundary of the cold, dry air masses from the interior and the maritime air over the Ross Sea. There is a substantial body of literature on TNB mesoscale cyclogenesis e.g. see Carrasco and Bromwich (1996).

Surface wind and the pressure field

The surface wind field is one of the most important factors to forecast correctly at TNB as winds can be very strong and highly variable over short distances. An examination of TNB AWS measured synoptic wind observations for between 1988 and 1997 inclusive has shown that there were 97 occasions during the colder (March to October) part of the year when the maximum wind gust on the synoptic hour was 52 m s^–1 (~101 kt) or more. On 31 occasions the mean wind was 41 m s^–1 (~80 kt) or more. The highest gust recorded was 64.9 m s^–1 (~126 kt) and the highest average speed was 51.5 m s^–1 (~100 kt). The gust ratio (10 minute‑mean to gust) was varied considerably, for example from 1.18 at 2000 on 6 June 1997 when the average 10 min wind was 51.5 m s^–1 (~100 kt) and the gust 60.8 m s^–1 (~118 kt); to 1.88 at 2100 on 23 August 1989 when the mean wind was 34.5 m s^–1 (~67 kt) and the gust 64.9 m s^–1 (~126 kt) to 2.32 at 1800 26 June 1991 when the mean wind was a mere 22.7 m s^–1 (~ 44 kt) but the gust 52.5 m s^–1 (~102 kt). In all of the 97 observations the wind direction was between 240 and 300 degrees true, with most observations being almost due westerly. Two important types of wind flow that occur in the TNB area are as follows.

(i) Katabatic winds:

The terrain slopes sharply at a number of sites in this area increasing towards the coast, and several glacier valleys channel the flow: the combined effect of channelling and increased slope greatly increases the wind speed. There is therefore a strong wind shear, both horizontal (due to the presence of glacier valleys) and vertical (due to the fact that the layer affected by the wind follows the orography and does not extend to higher levels), which is a severe limitation to aircraft operations. The Tethys Bay airstrip can be affected by both the Reeves and Priestley katabatic flow even if the Northern Foothills offer in general a better shelter for the latter. In the ECMWF model the direction of the two flows is well represented in direction but the wind speed is normally underestimated. Since a downslope wind along the two glacial valleys is quite common and being unable to predict the speed, the ECMWF model in most cases provides more valuable information in the reversal case when an upslope wind is predicted, which is a severe warning for an incoming weather system. In any case the extent of the area involved in the katabatics, which is also directly related to the strength of the phenomenon, is well outlined through the air–mass vertical velocity (ω) near the intersection between the 700–hPa surface and the terrain that is located just above the Antarctic Plateau escarpment and that exhibits down–welling values for katabatic flows.

The wind along the Priestley Glacier is monitored by two AWSs, 7355 (see Figure 7.12.2.4.5 in Appendix 2) and 7352 (see Figure 7.12.2.4.6 in Appendix 2), installed respectively on the top and at the middle–length of the glacier. A speed measurement higher than 18 m s^–1 (~35 kt) on the latter provides a sufficient alert condition for the wind to reach the station and the Tethys Bay airstrip. In fact if the cold flow has enough kinetic energy to climb over Black Ridge and the Northern Foothills, that is to say that the Froude number for these barriers is greater than 1, it will also have an impact on the air operations in the Gerlache Inlet where the coupling with the local orography generates characteristic turbulence, wind–shear patterns and cross–wind along the glide path. The occurrence of strong katabatic events along the Priestley Glacier is also remarked by high wind speed recorded by the AWS 7356 (see Figure 7.12.2.4.7 in Appendix 2) that dominates the AWS 7352 from the height of 1,700 m (~5,500 ft) on the steep northern side of the valley.

At TNB Station (AWS 7353) (see Figure 7.12.2.4.4 in Appendix 2) the Priestley steady flow condition is in general preceded by short peaks of strong wind followed by long lulls and its distinctive signature is the flow direction ranging between 290° and 340° with the higher average wind speed occurring for greater western components.

The highest wind speeds ever recorded at TNB are in the range 45 to 50 m s^–1 (90–100 kt) and are related to the Reeves katabatics since the Northern Foothills gentle relieves offer on this side an easier access into the bay. Due to its orographical deployment, the Reeves Valley channels the katabatic flow along the east–west direction and this is well represented on the wind–rose of the AWS 7350 (see Figure 7.12.2.4.8 in Appendix 2) installed down–wind of the glacier on the Nansen Ice Sheet. Since the weather station has been positioned a few km away from the glacier, the measured wind speed represents an under–estimate of the effective value that the sensor would feel just along the axis of the glacier. On the other hand the AWS in that position is able to monitor also the northerly barrier wind (see below) generated by the Priestley katabatic flow against the Northern Foothills and Black Ridge.

Other indirect evidence provides the forecaster with a good guidance for katabatic wind prediction on the nowcasting time scale:

·                         The clear skies that normally accompany strong katabatic flows favour the detection of blowing snow in the imagery from the AVHRR band–3 sensor from the infra–red channel of the OLS.

·                         Similarly the wind–induced ruffling of the surface of the open water in the polynya determines an abrupt change in reflectivity and emissivity compared to the surrounding calm water. Such a difference is easily detected respectively in the VIS channels of AVHRR and OLS and in the micro–wave channels of the SSM/I.

·                         If the ground level flow is coupled with northwesterly large–scale winds, typical orographic (lenticular) clouds can be observed downwind of orographic obstacles: these clouds can be stationary on the same site for several days. Wind peaks are always preceded by blowing snow above the surrounding mountains.

(ii) Barrier winds:

During summer, a cyclonic circulation is semi–permanent over the Ross Sea and when the easterly flow along the southern Ross Sea approaches the Transantarctic Mountains it follows the orographic barrier as a southerly. There is a clear evidence of this behaviour in the wind–rose of the AWS 7357 (see Figure 7.12.2.4.9 in Appendix 2) installed at Cape Ross, which exhibits a characteristic predominance of southerlies that are normally considered as a first warning of low–pressure systems developing in the south–eastern part of the Ross Sea. Since there is a break in the Transantarctic Mountains in the central part of the Victoria Land corresponding to the Drygalsky Ice Tongue and Nansen Ice Sheet, the barrier winds mix their southerly character with a not negligible easterly component that can let them penetrate well inland and also ascend the glaciers. In fact an 8 m s^–1 (~15 kt) upslope wind at the Medium Priestley AWS 7352 (see Figure 7.12.2.4.6 in Appendix 2) is not unusual and is normally associated with low clouds and quite often with precipitation. A similar behaviour is less evident in the Nansen Ice Sheet AWS 7350 (see Figure 7.12.2.4.8 in Appendix 2) data because of its slantwise position with respect to the axis of the glacier.

In the region between Mount. Melbourne and Cape Adare, the Transantarctic Mountains bend towards the 170° E meridian and induce southwesterly barrier winds that can be easily identified in the wind–roses of the AWSs installed at Cape King (7351) (see Figure 7.12.2.4.10 in Appendix 2) and Cape Philips (7379) (see Figure 7.12.2.4.11 in Appendix 2) where the channelling effect determined by the parallel deployment of Coulman Island orography, makes them stronger than anywhere else along the coasts of the Victoria Land.

The barrier winds are generally well forecast by the ECMWF model while the wind reversal in the glaciers is predicted only when the larger and well developed weather systems affect the Eastern part of the Ross Sea.

Upper wind, temperature and humidity

Upper conditions are predicted using the ECMWF model fields, modified in light of the radiosonde data (see Figure 7.12.2.4.12 in Appendix 2). Moreover the upper atmospheric evolution envisaged by the ECMWF model is cross–checked with the TOVS data that are processed according the Wisconsin University algorithm retrieval. Valuable information on the wind field, such as the position of jet–streams, orographic clouds etc., is also inferred from the satellite imagery.

Clouds

Cloud can be associated with the synoptic–scale weather systems in the Ross Sea or with the more local mesoscale lows. In addition, the barrier wind moving northwards along the coast can cause stratified cloud cover on the coast and for tens of kilometres inland. In the area around TNB an increasing cloud cover extending also inland is always associated with an easing of the katabatic flow in the glacial valleys. Clouds are more likely to develop at the interface between the ice and the open water especially during the summer season when an equilibrium condition between the sea breeze and a weak katabatic flow is reached quite often and low clouds follow the coastline of the Victoria Land. This condition is quite critical for air operations because a local sudden break of the dry and cold katabatic wind may lead to a foggy condition for the airstrips in Tethys Bay and in Browning Pass. For the same reason, in December and January when the ice edge gets closer to the Transantarctic Mountains in northern Victoria Land, that coastal region has a lower ‘degree of accessibility’ with respect to the beginning of the season in October or November especially for single–engine helicopters that require a constant visual contact with the ground.

Since the clouds coverage has a direct influence on the surface and horizon definitions particularly where there is no orographical reference, a correct forecast for this parameter has a strong impact on air operations in Antarctica. The cloud coverage fields produced by the ECMWF model to initialize the internal radiation and precipitation computation provide the forecaster with an approximate guidance. Particular care needs to be taken when using such for operational forecasting due to the above–mentioned underestimate of the dry katabatic flow action particularly on low and medium clouds.

More precise information can be inferred from satellite imagery on the nowcasting time–scale; the top height can be deduced by the top cloud temperature provided by the AVHRR applying the dry or moist lapse rate to the surface temperature. When the cloud edge and its shadow on the surface are clearly discernible an estimate of the top height can be calculated using elementary trigonometry based on the sun elevation at that time and the terrain altitude. The stratiform feature of the Antarctic clouds makes the results of this simple computation considerably realistic.

Visibility: snow and fog

For the most part of the summer period the visibility at Terra Nova Bay Station is good but sometimes it may be reduced by blowing snow and precipitation, and only infrequently by mist and fog. There are plenty of orographical references in the area, such as rocks and peaks, which are helpful even in low visibility conditions.

Katabatic winds stronger than 20 m s^–1 (~40 kt) may induce drifting or blowing snow that may lead the visibility to go down to 6–7 km during which the rocky coasts of the Tethys Bay are always clearly discernible. Only with the occurrence of extraordinary katabatic winds may the visibility be reduced below 3 km. These events are more frequent in winter months and during the short transition seasons. The most dramatic decrease of visibility is caused by moderate to heavy precipitation that can reduce the horizontal visibility to 50–100 m. and determine a white–out condition.

The occurrence of mist and fog during the summer in the area between the Nansen Ice Sheet and Mount Melbourne is related to the above mentioned equilibrium between the katabatic flow and sea breeze in which advection fog forms over the sea and drifts towards the coast by light marine winds when the equilibrium area for short periods is pushed inland. Such a rare phenomenon normally happens during the hottest days in December and January when the majority of the sea ice in the Bay has melted so that the transfer of moisture from the open water to the surrounding air is favoured.

Low clouds and fog may also be driven into the Gerlache Inlet by a low–pressure system developing nearby, but in this case the main limitation for the visibility is determined by the precipitation. The detection and the evolution of fog patches can be investigated through AVHRR channel–3 coupled with the analysis of the fine resolution wind field predicted by the ECMWF model and observed in satellite imagery.

Surface contrast including white–out

Beyond the usual lines of drums or poles used to mark the airstrip location, the variegated orography of the Gerlache Inlet provides plenty of reference elements for the pilots on final approach to the Tethys Bay airstrip even in situations involving extensive featureless clouds or fog. The local orography greatly enhances the surface and horizontal contrast. In fact the relationship between cloud coverage and contrast is relative to the local environment: a low cloud overcast sky provides different conditions over the featureless Antarctic plateau compared to the better–referenced Tethys Bay. It may also happen that if all the geographical references are not well distributed in all the directions, the contrast can be different depending on the direction you are looking at. In the Dumont d’Urville area, for instance, there are a lot of islands but no distinguishing features over the plateau where the airstrip used by Twin Otters has been set up.

In Tethys Bay, when marginal weather conditions prevail, the contrast between the sea ice and open water is normally used to recognize the obstacles–free entrance of the bay, while the contrast of the rock–sides during the "short–final" approach provides an helpful reference during the landing. In summer the katabatic winds generally determine a fair to poor surface contrast, but never reaching white–out conditions, which on the contrary, may be reached in case of moderate or heavy precipitation.

Horizontal definition

There are many geographical references are well distributed all around TNB Station. As for the surface contrast in summer, the katabatic wind is never responsible for white–out condition, while mist, fog and precipitation can determine a featureless boundary between the ground and the sky. For aeronautical use the horizontal definition is provided in all the directions relevant for air operations.

Precipitation

Both synoptic scale and mesoscale lows can give precipitation in this region. In addition, the coastal cloud band associated with the barrier wind can give precipitation in the form of snow with up to 60–80 cm in a few hours being reported. Since the Italian Base has been established in 1985, no reports have been made of liquid precipitation during the summer season, while cumulus and stratocumulus clouds are frequently reported in December and January as indication of limited convective activity. The convection is mostly induced by heating from the sea rather than by the orography that for air masses coming from the ocean has a negligible relevance. The solid precipitation is mostly in the form of snowflakes, but snow grains and snow pellets reports are not unusual.

Cumulated 12 hour total precipitation fields are drawn from the ECMWF model and provide valuable information about the amount and about when and where it may occur. The precipitation is indicated in kg.m^–2 of equivalent water and no information is inferred about the snow depth due to its highly variable density. A somewhat qualitative validation of these fields is regularly performed on the satellite imagery through the estimates of form, texture, and top temperatures of clouds.

Temperature and chill factor

During the period of station activity the temperature has no impact on operations. In October and February the combined effect of wind and temperature may determine extreme chill factors that may go beyond –50°C particularly when the sun is below the horizon. In this period the diurnal temperature variation may also be of the order of 15°C in clear sky condition. From November to January the maximum daily temperature can be well above 0°C reaching as high as 7–9°C and the daily variation decreases to 4–8°C.

A recent verification has shown the 2 m summer temperature derived from the ECMWF provides a considerably accurate description of the data measured by the AWS 7353 for forecasts up to 72 hours. The data at all the prognoses times show a negative bias from the second half of October to the first decade of December, while the bias is positive from December to February. This behaviour corresponds to the two different regimes determined before and after the sea ice melting near TNB Station.

Icing

The low temperatures commonly found at latitudes south of 70°S generally allow for only very low cloud water content and consequently a very low risk of airframe icing. Nevertheless moderate icing being reported by the pilots is not unusual particularly when descending in unstable air masses near the ocean. For this reason during the scientific experiments requiring the installation of external devices on the airframe, the in–cloud flight is prohibited by the operating companies.

The radiosonde data and the forecast sounding drawn by the ECMWF model are used to issue ice warnings and the –23°C isotherm is the threshold under which the icing risk is negligible. Such a choice is determined by the parameterisation of the hydrological cycle performed in the model that does not allow any liquid water below this temperature. This criterion which, according to the pilots is applicable to the central and southern Victoria Land, has been contradicted many times by crews flying to Dumont d’Urville, that experienced in January and February moderate icing and reported temperature as low as –28°C. For this reason before each flight, Dumont d’Urville radiosonde data are required and –30°C is used as threshold for warnings issued during these two months.

Turbulence

Turbulence warnings included in the flight documentation produced for fixed–wing aircraft are drawn from the ECMWF model. Also the position and strength of jet–streams are specifically outlined in the upper–level maps provided for the cruise flight.

Moreover, a diagnostic model containing a fully detailed orography of the area close to Terra Nova Bay Station is used to infer the wind field at various levels up to 600 m (~2,000 ft) from spot measurements. AWS data, upper soundings, and possibly SODAR measurements are assimilated by the model that performs the Richardson Index computation between each pair of levels. Since the information drawn by aircraft pilots operating in the area has shown a good agreement with the model output, the AWS measured wind have been clustered and associated with turbulence patterns that nowadays are considered as the basic rules for flying in this area. The model is generally run close to the radiosonde launch hours to have a closer relationship between the evolution of the surface and of the upper–air parameters.

Hydraulic jumps

The glacial valleys around TNB have the characteristics required for hydraulic (or katabatic) jumps, i.e. strong katabatic flow and rapid changes in orographic gradients. This phenomenon is very common on the edges of the confluence areas just before channelling along the Reeves and Priestley Glacier. In the period of the station activity the highest probability for the occurrence of hydraulic jumps is in October and in the first days of November when the still strong katabatic flow makes them grow up and persist. In most cases the high–resolution OLS imagery provides a quite impressive description of these events for which no routine prediction is carried out.

Sea ice

The most part of the success of the scientific season of the Italian Programme depends on the seasonal behaviour of the sea ice in the Gerlache Inlet and in the Tethys Bay. From April to October, all the area between Cape Washington, the Campbell Ice Tongue and Adelie Cove is fully covered with first–year sea ice. The first drillings taken in October indicate an increasing sea ice thickness along the Tethys Bay airstrip ranging from 3.5m under the eastern threshold to 4.5m at the opposite side. The temperature measured 2 metres below the surface fluctuate in this period around –10°C. Hence on the sea ice cover undergoes a lot mechanical, chemical and thermodynamical processes that will lead to the final breaking. The mechanical action is partly produced by the wind and partly by the sea swell induced by the tide and storms crossing the region.

The katabatic flow on one hand falls against the ice surface and contributes to crack it and on the other hand is primarily responsible of drifting the floes away. The tide induces a low frequency sea swell that greatly enhances the ice frailty, while the higher frequency primary and wind swell raised by storms mainly crumble the ice edge.

The periodical check performed along the airstrip shows that from October to December the ice temperature (T_–2m) increases up to –2°C/–3°C and the ice thickness undergoes a 1‑1.5 m reduction. These may be considered the macroscopic effects of the seasonal increase of temperature, but also other effects occur in the microscopical structure of the sea ice. The incorporation of air and brine determines a dramatic change in the physical properties of the sea ice that weakens the structure of the ice–crystals.

An accurate prediction of the sea icebreaking period is considerably important because sometimes it takes only 2 or 3 hours for the cracks to percolate in the Tethys Bay and in the Gerlache Inlet. Since this phenomenon is determined by the concurrence of the above‑mentioned elements they all must be taken into account; a good guidance for the prediction is the periodic check of the detachment of the sea ice edge from the rocks delimiting the bay. When only a weak constraint is applied at the boundaries, the ice sheet may be easily and quickly broken by the tidal and/or the primary swell. After that, a katabatic event may sweep away all the floes in less than 4 to 5 hours. This generally happens in January with highest probability in the 3^rd or 4^th week, but the fraction of the sea ice that will be drifted away is difficult to predict and varies considerably season by season. At this stage many floes may be re–pushed in by southerlies winds and swell, but the evidence that no iceberg has ever been found in the Bay in October at the beginning of the season, suggests that the refreezing phase, which at the end of February is already on the way, is preceded by strong katabatic winds.

Wind and swell

For ocean wave forecasting the ECMWF WAM model on 2.0° by 2.0° grid is used. On this basis forecasts up to 72 hours of significant height and direction of primary and wind swell forecasts are issued for the Italica Research Vessel and on demand for any ship in transit in the region. Such predictions have been also tailored to the route to and from Lyttelton (New Zealand) for which specific weather maps are produced.

In the Ross Sea the underestimate of the katabatic forcing requires some corrections that are computed from the speed, fetch and duration of the observed wind. A good estimate of the wind forcing on open water surfaces is provided by the SSM/I imagery that, in turn, permits one to validate the model indications.

7.12.3                            McMurdo Station (inc. Scott Base)  

7.12.3.1                      Orography and the local environment

McMurdo Station (77.86° S, 166.67° E, 24 m AMSL) is located near the tip of Hut Point Peninsula on Ross Island. The New Zealand Scott Base is located very close by (see Figures 7.12.3.1.1, 7.12.3.1.2 and 7.12.3.1.3). The peninsula extends about 13 km (~7 nm) southwest of the lower slopes of Mount. Erebus, an active volcano with a summit of 3,794 m (~12,488 ft). Mounts Terra Nova (2,130 m) (~6,989 ft), Terror (3,250 m) (~10,663 ft) and Byrd (1,800 m) (~5,906 ft) make up the remainder of Ross Island (Figure 7.12.3.3). The McMurdo Sound lies directly west of McMurdo and is the southern most extension of open water in the late summer (Figure 7.12.3.1.4). The Royal Society Mountains (in the Asgard Range (see Figure 7.12.1), and its backdrop of the ice plateau, provide intrusions of Antarctic air mass within the region. Each environment is contrasting in terms of temperature and moisture profiles with modification and mixing of these air masses in the near proximity to McMurdo. Extremes of this contrast are at a maximum during the height of the summer season when open water is present, allowing for greater capabilities of cloud enhancement or fog development.

7.12.3.2                      Operational requirements and activities relevant to the forecasting process

McMurdo provides weather data, observations, and forecasts for all United States Antarctic Program operations. Typical weather facility operations include:

·                         Area observations and forecasts for safety for McMurdo and all operating camps to include the South Pole Station;

·                         Climatic information;

·                         Transoceanic flights from Christchurch, New Zealand to McMurdo:

–Air Force C–5

–Air Force C141

–Air National Guard and US Navy LC130 Hercules

–NZRAF and Italian C130 Hercules;

·                         Continental Flights from any point to any point by USAP Aircraft (normally originating from McMurdo Station):

–Air National Guard and US Navy LC130 Hercules

–Twin Otter

–PHI Helicopter;

·                         Information for Safety of Flight purposes is provided to any requesting aircraft within the McMurdo Area of Responsibility;

·                         Ship forecasting is provided for USAP vessels within the McMurdo Area of Responsibility.

7.12.3.3                      Data sources and services provided

McMurdo predominately utilizes USA Navy Fleet Numerical computer products available via the Internet. The global model is updated twice daily. Real time data from AWSs, composite satellite imagery, and computer products are available and frequently down–loaded from the University of Wisconsin web site. 

NOAA, DMSP, and Meteor satellite data are received and processed providing McMurdo Station and the Ross Ice Shelf with nearly continuous coverage. The TeraScan image processor enables overlay, and still animation for the DMSP and NOAA images. Cooperation with other existing TeraScan sites periodically assists with images outside of the receiver’s swath.

Products and satellite images are normally distributed by the McMurdo or Christchurch Office. Requests are made periodically to fax or e–mail computer information to various sites to support USAP related programmes.

7.12.3.4                      Important weather phenomena and forecasting techniques for McMurdo

General overview

McMurdo is normally within the boundaries of the barrier wind set up as a thermal contrast between the plateau and Ross Ice Shelf (see Sections 2.6.7.3 and 6.6.1.4). The barrier winds move north along the Transantarctic Mountains and are forced around Ross Island providing McMurdo Station with a northeast wind. The plateau high frequently extends across the McMurdo Sound into the Ross Island and the western portion of the Ross Ice Shelf. This situation provides for generally fair conditions at McMurdo. Because of this pattern, low–pressure systems located on the Ross Ice Shelf cause a large pressure, temperature, and moisture contrast along the barrier winds. With a breakdown or over–running of the barrier winds, the low and associated weather is allowed to move into the Transantarctic Mountains and McMurdo Sound.

Figure 7.12.3.1.3     A map showing the location of McMurdo Station and Scott Base in relation to Mount Erebus and the other nearby mountains. (See also Figures 3.4.3.2 and 3.4.3.3.) (From http://www.ees.nmt.edu/Geop/mevo/erebus_info.html,, courtesy of Philip Kyle.)

Several general synoptic patterns effect the weather at McMurdo and are the product of the zonal or meridional upper–level flow between Antarctica and the mid–latitudes. The synoptic flows in the McMurdo area are enhanced or hindered from normal progression due to large geographic obstructions and wide variations within the local environment. Synoptic‑scale highs and lows may directly effect the area’s weather but more often it is the indirect relationship of mesoscale systems or the initiation of local winds that cause temperature and humidity variations around McMurdo.

The upper–level pattern frequently referred to as the Ross Sea Low provides perhaps the best indications of the movement of synoptic scale highs and lows within the Ross Sea area. The upper‑level low is normally produced from a continuation of decaying waves within the region. From this origin it contains many characteristics of the wintertime Aleutian Low.

Figure 7.12.3.1.4     This photograph of Hut Point was taken from the deck of the U.S. Coast Guard Cutter Polar Star. (Vince's Cross and Scott's Discovery Hut are visible. From http://www.geocities.com/~kcdreher/mcmurdo3.html © 1998 Keith C. Dreher.)

When the low is within about 600 km (~300 nm) of McMurdo, high humidity is expected from the surface to 3000 m (~10,000 ft). This increase in humidity will relate to layers of stratified clouds, and frequent fog patches over most of the Ross Ice Shelf and Ross Sea. Results from this pattern would drive an occluded low, migrating from the Indian Ocean into the Ross Sea, in a more northern direction. This would make a “direct hit” on Ross Island highly unlikely since the surface low would be directed away from Victoria Land. It would also disfavour initiation of katabatic winds allowing the plateau high to spread but not build in intensity. This system may advect additional moisture into McMurdo if the warm air advection side of the low extends into the Ross Sea as it moves north of Cape Adare.

A Ross Sea low within about 600 km (~300 nm) of McMurdo generally equates to long periods of stable conditions, light winds, and low stratus ceilings. Occasionally an easterly wind pattern will force a piling of moisture into the Royal Society Range, which develops nimbostratus clouds and light snow progressing back across the McMurdo Sound. The best forecasting tools are:

·                         Do not expect any long–term clearing while a 400–hPa low is within this range.

·                         Minor ridges may provide a southwest wind from 1,500 to 4,500 m (~5,000 to 15,000 ft). This will provide the best possible weather pattern, normally working the moisture away from McMurdo Sound. This flow would be abnormal and should not be expected to last for any length of time considering the location of the Ross Sea low.

·                         Surface to 3,000 m (~10,000 ft) winds from the southeast push moisture from the Ross Sea into the McMurdo Sound. Gathering of the moisture and additional lifting is provided with the Royal Society Range west of McMurdo. This will develop the clouds from the bases downward in a nimbostratus cloud formation. The clouds will develop across the sound and light snow can be expected.

With a Ross Sea low 600 to 1,100 km (~300 to 600 nm) from McMurdo wide variations can be expected within the weather patterns. With wide spacing between McMurdo and this steering low centre, synoptic scale systems may affect the area.

Polar highs will split off the Antarctic air mass over Victoria Land to separate migratory lows. In this situation the McMurdo area is indirectly affected by the expansion of the plateau high, providing clear skies with normal northeast winds. A strong upper–level ridge west of McMurdo will indicate the set up of this situation. The 500–hPa vorticity chart will normally pick up an associated channel jet over Victoria Land into the Ross Sea up to 48 hours in advance of the formation.

When the Ross Sea low drifts eastward, occluded lows may migrate into Victoria Land prior to entering into its steering flow. This will normally provide enough upper–level cyclonic support to develop a triple–point low that will proceed around the Ross Sea low dragging the existing occluded front west to east across the Ross Sea. In this situation high pressure is developed over McMurdo and fair conditions prevail. Moisture and warm air is limited to the flow around the newly formed low well to the north keeping cold dry air in the region.

When the Ross Sea low is in this location mesoscale lows may develop near the transition from the Ross Sea to the continent’s coast near 150º W. This is normally initiated by a short wave progressing around the eastside of the low. With a natural trough and warmer temperatures in the Ross Ice Shelf as opposed to the surrounding continent, lows can further develop as they move around the south end of the Ross Sea low. Based on the steering level the low may move into the McMurdo area causing increasing clouds and snow if they can break the protective barrier wind pattern. If the upper–level flow forces the low into the Royal Society Ranges, the low may stall near McMurdo Station and heavy snowfall can be expected for an extended period. One such system in December 1997 deposited over 350 mm of snow during a 27–hour period.

The absence of a Ross Sea low will produce normally the best weather for extended periods. The plateau high is allowed to naturally flow off the continent allowing for cold dry air to dominate over the Ross Ice Shelf and into the Ross Sea. This is the normal flow of the winter months as expected. It may also be seen in many seasons during the early summer months when there is a gradual disparity between the plateau temperatures and warmer Ross Sea temperatures and absence of any upper–level cyclonic behaviour preventing surface low development.

Katabatic winds are widely studied around the McMurdo area. They are known to cause clearing weather patterns as dry air descends into the region. The katabatic flow can be divided into two categories of weather–related phenomena and two separate categories noted in forecasting the onset and intensities.

The building of the plateau high over Victoria Land will normally produce a glacier flow in the Ross Ice Shelf or Ross Sea affecting McMurdo on a regular basis. The katabatic winds are produced from an imbalance of the horizontal pressures between the plateau and the Ross Sea or Ross Ice Shelf. An increase in subsidence resulting in higher surface pressure over the eastern plateau will normally promote the onset of this wind pattern. As the pressure increases the flow around the anticyclone builds and forces the air outward. When the high builds to the west of the Transantarctic Mountains (eastern ice plateau) the air flows down the glacial passes spilling onto the ice shelf to the south, or Ross Sea to the north. A number of factors can cause the increase in the subsidence from low level cooling to upper‑level convergence.

The most noted and easiest forecasting event is the increase of upper–level ridging to the west of the Transantarctic Mountain Range. The upper–level confluent area will exert pressure on the underlying layers forcing downward vertical motion. With its decreased compressibility from cold air properties it has a more immediate reaction to subsidence. Surface air over the plateau will move anticyclonically and outward. When the outflow extends across the Transantarctic Mountains the wind will be forced down the glaciers, warming adiabatically as it descends. Stronger flow is noted when the ridging creates a perpendicular component to the mountain ranges promoting an aligned upper–level force with the surface wind direction. When this situation occurs over the Ross Ice Shelf, McMurdo will have an initial strong northeast wind and may be accompanied by reductions to visibility for one to three hours in blowing snow. This reduction is normal except during the mid– to late–summer when the snow is too moist and heavy to be carried. Due to the orientation of Ross Island to the wind flow the availability of loose snow is depleted within this limited time frame.

The second situation is the onset of katabatic winds north of McMurdo. This can have varying effects on McMurdo’s weather. The dependent variables are low–level winds and availability of open water in the Ross Sea. A dry katabatic wind pattern flowing into a low‑level cyclonic circulation with open water can result in an explosive low formation. The dry relatively cold air over relatively warm water creates this situation. Care is always taken in this situation to determine whether the low–level circulation will engulf Ross Island. When the pattern develops with outflow at Terra Nova Bay and moves cyclonically around Ross Island, heavy snow and light east to southeast winds can be expected. This situation has been commonly referred to as a Terra Nova Bay low. This weather will persist as long as the upper–level flow supports the glacier winds at Terra Nova Bay and upper–level support is provided to support the surface low.

More difficult to forecast is the effects of cyclones in the Ross Sea or Ross Ice Shelf. The low can equally cause the horizontal pressure disparity required for the onset of glacier winds. In this situation the low must be broad enough to encompass most of the Ross Ice Shelf or Ross Sea. It must redirect the barrier winds into its centre to replace ascending air. As this occurs the barrier wind pattern away from the Transantarctic Mountains are replaced by a light glacier flow.

The katabatic winds will provide a drying effect on the lower cloud masses either preventing them from entering McMurdo Sound or delaying them for several hours. This situation provides difficulties in forecasting the motion of weather producing low clouds. When this situation arises with a low to the south, McMurdo commonly will stay under a blanket of altostratus with cloud base near 2,400 m (~8,000 ft). McMurdo’s wind may change from northeast to light and variable, then swinging to the northwest towards 5 m s^–1 (~10 kt) as the barrier winds are redirected into the low. Temperature disparities commonly exist between the low and the drier air boarding McMurdo and the foot of the Transantarctic Mountains. When the temperatures become modified closer to that of the low the katabatic winds may be subsiding. This could be in part from the entrainment of the barrier winds filling the low–pressure system and eliminating the horizontal pressure contrast that created the katabatic flow. At this point the low clouds will meet no resistance and if the low–level flow supports it they will move into McMurdo area.

When the same situation is presented with a low to the north the dynamics of the low change. This low will be moist and warmer than its ice shelf counter part. The relatively cold dry katabatic air will become cold and moist as it modifies over the Ross Sea. This will induce new instability into a decaying wave, or rapidly modify a mesoscale cyclone (a Terra Nova Bay low). The concave curvature of the Transantarctic Mountains within the region will also assist in confining the cyclonic motion in the lower levels. In this situation it is required to know what the upper–level wind pattern is doing. If this system is directed into the McMurdo area around the Ross Sea low, or if it is vertically stacked under a Ross Sea low within the proximity of McMurdo, heavy snow and gusty winds can be expected. The onset of severe weather is marked by the transition of the upper–level ridge moving across Ross Island. This can be easily identified in the accompanied cirrus shield in all observed decaying wave situations.

Surface wind and the pressure field

Prevailing wind over the eastern Ross Ice Shelf is southerly due to the barrier wind as discussed. Although Ross Island is located in this band of southerly winds, the predominant McMurdo winds are easterly as the cold and denser air flows around the orography of Ross Island rather than over it (see Figure 7.12.3.4.1 (in Appendix 2)). With a strong increase in the pressure gradient the flow will begin to scale over rather than around Ross Island causing a shift of the wind direction toward the south.

Low–level jet winds can develop if winds below 500 hPa shift to southerly and strengthen to above 20 m s^–1 (~40kt). This southerly jet can cause surface winds above 25 m s^–1 (~50kt) producing severe low–level turbulence. This condition will frequently be accompanied by extensive cloudiness. The onset of the strong surface winds usually will be within 12 hours of the development of this jet.

McMurdo’s wind direction may also be affected by high and low–pressure systems within the region. These systems can modify or eliminate the prevailing wind flow.

Table 7.12.3.4.1 (in Appendix 2) lists mean–monthly MSLP values at McMurdo Station.

Upper wind, temperature and humidity

Ross Island normally falls within a westerly upper–level wind pattern. This pattern follows the normal polar vortex wind direction. The formation of the Ross Sea low and its position frequently disrupt the flow, and intensity of the Ross Sea low will drive low–level features.

Temperature and humidity fluctuations are common. Dry cold wind from the continent normally will dominate the region. During the summer months large areas of open water exist on the north side of Ross Island introducing a ready source of humidity for the cold dry air. Low–level wind patterns, surface inversions, and wind speeds dictate the results that normally equate to low clouds or fog when the moisture is advected over the cold surface of the Ross Ice Shelf.

Clouds

Clouds within the region are generally limited to stratiform at various levels. Cap clouds are common over Minna Bluff, Ross Island, and Mount Discovery indicting increasing winds at those altitudes.

Visibility: blowing snow and fog

Visibility normally exceeds 50 km (~27 nm) with limits by land obstructions and the earth’s curvature. Fata Morgana (a shimmering inverted and elevated mirage) is common and on occasion Superior Morgana* can be observed. The most common obstruction is light snow and blowing snow. The snowfall is generally light enough that under calm winds the visibility will remain above 4,800 m (~2.5 nm). Since the snow is so light and dry it is easily displaced by the wind. Winds above 5 m s^–1 (~10 kt) will cause the visibility to lower to less than 1,600 m (~0.9 nm) for hours after the snowfall has ended. Wind directions from any southerly component provide the worse reductions. *(The “superior” mirage forms when cold air lies beneath relatively warmer air. In these inversion conditions, light rays refract, or bend, toward the colder (and denser) air, that is, downward. This bending causes the image of the object to appear to be above its actual position because one’s brain assumes the light rays have taken a straight path from the object to one’s eyes. The rate of increase of temperature with height (the lapse rate) affects how the light rays travel from the object to the eyes and thus how one might see the resulting image pattern.)

During the summer months snowfall becomes moist, the flakes grow in size and adhere to the surface easier and faster. Winds in excess of 15 m s^–1 (~30 kt) are normally required out of the south to provide any prolong reductions in the visibility from blowing snow alone. With the increase in flake size reductions to less than 3,200 m (1.7 nm) will be observed even in calm winds.

Fog has been noted in all months of the year but most frequent occurrences are during the peak summer months. The cold surface of the Ross Ice Shelf provides the temperatures to condense moisture advected from the Ross Sea. The Windless Bight with its natural divergent wind pattern is a common area for fog to form and rapidly expand. Wind directions pushing moisture into this area are cause for forecasting fog in McMurdo. Fog most frequently occurs during the middle to late morning hours. Although diurnal effects are minimal it tends to assist fog development in these situations.

Fog normally develops when McMurdo falls on the fringe of any mesoscale high that promotes moisture advection into the windless bight. Fog has also been observed in advance of mesoscale lows moving from the Ross Sea into the Ross Ice Shelf. This situation pushes low–level moisture around Ross Island into the Windless Bight. The latter of these situations causes only a short–term fog condition that is quickly removed as the low skirts just east of Ross Island and then switches the wind direction as it passes.

Surface contrast including white–out

McMurdo has an established runway with markers, a tactical air navigation (TACAN) system, approach lights, and strobes. Even with all the amenities surface definitions are used and transmitted on all hourly METAR observations. Because of the runway markers and large non–snow covered landmasses within the area white–out conditions from optics alone is not a factor.

Horizontal definition

Contrast in the horizon definitions is easily discernible against the numerous landmarks within the region. Snow, blowing snow, and fog can frequently obscure it. Horizon definitions are used and transmitted on all hourly METAR observations.

Precipitation

Snowfall occurs all year with a maximum from December to March associated with the availability of moisture and the warmer air temperatures. Snowfall is normal with any meso or synoptic–scale low within the region. Heaviest snowfall accompanies decaying waves that retrograde across the Ross Sea and move toward Ross Island from the southwest. Terra Nova Bay lows (formation discussed earlier) will provide a continuous snowfall for extended periods.

Liquid precipitation is uncommon but has been observed. To forecast the occurrence parameters for forecasting a cold rain process should be used.

Ice needles, snow grains, and ice crystals have all been observed at McMurdo during the coldest months. Mean–monthly precipitation values at McMurdo Station are given in Table 7.12.3.4.2 in Appendix 2.

Temperature and chill factor

McMurdo’s diurnal effects are limited to the sun angle and obstructions of mountainous terrain. Large temperature variations are attributed to temperature advection. Extreme maximum temperatures have been noted when warm air is advected from the Ross Sea to the Ross Ice Shelf, when the air rises moist adiabatically over the Transantarctic Range and then falls dry adiabatically into the McMurdo Sound. Wind chill is monitored during all months and special weather warnings are in place to limit exposure under harsh wind chill conditions. Mean–monthly maxima and extreme temperatures for each month for McMurdo are given in Table 7.12.3.4.3 in Appendix 2.

Icing

Aircraft icing is normally restricted to the summer months with nimbostratus cloud formations. When suitable conditions exist light occasional moderate rime icing can be seen and on rare occasions moderate mixed icing has been observed.

Turbulence

Gusty surface winds can provide light to severe turbulence normally from the surface to near 1,500 m (~5,000 ft). Funnelling of air between Ross Island and the continent provides gusty winds in excess of what the pressure gradient would suggest.

Hydraulic jumps

McMurdo experiences periods of sudden pressure changes, squall line type weather patterns, and changes of inversion heights. This may be the attribute of atmospheric hydraulic jumps and any correlation is being studied at this time.

Sea ice

Sea ice in McMurdo Sound normally allows for shipping to be practical by late December. The sea ice naturally breaks by this time to south of Cape Royds. Coast Guard cutter operations begin near the first of the year breaking southward into McMurdo Station. McMurdo Sound will completely refreeze by early April, with the exceptions of open areas driven by strong off shore winds.

Wind waves and swell

Wind waves within the McMurdo Sound are normally slight to nil due to limited fetch. Sea ice concentrations and land–mass restrict the wave development.

7.12.4                            Transantarctic Mountains  

7.12.4.1                      Orography and the local environment

The Transantarctic Mountains provide a primary glacier source and western barrier for the Ross Ice Shelf. This formidable barrier extends from the continent’s edge at Cape Adare southward dipping below 87º S (Figure 7.12.1). Individual mountains average nearly 3,000 m (~11,400 ft) with the ice plateau covering the western slopes dropping to sea level on the eastern side. The Antarctic air mass is the predominate weather feature of the Transantarctic range. The dense characteristic of this mass only requires the slightest of external forces to begin the katabatic flow from the ice plateau into the Ross Ice Shelf. The Ross Sea to the north provides frequent intrusions of maritime air over the Ross Ice Shelf and may extend into various levels over the Transantarctic range. This will normally set up a density imbalance increasing cyclonic behaviour over the ice shelf and frequently forces the air with modified maritime properties into the Transantarctic region.

7.12.4.2                      Operational requirements and activities relevant to the forecasting process

The United States Antarctic Program will frequently establish camps within the Transantarctic range south of McMurdo. Various fixed and rotor wing air support is directed from McMurdo Station.

Tourists flights (Boeing 747–400 aircraft) originating from Sydney or Melbourne, Australia during the summer season aim to over–fly the Transantarctic Mountains as far south as about 75º S. (see http://www.anzac.com/at/ant/antdd.htm and Figures 6.6.2.1 and 7.12.4.2.1). The forecasts for these flights are provided by the Antarctic Meteorological Centre at Casey (when staffed) or by the Australian Bureau of Meteorology Regional Forecasting Centre in Hobart, Tasmania, Australia.

7.12.4.3                      Data sources and services provided

USAP weather operations are limited to camp personnel providing basic weather observations upon request to the McMurdo weather facility via HF radio. Camp personnel will transport portable weather sensors to provide temperature, winds, and pressure.

7.12.4.4                      Important weather phenomena and forecasting techniques for the Transantarctic Mountains south of McMurdo Station.

General overview

The variation in altitude and orography makes this a complex weather area: the complexity ameliorated to some extent by the remoteness of the area from moisture sources.

Surface wind and the pressure field

The eastern slopes are normally within the boundaries of the barrier winds set up as a thermal contrast between the plateau and Ross Ice Shelf. These higher elevations fall within the established cold and dry Antarctic air mass with a normal light to moderate downslope wind feeding into the barrier wind flow. The predominant wind direction for unsheltered locations is typically a pattern of funnelling to the closest glacier.

Extreme elevations of this region and modification of advected maritime air into the Ross Ice Shelf limited the duration of clouds and snowfall. Only deep–pooled moisture patterns when coupled with a strong up slope wind field produce ample energy to overcome the massive terrain feature. These wind patterns can be attributed to a decaying wave in the Ross Sea or along the Hobbs Coast. The influence of such a system can produce enough horizontal temperature contrast to sustain jet level winds and vorticity lobes far into Marie Byrd Land. This wind will support and often enhance low–level cyclonic features in the southern Ross Ice Shelf. With the proper wind field the moisture can be driven into any portion of the Transantarctic Mountains spreading stratus clouds, light snow, and strong surface winds far into the ice plateau.

Upper wind, temperature and humidity

The 500 to 400–hPa winds provide a good source for forecasting progression of low‑level moisture over the Transantarctic Mountains onto the plateau. A Ross Ice Shelf depression in a tight easterly 400–hPa flow that extends over the plateau provides the perfect means to push the mass over the area.

Clouds

Mesoscale lows migrating northward along the eastern slopes of the Transantarctic Mountains will frequently drive low cloud or fog up the glaciers as the low moves north. Any katabatic outflow will restrict this cloud movement or force the depression and associated clouds eastward into the central ice shelf continuing fair skies over the much of the region.

Clouds are dependent on specific location and elevation. The primary moisture source is the Ross Sea. Cap clouds and lenticular cloud formations are the norm for advancing systems. Note the direction of the lenticular or cap clouds to assist in forecasting whether low level stratus will be forced into a particular location. The more easterly the clouds extend the greater the threat of advancing low clouds becomes.

Visibility: blowing snow and fog

Snowfall will normally be light and fog will exist in higher elevations. With many protected areas it is not unusual for the low stratus or fog to stall over the region for extended periods. The onset of any katabatic wind flow is the normal means to decrease the cloud cover.

Surface contrast including white–out

No specific information on forecasting has been obtained.

Horizontal definition

No specific information on forecasting has been obtained.

Precipitation

No specific information on forecasting has been obtained.

Temperature and chill factor

Temperature ranges are dependent on the elevation and latitude. In such a large region with these two items are too vast to be specific.

Icing

No specific information on forecasting has been obtained.

Turbulence

Turbulence is rare and normally only exists in the presents of cap clouds or jet cirrus. When these items are present, light and on occasions, moderate turbulence has been observed.

Hydraulic jumps

No specific information on forecasting has been obtained.

Sea ice

Not relevant to this location.

Wind waves and swell

Not relevant to this location.

7.12.5                            Amundsen–Scott (South Pole) Station

7.12.5.1                      Orography and the local environment

Amundsen–Scott (South Pole) Station is located within the proximity of the geographical South Pole. At an elevation of 2,800 m (~9,000 ft) and near 13,00 km (~700 nm) from the closest water source, commonly only a single air mass effects the local area. Periodical influences of modified maritime polar air can migrate into the region on occasion bringing some snow, increased wind velocity and a gradual increase in temperatures. With all cardinal points directing north, a grid system of navigation is used. All referenced directions will be in the grid system, using prime (Greenwich) meridian as north and the date line (180° longitude) as south. The area to the west of the South Pole, typical of western Antarctica, provides a gentle slope to sea level. The Ronne Ice Shelf and Weddell Sea lie just past the Pensacola Mountains to the grid northwest, providing the closest moisture source. The slope continues to rise in nearly all directions to the east. The Transantarctic Mountains extend to the south, dropping to lower elevations over either Marie Byrd Land or the Ross Ice Shelf.

Elevation and seclusion nearly eliminate intrusive air masses at this location. The polar high dominates the continent with common high–pressure centres situated over the extreme elevations of Eastern and Western Antarctica. The high over the greater plateau to the east normally extends over Amundsen–Scott Station providing cold dry conditions throughout the year. The most common intrusion of moisture is accompanied by a decaying wave in the Weddell Sea. This locale is the most direct route for a maritime flow and will normally only progress over the extended distance and scaling the elevation barrier when ample jet winds and dense multiple layered cloud cover are still associated with the depression.

7.12.5.2                      Operational requirements and activities relevant to the forecasting process

Amundsen–Scott Station provides surface synoptic observations every 6 hours and upper–air observations every 12 hours during the normal operating summer season. Hourly METAR and special weather observations are taken as needed during flight operations. Amundsen–Scott synoptic and upper–air data transmitted via McMurdo Weather to MET New Zealand over the Aeronautical Fixed Telecommunications Network (AFTN). Data are also available for climatic purposes by accessing the National Science Foundation Web Page. Forecasting facilities are located at McMurdo Station. South Pole weather operations include:

·                         Area observations for safety at Amundsen–Scott Station;

·                         Climatic information;

·                         Continental Flights from McMurdo Station:

–Air National Guard and US Navy LC130 Hercules;

–Twin Otter.

·                         Information for Safety of Flight purposes is provided to any requesting aircraft via HF broadcast.

South Pole monthly climatological data are also placed on the ATSVAX in Miami in the anonymous.polewx directory, and can be retrieved by anyone that has the ability to FTP. Users wishing to download the files should log on ATSVAX.RSMAS.MIAMI.EDU using the password "anonymous". After changing to the (polewx0 directory, the desired file can then be retrieved. Typically the files are kept on the ATS VAX for a period of one month. In addition to the files that are stored on the ATS VAX in Miami, various research groups will occasionally make direct requests to this department for climatological data dating back to 1957.

7.12.5.3                      Data sources and services provided

Amundsen–Scott provides observational information for operational use and climatology. McMurdo Station provides all forecasting support using numerical products and satellite imagery. Communications with McMurdo exists on a 24–hour basis via HF communications and periodical windows of e–mail service.

7.12.5.4                      Important weather phenomena and forecasting techniques for South Pole Station

General overview

Amundsen–Scott Station is dominated by the plateau high with periodical influences of modified maritime air from the Bellingshausen Sea, the Weddell Sea, and on rare occasions, residual moisture from the Atlantic Ocean via Dronning Maud Land. The most typical and intense systems are those from the Weddell Sea due to the proximity and gradual slope.

The predominating plateau high extending from Eastern Antarctica produces a light northeast wind normally less than 5 m s^–1 (~ 10 kt). With a compressed upper–air pattern following the polar vortex, clouds in the region will lack the basic dynamics to develop providing mostly clear skies and unrestricted visibility as the norm.

The normal northeast wind, clear skies and cold temperatures can abruptly end when a jet finger propagating around a decaying wave moves over the South Pole transporting moisture from the expanded decaying system. The upper–air sounding may herald poor weather by showing slight low level wind shifts in the direction of the advancing system, warm air advection indicted by height rises, and increased moisture patterns. More frequently is the lack of telegraphing signatures in the upper–level due the extreme barotropic nature of the air masses. Poor weather normally accompanies or is in advance of all changes to the upper–level pattern.

Residual stratified clouds may become trapped in inversion layers over the plateau and meander with the lower level winds. The stratified cloud masses frequently move slowly and may produce snow grains or ice needles reducing both ceiling and visibility at the station.  The slow nature its motion will allow a small cloud mass to hinder flight operations for several hours.

Forecasts from McMurdo Station are typically developed using satellite imagery and computer model output. Look for an upper–level ridging pattern and associated cloudiness intruding from a coastal location. The low–level wind directions are monitored for shifts from the norm (Figure 7.12.5.4.1). When all the above are present an expected shift is forecast normally to the grid northwest. The thickness of the cloud pattern, forecasted wind fields from computer charts and IR temperature patterns off the satellite provide guidelines to the intensity of poor weather. Frequently a thick smooth arching cloud pattern will equate to visibility dropping under 1mile for the duration of the cloud pattern. Overcast to broken clouds will persist throughout the period of reduced visibility with a slow rise in heights above 300 m (~1,000 ft) as the cloud pattern thins from precipitation and air–mass modification.

Surface wind and the pressure field

Prevailing winds over the South Pole migrate around the plateau high centred over Eastern Antarctica with a downslope influence into Western Antarctica. This produces a grid northeast wind, which may vary from 010º grid to 080º grid depending on the potential temperature difference from west to east Antarctica. Table 7.12.5.4.1 (in Appendix 2) shows mean–monthly wind speeds and directions for the “Clean Air” AWS at the South Pole.

Winds from the northwest are typically influenced by the remnants of a decaying low advancing or spreading out from the Weddell Sea. The degree of wind shift truer to the moisture source normally equates to the intensity of the weather. A depression moving or expanding in such a manor producing a wind direction that would provide the maximum amount of moisture with the least amount of modification with near equal dependency on the wind speed.

Wind directions from the south are rare as stated in Figure 7.12.5.4.1 but may occur if a depression is forced inland over Marie Byrd Land. This may occur from blocking highs extending into Ellsworth Land (Figure 7.13.1). These systems are intense over the southern portion of the Transantarctic Mountain Range as indicated by satellite depicting turbulent wave clouds with identifiable directions toward the Pole. With sufficient force and strong reinforced temperature advection over Marie Byrd Land the system can scale the geographic incline and move into the South Pole. Winds will slowly progress to the grid southeast. These systems can produce snow grains and even snow over the area. The effects of the system are normally short lived with rapid modification as the flow typically redirects northward toward McMurdo Station.

Table 7.12.5.4.2 (in Appendix 2) shows mean–monthly station–level pressure for the “Clean Air” AWS.

Upper wind, temperature and humidity

Mean January and July upper–level wind roses for Amundsen–Scott are included Figures A3–9 (a) and A3–9 (b) (in Appendix 3) while mean–temperature profiles for Amundsen‑Scott are shown in Appendix 3 as Figures A3–4 (a) and (b).

Amundsen–Scott upper–level winds are typically a reflection of plateau high in the lower levels changing to a influenced direction from coastal lows in the mid levels, and changing to the polar vortex pattern in the extreme upper levels. The mid levels (500 to 300 hPa) drive the majority of changes within the surface weather features as discussed in the general weather description.

Temperature fluctuations are subtle and they are normally preceded by poor weather. Surface humidity is not reported at the site but influences of maritime air are consistent with obscuring weather phenomena. The most common are ice crystals, and ice needles, followed by snow grains, which are present when an intense system migrates into the pole.

The data from the radiosonde programme provides information on conditions at upper levels. In addition, these observations are assimilated into the numerical analyses for the model fields around the station should be fairly reliable.

Clouds

The surface observations indicate that the annual average cloud amount is close to 45%.  However, there are problems for the observers in deciding how to report the semi–permanent veil of thin cirrus that is found over the area. This can either be reported at zero oktas or eight oktas of cloud, depending on the thickness of the cloud. Intrusions of lower latitude air associated with synoptic disturbances obviously introduce thicker cloud to the area. The most frequently reported clouds at the station are altostratus and cirrus.

Visibility: ice crystals and fog

Ice fog and ice crystals are the only conditions that produce "instrument flight rules (IFR) weather" at the South Pole. Otherwise the visibility is excellent. The forecaster needs to watch closely the effects of surface temperature, wind speed, and direction and inversion strength on the formation and dissipation of ice fog and ice crystals. Cloud cover is normally cirrus (above the saturation layer) but altostratus is occasionally present during ice crystal formation. Small ice crystals from the cirrus fall into the saturated layers, serving to nucleate columnar ice crystals, which then precipitates.

Winds from GRID 260º to 020º at greater than 6 m s^–1 (~ 12 kt) will normally accompany a storm from the Weddell Sea and produce ice crystals followed by blowing snow. Winds in excess of 10 m s^–1 (~ 20 kt) are normally restricted to this sector. With this wind speed and direction the weather conditions will lower to less than 150 m (~ 500 ft) ceilings and/or less than 900 m (~ 0.5 nm) visibility restricting flying conditions. When the winds are between 6 and 10 m s^–1 (~12 and 19 kt), slant range visibility may allow a distinction of runway markers. Winds within this speed range will vary in effects based on the direction from which it is coming, and on the availability of newly fallen snow.

Another common obstruction to visibility at the South Pole is fog. Ice fog or freezing fog can develop from a flux in moisture most commonly found after a wind change to the southeast to south. After a 6 to 18 hour period of this wind component freezing fog is likely to occur provided the winds remain under 3 m s^–1 (~6 kt). A second common occurrence is the production of fog from ample moisture, extremely cold temperatures and the availability of nuclei from combustion engines, otherwise known as "camp fog". Combustion and aircraft engines will produce similar conditions to those that produce contrails. Amundsen–Scott can become IFR due to the landing of an aircraft.

Surface contrast including white–out

Contrast in the surface definitions is observed and incorporated in the remarks column of the METAR observation. Cloud cover over the uniform surface will make frequent and major changes to the surface contrast. Overcast skies, snow, blowing snow, and fog will typically cause poor to nil contrasts. 

Horizontal definition

Contrast in the horizon definitions is observed and incorporated in the remarks column of the METAR observation. As all plateau locations the region offers little assistance in discerning the white surface against a white sky when any cloud cover is present.  Snow, blowing snow, and fog will typically obscure the horizon causing poor definitions at the surface and at aircraft approach altitudes. 

Precipitation

Precipitation over the interior of Antarctica is extremely light, and is difficult to measure due to the nearly constant drifting of snow across the plateau. Ice crystals, or diamond dust, are the most common form of precipitation and are observed on the majority of days. Unlike other forms of precipitation, ice crystals often precipitate out of a clear sky, glittering in the sunshine or moonlight, sometimes creating spectacular halos and arcs. Snow grains are commonly observed during strong upper–level storms, while actual snowflakes (branched crystals) are much less common, usually only occurring in the summer months. The intensity of precipitation, almost without exception, is light. Snow accumulation in the vicinity of station, which is comprised of both fallen and deposited drifting snow, averages about 280 mm annually.

Temperature and chill factor

Table 7.12.5.4.3 (in Appendix 2) shows mean–monthly maximum and minimum temperatures from the “Clean Air” AWS. During the summer months of November–February, temperatures usually remain above –50ºC/–58ºF, and may even approach –18ºC/0ºF during the warmest weeks of late December and early January. With the loss of incident solar radiation in March, temperatures cool rapidly, usually dipping below –73ºC/–100ºF at least once during the austral winter. The coldest temperature ever recorded at the South Pole is –82.8ºC /–117.0 ºF, while the warmest is –13.6ºC/+7.5ºF. The average annual temperature is  –49.4ºC/–56.9ºF. Temperatures begin to climb rapidly after sunrise in late September. Apart from the elevation of the sun, the largest factor affecting surface temperatures is cloud cover. Cloud cover immediately changes the surface radiation balance, and therefore can have a rapid and significant effect on the observed temperature.

One of the most pronounced features of the vertical temperature structure is a strong low–level inversion that is especially prominent during the winter months. The temperature often increases by over 20ºC in the lowest few hundred metres of the atmosphere, and then steadily declines again to a well–defined tropopause at 7,000 through 10,000 m above sea level. (See, for example, January and July mean–temperature profiles Figures A3–4 (a) and (b)) in Appendix 3.)

The stratospheric temperatures also exhibit a strong annual cycle, with the minimum slightly below –90ºC during July and August.

Icing

Aircraft icing is rare but has been observed during intense influxes of maritime air over the region. A low cloud deck may provide a warm inversion falling within the prescribed temperature regime to promote rime icing. During summer months the upper–air temperatures must be monitored closely to determine the capabilities to form rime icing in a low cloud deck.

Turbulence

Turbulence in this region has historically been restricted to the areas near the Dronning Maud Mountains.

Hydraulic jumps

Do not affect the region around the South Pole.

Sea ice

Not relevant at this location.

Wind waves and swell

Not relevant at this location.

7.12.6                            Ross Ice Shelf Camps

7.12.6.1                      Orography and the local environment

Siple Dome camp is the most widely used base during recent years. Siple Dome is located near 81.7º S, 148.8º W near the Siple Coast on the western side of the Ross Ice Shelf (Figure 7.12.1). The camp has an elevation of approximately 620 m (~2,030 ft). At this increased altitude, Siple Dome receives cloud bases at a much lower level than surrounding camps. A cloud base passing over Roosevelt Island at 600 m (~ 2,000 ft) will be fog when it moves over Siple Dome. A lower stratus cloud or fog over the ice shelf may not reach Siple Dome or be prolonged in its advance due to the increased elevation. The gradual increased slope normally does not preclude any cloud formations from moving over the Dome, unlike the Transantarctic camps. Climatology shows a mean wind from any GRID southerly component will produce snow, fog or extremely low cloud bases. This wind direction is a direct result of a low–pressure system being within close proximity to the camp.

Down Stream Bravo is another camp commonly used, and is located near 84º S, 155º W, on the Gould Coast. This location is near the Transantarctic Mountains to the south and Siple Dome to the north. The elevation is approximately 110m (360 ft). At this lower elevation and with higher landmasses on three sides, it provides some protection from advecting moisture off the Ross Sea. The glacial winds from the Transantarctic Mountains and the barrier wind flow provide favourable weather conditions at this location. When low–pressure centres find their way onto the southern ice shelf, it normally is only a matter of time before all of these protecting factors give way, and flight restricting weather moves over the camp.

7.12.6.2                      Operational requirements and activities relevant to the forecasting process

No specific information on forecasting requirements has been obtained.

7.12.6.3                      Data sources and services provided

No specific information on data or services has been obtained.

7.12.6.4                      Important weather phenomena and forecasting techniques used at the location

General overview

Low centres that migrate east of Cape Colbeck and take a track toward the southern portion of the Ross Ice Shelf will produce a moderate grid north wind, turning westerly as the low passes. Snow and blowing snow may accompany the system, but the ceiling and visibility remain predominately above 300 m (~1,000 ft) and 5,500 m (~ 3 nm), due to the higher elevation and colder surface the system had to pass over before moving onto the Ross Ice Shelf. If the low is large enough, it may engulf existing low cloud cover or pre–existing fog, providing a temporary reduction to ceilings and visibility.

Low centres that move over or to the west of Cape Colbeck have a direct impact on the southern ice shelf. Large amounts of moisture up to 3000 m (~ 10,000 ft) will be advected over the cold, dry surface causing condensation at a multitude of levels, which may include the surface. Camps such as Roosevelt Island and Ford Range will be immediately affected with low ceilings, reduced visibility and increasing southerly winds. As the low moves onto the ice shelf, the cloud cover will advance over the remainder of the ice shelf to the south, and then circle up the west side of the low along the Transantarctic Mountains toward McMurdo Station. Cloud bases can range from the surface to 600 m (2,000 ft) and stay over the ice shelf until a new synoptic pattern pushes the clouds out to sea.

Surface wind and the pressure field

No specific information on forecasting has been obtained, however, Table 7.12.6.4.1 (in Appendix 2) shows mean–monthly wind speeds and directions for the Siple Dome AWS while Table 7.12.6.4.2 (in Appendix 2) shows mean–monthly station–level pressures for this site.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained.

Clouds

No specific information on forecasting has been obtained.

Visibility: blowing snow and fog

Schematic forecasting rules are shown in Figures 7.12.6.4.1 and 7.12.6.4.2 for Siple Dome and Down Stream Bravo respectively. In summary the guidelines are:

·                         Wind directions from west–northwest to north and speeds between 3 to 7 m s^–1 (~ 6 to 14 kt) are the best case for good visibility.  But when the wind exceeds 8 m s^–1 (~15 kt) and loose snow is available, visibility becomes poor due to blowing snow.

·                         Wind directions from north–northeast to east produce both light snow and light fog, with visibility normally not reducing below 3,700 m (~2 nm);

·                         Wind directions from east–southeast to south produces heavy snow and dense fog with visibility normally below 3,700 m (~ 2 nm);

·                         Fog can develop at Siple Dome with light wind from any direction;

·                         Wind directions from south–southwest to west produce the greatest chance of fog development with very low visibility, especially when the wind speed is between 3 to 6 m s^–1 (~6 to 12 kt) (fog onset is approximately 3 to 6 hr).  This area also produces blowing snow when speeds exceed 8 m s^–1 (15 kt) and will last until the wind either dies or shifts direction. 

Surface contrast including white–out

No specific information on forecasting has been obtained.

Horizontal definition

No specific information on forecasting has been obtained. 

Precipitation

No specific information on forecasting has been obtained.

Temperature and chill factor

No specific information on forecasting has been obtained.

Icing

No specific information on forecasting has been obtained. 

Figure 7.12.6.4.2          Forecasting aid for Down Stream Bravo.

Turbulence

No specific information on forecasting has been obtained. 

Hydraulic jumps

No specific information on forecasting has been obtained.

Sea ice

Not relevant at this location.

Wind waves and swell

Not relevant at this location.