7.10                              Wilkes Land  

Wilkes Land spans meridians 102º to 136º E (see Figure 7.9.1). From west to east, key features or stations/bases referred to in this section include:

·                         Casey Station                                       (66° 16´ 48″ S, 110° 31´ 12″ E, 42 m AMSL);

·                         Law Dome                                           (66° 42´ S, 112° 42´ E, 1,395 m AMSL);

·                         Vostok Station                                      (78º 28´ S, 106º 48´ E, 3,488 m AMSL);

·                         Concordia Station (Dôme C)  (74° 30´ S, 123° 00´ E, 3,280 m AMSL).

7.10.1                            Casey Station including Law Dome Summit 

7.10.1.1                      Orography and the local environment

Casey Station is situated near 66° 16´ 48″ S, 110° 31´ 12″ on the Antarctic coast in an area known as the Windmill Islands. The station is very near sea level on a section of coast oriented north/south, with the plateau rising to the southeast of the station onto Law Dome. The Dome rises to a height of 1,395 m some 120 km from the station. The Vanderford Glacier, situated 25 km south of Casey, and the Adams Glacier immediately west of the Vanderford, empty into Vincennes Bay and act as a drainage basin for the trench south of Law Dome, as does the Totten Glacier to the east of Law Dome (Figure 7.10.1.1.1). The Dome has a significant effect on the weather in the Casey area.

The Law Dome drilling site is at the summit of Law Dome. At the time of writing (2002) there was no active drilling programme on Law Dome and the only visits to the site are to maintain entrances, measure strain grids and maintain cane lines. Forecasting for the site is ad hoc and mostly based on the synoptic features forecast. Being at the top of a significant orographical feature the site is not affected by katabatic flow so the wind regime is assumed to be dominated by the gradient flow predicted by the models.

7.10.1.2                      Operational requirements and activities relevant to the forecasting process

Casey houses an Antarctic Meteorological Centre (AMC) that, when staffed, acts as the primary forecasting centre for all Australian activities in East Antarctica. During the summers of the 1990s up to three meteorologists provided an analysis and forecasting service to both personnel at Casey and users anywhere in East Antarctica or the southern Ocean south of 50º S, from 0º E to 180º E. Local activities included field parties working in the surrounding Windmill Islands and coastal areas and glaciologists working on Law Dome. Other forecasting duties included flight forecasts for helicopters operating either locally or transiting between Casey and Davis, some 1,400 km to the west.

Australia has the research vessel Aurora Australis providing marine science capabilities and providing the resupply for the Australian stations, Mawson, Davis, Casey and Macquarie Island. Australia relies on helicopter support for field work around Davis and the Prince Charles Mountains with Casey providing data and scientific support for the meteorologist based at Davis.

From the 2004–05 season the AMC will provide all forecasting services for operations at Australian stations and field camps, with the primary task being support for the intra‑continental flying program undertaken using two CASA 212 aircraft and helicopter operations at Davis. The AMC will also be responsible for providing forecast support for the three Australian continental stations and field camps, shipping support the Australian operations and any international obligations that may arise.

7.10.1.3                      Data sources and services provided

The Casey AMC is connected to the Australian Bureau of Meteorology network (and to the internet) via a 128 kbps satellite link. Through this link Casey has access to hourly geostationary data from the GOES-9 platform, including IR, visual and water vapour images. Three–hourly METEOSAT data are also available. The NCEP GFS model data are routinely transmitted to the AMC with three hourly data out to +96 hours made available from the 0000 UTC, 0600 UTC, 1200 UTC and 1800 UTC runs. Surface and upper–air data from the Southern Hemisphere GTS network are also transmitted to the AMC. The World Wide Web also provides forecasters access to high resolution NWP specifically designed to support Antarctic weather forecasting operations with such systems provided by both the Australian Bureau of Meteorology and the US Antarctic Program.

The AMC has an HF Metfax system for transmitting products to end users with locally prepared products being broadcast during the summer months and Australian derived charts being broadcast over the winter months when no meteorologists are on base. The primary frequency used is 7,468.1 kHz with the secondary frequency being 11,453.1 kHz. This system is likely to be phased out during the second half of this decade.

7.10.1.4                      Important weather phenomena and forecasting techniques used at the location  

General overview

Table 7.10.1.4.1 (in Appendix 2) gives a summary of mean–monthly values of certain weather elements at Casey while Table 7.10.1.4.2 (in Appendix 2) below is taken from work by D. Shepherd (personal communication) and summarises the suitability of Casey as an aircraft landing site based on the incidence of adverse cross–wind, cloud, white–out and adverse visibility. While these parameters are discussed in more detail below it may be seen from the table that potential white–out aside, Casey enjoys a relatively low percentage of weather that might be adverse to aviation. December and January (only 15 % adverse conditions (excluding possible white–out)) are the best months while the period March through to September experience uniformly around 25 to 35% adverse conditions.

The wind and weather regimes at Casey are strongly modified by the close proximity of Law Dome. Rather than a general katabatic outflow from the plateau giving rise to strong southeasterly flow, as experienced at other coastal stations on the plateau verge Casey has, in general a light wind regime and only a weak katabatic signature. The wind is most commonly from the southeast, although on occasions a weak northeasterly outflow is experienced. Strong wind events at the station are generally associated with deep synoptic scale cyclones moving north of the station. On a few occasions mesoscale lows forming in Vincennes Bay give rise to gale to occasionally storm force wind events and on rare occasions the strong cold outflow typical of the Vanderford Glacier area makes it far enough north to encompass Casey. These southerly gales are typically in sunny conditions with just low ground drift.

Mesoscale low–pressure systems forming in Vincennes Bay are almost impossible to forecast as they are typically of a scale well under the resolution of the available numerical models. The only way the systems may be detected is by the AVHRR imagery available. The dynamics of formation of these systems is not well understood but it appears that under some circumstances the strong southerly outflow from the Vanderford Glacier interacts with the weak, moister northeasterly gradient to spin up a small scale low–pressure system. The systems are capable of accelerating the incipient northeasterly flow from a light to moderate breeze up to gale to storm force and typically have a life time of the order of 12 hours.

The gale to storm force katabatic wind events at Casey are infrequent and not well forecast. The dynamics behind why the katabatic makes it up the coast to Casey on some occasions is not known. However, it would appear that the situations in which these events occur are just after the passage of a synoptic scale low–pressure system where the gradient flow becomes southerly. Perhaps the weak southerly flow assists the downslope flow of a pool of cold air up on the plateau inland of Casey. However, it is more common that after the passage of low–pressure systems the wind remains only light from the southeast.

Casey is renowned for the strength of the blizzards that strike with the passage of deep cyclonic systems. The wind speed experienced at Casey is typically near double that of the gradient level flow. As low–pressure systems approach Casey, generally from the northwest, and the gradient level flow (easterly or northeasterly) increases the wind at Casey becomes extremely variable with the direction swinging through all directions, with stronger gusts likely from the east. At some critical point the wind direction becomes stable at 090º and the wind speed dramatically increases to somewhere near twice the gradient level wind. Once the gale to storm force wind has become established the direction is very stable with only about a 5º fluctuation either side of the mean position (normally 090º). The storms typically last from around 12 hours through to 3 or 4 days. Once the low–pressure system begins to move away and the gradient level wind shifts from being easterly the wind at Casey drops dramatically with strong fluctuations in direction as observed at the commencement of the strong wind event. The storm force wind events are very much direction dependent and any wind shift of more than about 10º from 090º generally results in a dramatic drop in wind speed.

The difficulty in forecasting these storm events at Casey lies in ascertaining whether an approaching low has the right characteristics to develop a suitable gradient level flow for the wind to accelerate in the Casey area. Over the years several forecasting aids have been developed to assist in the forecast process. These rules are listed out below.

In the medium term, look for:

·                         a well–developed synoptic scale low visible on the satellite imagery approaching Casey, with the centre of the low moving to be south of 60º S;

·                         and a strong upper–level westerly jet forming over Casey.

In the very short term (anywhere between about 20 minutes and several hours warning) look for:

·                         strongly fluctuating surface wind at Casey with the wind direction changing between westerly and easterly on a time scale measured in minutes.

·                         very obvious drift tails of snow above the moraine line some two kilometres east of the station.

All of the above signs may become obvious at the station but unfortunately do not necessarily lead to a 100% reliable forecast of a storm. There appears to be a definite critical point (see, for example, the references cited in Section 6.6.1.5) that has to be reached for the storm to move down off the plateau and over the station. On occasions there can be visible signs of strong wind on the plateau some five kilometres away from the station and a deep low just to the north, yet the strong wind never reaches down to the station. Deciding whether a storm event is going to reach the station remains one of the challenges at Casey.

A snow compacted blue–ice runway is under construction some 63 km south east of Casey at 66^o 41´ 35″ S 111^o 30´ 14″ E in support of the proposed inter-continental flights from Hobart to Casey, expected to commence in the 2005–06 season. Other possible landing sites that are located near Casey Station, and that are suitable for fixed–winged aircraft, include sites known locally as the "Casey Airstrip" and Lanyon Junction:

·                         Casey Airstrip and Casey: A comparison of available conditions for these sites is provided in Table 7.10.1.4.3 (in Appendix 2). Overall, the mean temperature at Casey Airstrip is about 2°C colder than at Casey, but during summer months the difference is closer to 3°C. The mean wind speed is slightly stronger at Casey Airstrip than at Casey. It is likely that if more data were available the mean wind speed for Casey Airstrip would be similar to that for Lanyon Junction, given their elevated locations, and that the mean speed for Casey would be less as a result of the rotor effects experienced there at times. The percentage occurrence of adverse cross–wind is similar for the two sites. In the absence of visual observations at Casey Airstrip, it is difficult to compare other aspects. However, given the higher elevation of Casey Airstrip compared to Casey, and bearing in mind the data for Lanyon Junction, one would expect a higher frequency of adverse low cloud and visibility conditions at Casey Airstrip than at Casey.

·                         Lanyon Junction and Casey: A comparison of conditions for these two sites is given in Table 7.10.1.4.4 (in Appendix 2). The mean temperature at Lanyon Junction is about 4°C colder than at Casey, largely a result of the elevation difference between the sites. The overall mean wind speed at Lanyon Junction is nearly 40% stronger than that at Casey. This would seem to reflect the times when wind speeds are much less at Casey as a result of rotor effects that affect Casey but not Lanyon Junction. However, it should be noted that at other times the winds at both places are similar, or indeed stronger at Casey. The frequency of adverse cross–wind is higher at Lanyon Junction than at Casey. This may just be the result of limited data, but it would appear that the generally more consistent wind direction of Lanyon Junction is offset by the stronger wind speeds involved. The frequency of both adverse low cloud and poor visibility at Lanyon Junction is about three times those at Casey. The occurrence of adverse weather type is about twice as much at Lanyon Junction. The percentage occurrence of at least four oktas of total cloud is slightly higher at Casey, as is the occurrence of times when adverse criteria were met for at least one of all the elements considered. The frequency of occasions when adverse conditions were met for at least one of cross–wind, low cloud or visibility is nearly three times higher for Lanyon Junction than it is for Casey.

Surface wind and pressure field

As mentioned above the wind at Casey is generally light (less than 7 m s^–1 (~15 kt) and from the easterly sector, either southeasterly or northeasterly (see the wind rose shown in Figure 7.10.1.4.1 (in Appendix 2)). Flying operations and fieldwork at Casey itself are typically not affected by wind. For example, in terms of cross–wind component effects on aircraft, Table 7.10.1.4.5 (in Appendix 2) shows the percentage frequency of occurrence of wind components normal to the mean wind direction of 090º at Casey greater than 7.7 m s^–1 (~15 kt). It may be seen that the frequencies are all less than 10 per cent.

However, to the south and the north of the station, significantly different wind regimes exist. At an old AWS site 40 km (~21 nm) north at the Balaena Islands the predominant wind is a northeasterly and it is typically much stronger than at Casey with a significant strong outflow signature. Similarly, to the south of Casey the predominant wind is a south to southeasterly outflow from the Vanderford Glacier. On many occasions this outflow makes it up the coast to Ardery Island (site of ornithological studies), some 10 km (~5 nm) south of Casey yet not to the station itself.

The wind events affecting Casey are the less common southerly gales that make it up from the Vanderford, mesoscale low–pressure systems forming in Vincennes Bay giving rise to gale–force northeasterly flow and synoptic scale low–pressure systems resulting in full storm force wind (typically averaging anywhere from 25 m s^–1 (~50 kt) through to 45 m s^–1 (~90 kt) with gusts recorded in excess of 60 m s^–1 (~120 kt).

The preparation of surface analyses and the accompanying study of both GMS and NOAA AVHRR imagery certainly is most beneficial in the forecasting of the synoptic scale events and in some cases may even assist in picking the formation of the cloud signatures associated with mesoscale lows forming in Vincennes Bay. However, such analyses do not appear to be at all useful in picking strong southerly wind events.

The numerical models available to the forecasters are all global in scale and so of relatively low resolution. The best model available to the forecaster is the NCEP aviation run with 1.0 degree data available on pressure levels and the first sigma level. However, as with the ECMWF and GASP models, the orography around Casey, and in particular Law Dome, is not represented at all well. In fact Law Dome appears as little more than a ridge pushing slightly out into the ocean. Because of the poor representation, the surface wind fields from the model need to be treated with care. One of the main problems arising from the poor model orography is the tendency for the NCEP model to over estimate the southeasterly outflow at Casey – quite often by as much as 100%, and not pick those occasions when Casey experiences a northeasterly outflow from around Law Dome. Storm–force easterly wind at Casey is also not well handled with the models not picking the accelerations experienced at Casey. These model shortfalls are fairly consistent and can be accounted for when interpreting the model data.

Upper wind, temperature and humidity

These fields are taken directly from the model for flight forecasting or cloud prediction. Aerological diagrams may be generated from any model time–step for the GASP and NCEP for assistance in cloud prediction and upper flow.

Clouds

The location of Casey makes for interesting wind effects but unfortunately the location also acts as a cloud trap. Once the weather begins to warm during the summer months and the sea ice breaks up, allowing large fluxes of heat and moisture from Vincennes Bay a bank of low cloud (typical base anywhere between 1000 and 6000 ft) forms over Casey.

As may be seen from Table 7.10.1.4.6 (in Appendix 2) low cloud of major significance to aircraft landing/takeoff at Casey has a frequency of occurrence below 10 per cent in all months. The extent of the lower level cloud in "cloud trap" situations is generally not large, with sunshine visible on the horizon from the south, through west to the north. On the AVHRR imagery the cloudbank can be picked up hugging the western sector of Law Dome. The signature is warm with the cloud tops quite low. The Vanderford and Adams Glaciers are typically cloud–free, as is much of the Peterson Bank. The cloud is typically thin but significant enough to reduce the visibility to the east and on occasions produce very light falls of snow. This phenomena does not typically upset flying as the locations the helicopters are normally heading for are cloud–free and there is only a short flight of some 20 to 30 km to clear the cloud bank. During summer months, when a lot of rock is exposed surface definition remains fair under the cloud even with the poor horizon definition.

More significant cloud events are associated with the synoptic scale low–pressure systems where warm and moist northeasterly flow produces large amounts of middle and low level cloud over Casey, often with good falls of snow. During storm events wave, or banner cloud, is quite predominant over the station with a stationary lenticular cloud positioned just to the southwest of the station with significant rotoring observable underneath.

Visibility: blowing snow and fog

Visibility at Casey is normally very good, particularly in summer as may be seen from Tables 7.10.1.4.7 and 7.10.1.4.8 (in Appendix 2). These tables show, respectively, frequencies of occurrence of poor visibility and of adverse weather types (all of which affect the visibility). It may be seen that over the summer months, and in December and January in particular, the visibility and related weather types have frequencies of occurrence that are less than about 10 to 14 per cent. Moreover, during the summer months a lot of rock is exposed to assist in surface definition. Visibility is generally only ever lost in falling or blowing snow associated with storm events. During the summer months of November to February the chances of a synoptic scale low inducing a blizzard are very low so visibility is normally only reduced in falling snow events and these are not very common at all during summer.

Fog is a rare event at Casey with perhaps two to three fog days a year, at most. The fog formations observed have all occurred during the height of summer and under the same regime. A light afternoon northwesterly flow has prevailed for a few days (quite possibly a weak sea breeze circulation) advecting moisture in from Vincennes Bay over Casey. This moisture packs in against the steeply sloping terrain above the moraine forming a thin deck of stratus. As the evening progresses, the weak northwesterly is replaced by a weak southerly flow and the stratus moves back towards the station, sinking as it does so and enveloping the station in a misty fog. Visibility typically reduces to around 200 to 300 m.

Surface contrast including white–out

Typically the surface contrast around Casey remains good due to the amount of exposed rock. However, as may be seen from Table 7.10.1.4.9 (in Appendix 2) the potential for white–out at Casey, should the rock be covered with ice or snow, at least in some sectors, is 66% or greater throughout the year. Furthermore, surface definition inland from the nearby moraine line, where there are no obvious ground features, does pose a problem on a number of occasions. The band of stratiform cloud that typically forms in the Casey area during summer does lead to white–out conditions on the plateau necessitating the use of GPS and radar for navigation. Flying inland on these occasions does not take place

Horizon Definition

Horizon definition in the Casey area is typically very good due to the visible landmarks up and down the coast and with the icebergs off the coast. The stratiform cloud formation typical in summer only really reduces the horizon definition to the east.

Precipitation

Most of the precipitation falling at Casey is as a result of synoptic scale low–pressure systems moving close to the coast. These systems are capable of producing sizeable amounts of snow in a short period of time. These falls can cause a significant reduction in visibility and stop flying operations. Such events are not common during the summer months and typically only effect flying operations during March and April when the helicopters are being used to expatriate the summer personnel. These large falls of snow have another adverse effect in that the loose snow is easily picked up by the wind and any subsequent strong wind event will almost certainly result in blizzard conditions.

The more typical weather patterns at Casey generally do not produce much snow at all and when precipitation does occur it is most typically observed falling out to sea over the icebergs or further south over the Mitchell Peninsula, leaving Casey relatively fine.

On a very few occasions over the summer months, when the temperatures are high enough, drizzle or very light rain may be observed. This may cause problems with riming on vehicles, or helicopters, but is uncommon.

Temperature and chill factor

The forecasters at Casey do not carry out any temperature forecasting, as the parameter is not considered important.

Icing

Forecasting airframe icing in Antarctica is quite difficult, as an assessment needs to be made of whether the clouds are fully glaciated or whether some supercooled liquid may still be present. Certainly helicopters operating in the Australian sector of the Antarctic have experienced icing on numerous occasions so the forecaster needs to be aware. As a matter of course if there is any cloud present where the temperatures are above –20ºC then icing is mentioned. Severe icing is considered a possibility in pre–frontal cloud near the coast where it is possible that the airflow has been strong enough to carry supercooled liquid to the Antarctic coast.

Turbulence

Turbulence is forecast at Casey using the wind profile from the twice–daily radiosonde ascents and the profiles from the numerical models. One of the output parameters from each radiosonde flight is a profile of Richardson’s number. This may assist in picking those levels where turbulence is most likely, however, strong shear is most commonly used to deduce turbulence.

Hydraulic jumps

Hydraulic jumps have been observed at Casey but they are not a common event. The commencement or cessation of strong wind at Casey is most commonly linked with synoptic scale lows and a critical Froude number were the flow shifts from being separated (light wind at the station) to a downslope flow (strong wind at Casey). Forecasting a hydraulic jump is not an issue.

Sea ice

Sea ice at Casey is quite variable. The regularity of strong wind events during winter (typically every 8 to 10 days and lasting for 2 to 3 days at a time) keeps a semi–permanent polynya in the eastern part of Vincennes Bay near Casey. At a smaller scale the blizzards quite often clear out the ice in Newcombe Bay north of a line between Kilby Island and the tip site at Wilkes with the inner part of the bay holding ice until early summer.

Wind waves and swell

Casey does not do any wind or swell forecasting for the local area. It is quite rare to get much of a swell into the Casey area. Extended periods of westerly wind to the west of Casey have generated swells that have penetrated into Casey but they are typically of very long wave length with amplitudes less than 1 m. Boating around the Windmill Islands is only allowed when the wind speed is under 7 m s^–1 (~15 kt) and wind waves are quite low.

7.10.2                            Vostok Station 

7.10.2.1                      Orography and the local environment

Vostok Station (78º 28´ S, 106º 48´ E, 3,488 m AMSL) (see Figure 7.9.1) was opened on December 16 1957. It is located on the snow surface of the ice plateau of central Antarctica 1,410 km distant from Mirny Station and 1,260 km from the nearest sea coast. The ice sheet thickness in this area is 3,700 m.

The highest part of the ice plateau is at a distance of 450 km to the west–southwest of the station at a height of more than 4000 m above sea level. The station is located on the snow plane that has a generally insignificant slope to the main ice divide of the continent.

Snow sastrugi and naduvy (blown snow fields) are typical micro–relief features of the snow surface in the station vicinity forming a slightly undulated surface. The height of sastrugi typically varies between 20–40 cm and that of naduvy is not greater than 50 cm.

7.10.2.2                      Operational requirements and activities relevant to the forecasting process

The inland research station Vostok is one of the major Russian stations in Antarctica. The following year–round observations at Vostok are undertaken:

·                         Meteorological and actinometric observations, snow line measurements;

·                         Total ozone content measurements and observations of the anomalous phenomena in the atmosphere;

·                         Geomagnetic observations, including ionosphere studies and observations of atmospheric electrical field variations;

·                         Deep drilling of the Antarctic ice sheet. Based on the deep–drilling results, a sub–glacial lake was discovered whose study continues;

·                         Study of the influence of environmental factors and micro–social conditions on the health of staff.

7.10.2.3                      Data sources and services provided

A surface meteorological programme continues, although the upper–air programme has ceased.

7.10.2.4                      Important weather phenomena and forecasting techniques used at the location

General overview

The geographical location of the station, features of the underlying surface, solar radiation and atmospheric circulation regime give a severe climate. The ice sheet is perennially covered by snow that never melts. High transparency and dryness as well as a smaller atmosphere mass above the ice surface of Antarctica (compared to the coastal stations) result in relatively large quantities of total incident radiation to the ice surface. Intense cooling occurs on the Antarctic plateau and strong surface inversions develop throughout the year. Here the air temperatures are very low during the entire year. The mean annual air temperature at the station is –55.4ºC with the absolute minimum of –89.2ºC recorded at Vostok on July 21 1983, which is the absolute minimum surface air temperature record on the globe. The absolute maximum temperature recorded at Vostok was –13.6ºC.

Surface wind and the pressure field

Winds in central Antarctica are much weaker than at the coast. The wind regime at Vostok Station is characterized by weak katabatic southerly–southwesterly winds with a mean annual speed of 5.0 m s^–1 (~10 kt). Annual wind speed variations have two pronounced maxima (March and September) and one sharp minimum (January). The highest mean–monthly wind speed is observed in September (5.5 m s^–1 (~10.7 kt)). Table 7.10.2.4.1 (in Appendix 2) shows mean–monthly wind speeds for Vostok based on 13 years of record, but because of the short period of record available for this table the variations mentioned here are not evident in the data in the table.

The maximum wind speeds are low, in the order of 13–14 m s^–1 (~26 kt) and sometimes 16–18 m s^–1 (~33 kt). They can in individual cases reach 20–25 m s^–1 (~40–50 kt). The maximum wind speeds are observed in all seasons of the year being due to the most active cyclones moving from the Antarctic coast over the continent.

The probability of storm winds throughout the year is small (only 0.1%). The probability of calms and of strong winds of around 11–15 m s^–1 (~ 20–30 kt) is also small comprising 2.7% and 1.3%, respectively. Weak winds (0–5 m s^–1 (less than about 10 kt) are predominantly observed in summer and autumn whereas stronger winds (6–15 m s^–1 (~12–30 kt) occur in winter and spring.

Vostok Station is located near the eastern foot of a giant spur extending northward from the central dome of the ice sheet of East Antarctica. The prevailing weak southwesterly winds here are determined by downward cold air sinking along the ice spur slope. The persistent wind directions and speeds and dependence of their direction on the relief indicate their katabatic nature. These winds also possess the features of katabatic winds. Compared to the winds of other directions (gradient and cyclonic), they are colder and total cloud and humidity of these winds is smaller, with a greater frequency of occurrence of clear weather.

However, the main indicator of katabatic winds is absent in these winds, namely, the maximum of daily speed variations is observed in the daytime, rather than at night as with true katabatic winds. Such daily wind speed variations are observed at the station in all months except for April, June and September. This is because of the presence of a strong surface high in central Antarctica in whose system the gradient winds are developed due to the anticyclonic pressure field.

In spite of being significantly remote (about 1,300 km) from the shores of the Indian and Pacific Oceans with a high elevation of the station above the ocean level, this area as mentioned above, is subjected to the influence of coastal cyclonic activity. The cyclones penetrating the continent both from the Indian Ocean coast and the Ross Sea, reach the station. The influence of the latter in the form of the frontal zones reaching the station area, is primarily manifested in a sharp deterioration of weather conditions accompanied by increased cloudiness, temperature and westerly and southwesterly wind speeds up to 18–20 m s^–1 (~40 kt). The cyclones coming occasionally from the Indian Ocean also bring cyclonic weather to the station area but with easterly winds. Their speeds are attenuated by contrary katabatic winds and are hence low (8–10 m s^–1 (~20 kt), but the other indications of cyclonic features are quite pronounced.

The downslope flow in the station area is screened by a coincidence with the gradient wind and a superposition in some cases of cyclonic winds. These effects are so strong that they distort the daily speed variations typical of katabatic winds. However, the katabatic wind preserves all its other indications at the Vostok Station. In summer, the frequency of occurrence of cyclonic winds similar to the other Antarctic stations increases and that of katabatic winds decreases while in winter it is vice versa.

Due to a high station elevation above sea level, the atmospheric pressure here is very low with the yearly–average being 624 hPa. It varies little in individual years. The annual variations, as in other mountainous areas, have one maximum in summer (January) and one minimum in late winter (September). Table 7.10.2.4.2 (in Appendix 2) shows mean–monthly station–level pressures for Vostok based on 13 years of record.

The largest deviations are typical of the entire cold period (autumn–winter) with dominating anticyclonic weather conditions. However, the largest frequency of occurrence of cyclonic activity is also observed at this time governing thus the significant amplitudes of atmospheric pressure reaching 20 hPa.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained. Mean January and July upper‑level wind roses for Vostok Station are included in Figures A3–9 (a) and A3–9 (b) (in Appendix 3) while mean–temperature profiles for this station are also shown in Appendix 3 as Figures A3–5 (a) and (b).

Clouds

Due to extremely low air temperatures, the clouds present over Vostok consist of ice and only occasionally do clouds comprised of supercooled water droplets occur.

Lower level clouds (stratocumulus) occur very rarely above the high mountainous Antarctic plateau where the station is located. Their frequency of occurrence comprises only 1.2%, which is natural as the plateau surface lies above the boundary of lower–level cloud formation and only occasionally does convection from the surface occur contributing to the appearance of low cloud.

Stratus clouds have the largest frequency of occurrence in summer and in the intermediate seasons (1.3%) and the least (0.4%) in winter. With these clouds in summer, the atmospheric pressure (638.6 hPa), air temperature (–30.9ºC) and absolute humidity (0.37 hPa) are greater than in winter, when they comprise 623.1 hPa, –65.0ºC and 0.01 hPa, respectively.

Middle level clouds are recorded more often, their frequency of occurrence comprising 8.5%, on average for a year. The prevailing cloud type over the station area is high–level clouds, whose frequency of occurrence for a year comprises 52.9%. The frequency of occurrence of clear weather comprising 37.4% is likewise high. The highest frequency of occurrence of clear weather throughout the year is in winter (41.6%) with the least in spring (30.8%). In winter during clear sky conditions, the weather is characterized by high wind speed (5.4 m s^–1 (10.5 kt)), but low pressure (620.0 hPa), temperature (–68.8ºC) and humidity (absolute: 0.00 hPa and relative: 66%). In summer, there is a low wind speed (4.7 m s^–1 (~9 kt)), but high air pressure (631.0 hPa), temperature (–33.5ºC) and humidity (absolute: 0.30 hPa, relative: 74%).

The undulated cloud forms (Ci, Cc, Ac, Sc) have the largest frequency of occurrence in summer when the air above the Antarctic plateau warms noticeably. The largest (39–42%) frequency of occurrence of cirrus clouds is observed not only in the summer, but also during the intermediate seasons. The lowest frequency of occurrence of cirrus and cirrocumulus clouds is recorded in winter while that of high cumulus and stratocumulus in spring.

The frequency of occurrence of stratiform clouds (Cs, As) is characterized by inverse annual variations. Whereas for high stratiform clouds such annual variations are pronounced, the frequency of occurrence of cirrostratus has two phases during the year: a maximum in winter and spring (18.4–18.2%) and a minimum in summer and autumn (12.3–12.5%). With stratiform clouds, the atmospheric pressure is lower (624.2 hPa) on average for a year, compared to undulated clouds. The temperature is slightly less (–54.5ºC), but the wind speed (5.5 m s^–1 (~10.6 kt)) and especially total cloudiness (6.5 oktas) is much higher. The absolute humidity comprises 0.07 hPa with relative humidity of 72%.

Large values of total cloudiness and wind speed with stratiform clouds confirm that this form belongs to frontal system clouds. They result from an interaction between the cooled surface of the Antarctic plateau and a relatively warm air flowing onto it. Their prevailing frequency in winter points to dominance of meridional intrusions of comparatively warm air masses in winter from the ocean to the continent.

Visibility: blowing snow and fog

The atmospheric transparency in the inland region of Antarctica is very pronounced. Except for water vapour and ice crystals, the air is not polluted with anything. But humidity here is less than anywhere else since precipitation is insignificant and snowstorms are rather rare. Therefore, the meteorological visibility in the station area is quite high. The largest frequency of occurrence of good visibility (of more than 10 km) is observed in summer while poor visibility (of up to 1 km) is most prevalent in winter.

Snowstorms in the station area are rare due to the weak winds. Drifting snow occurs more frequently at wind speed of up to 8 m s^–1 (~15 kt) and snowstorms with winds of more than 9 m s^–1 (~17 kt). The frequency of occurrence of snowstorms changes depending on the snow–surface state by seasons of the year. In spring and in summer, when the snow surface is consolidated, the frequency of occurrence of snowstorms is minimum. The largest frequency of occurrence of snowstorms is recorded in winter when loose surface snow is present.

There are 106 days with drifting snow on average over a year. Blowing snow is less frequent (50 days a year). Snowstorms significantly deteriorate visibility being mainly caused by active cyclones moving along the meridional trajectories and penetrating deep onto the continent giving strong winds at the station. This is related to the situation where the New Zealand high–pressure ridge above the Ross Sea develops and intensifies. The development of this ridge leads to a cyclone near the Balleny Islands moving over the continent. Vostok in this case is located at the southwestern periphery of the depression. The transformed maritime air flows along the western ridge periphery to the continent bringing middle and high–level clouds to the station, causing snowfall, snowstorms, a wind change to stronger southerlies and a visibility deterioration of 2–4 km or less.

Fogs and haze produced by ice crystals are typical of the inland regions of Antarctica. There are 42 days with fog and 166 days with haze on average for a year. In summer, fog is observed one day a month while during the other seasons, fog in the station area is observed 4 days a month. The largest number of days with haze is 16 a month in winter and a minimum of 8 days a month in summer. The mean duration of fog and haze is also largest in winter and the least in summer. Fog and haze form in the Antarctic high during calm and cloudless weather.

The dangerous phenomenon of snow fog is also quite common. It is the formation of very fine (indiscernible to the human eye) snow particles and ice crystals above the snow surface due to water–vapour sublimation at strong surface inversions. The horizontal visibility decreases to 1 m. Snow fog, unlike ice fog and the clouds near the snow surface, never produces a halo around the sun.

During calm weather, frost or ice fog and haze occur in the central parts of anticyclones and ridges. At wind speeds of 3 to 5 m s^–1 (~6–10 kt), the phenomenon of “snow mist” can occur caused by fine snow dust in the air after a snowstorm or with the formation of snow fog. The visibility during “snow mist” as in the presence of snow fog disappears. The absence of visibility makes the snow mist an especially dangerous phenomenon.

Surface contrast including white–out

No specific information on forecasting has been obtained.

Horizontal definition

No specific information on forecasting has been obtained.

Precipitation

The mean annual total of precipitation is 37.9 mm while that of accumulation on the snow surface is 21.5 mm. The sum of precipitation is non–uniform throughout the year comprising on average 4.7 mm during one month in winter, 2.0 mm in spring, 0.6 mm in summer and 2.2 mm in autumn. The largest amount (75% of the annual sum) falls in winter, being especially intense in September, which is distinguished by increased cyclonic activity. On average over the year, there are 26 days with snow, 247 days with deposition of ice needles and 225 days with deposition of hoarfrost. The duration of precipitation is quite long, its intensity however, being so small that in respect of the total annual precipitation the station area can be compared to the most arid regions on earth.

The meteorological conditions under which it snows differ sharply from the weather accompanying the fallout of ice needles and deposition of hoarfrost. During snowfalls, the air temperature and humidity are much higher (with total cloudiness twice as large) than during other kinds of precipitation throughout the year.

The weather during fallout of ice needles is almost similar to hoarfrost deposition conditions, the only difference being a slightly greater frequency of clear weather and middle clouds (along with lower clouds in winter and spring) and higher air humidity and wind speed. Similar weather conditions for these types of precipitation are understandable as the fallout of ice needles and deposition of hoarfrost occur often simultaneously.

The occurrence of ice needles is determined by maritime air flowing to the inner regions of the cold mainland at elevations of about 500–1000 m (~1600–3200 ft) above the ice sheet surface and its sinking due to downward airflow.

Temperature and chill factor

Low air temperatures are extremely dangerous, the more so if they are accompanied by strong winds. For example, a temperature below –75ºC and wind speeds of up to 11 m s^–1 (~21 kt) were observed at Vostok in August 1978. Under such conditions, the time that people can stay in the open is significantly limited. The maximum permissible duration of stay outside buildings is the time for which the human body surface temperature will decrease to the value dangerous to health.

The annual temperature variations are typical with the maximum in summer and the minimum in winter. The natural seasons in Antarctica are divided conventionally by the character of the temperature and illumination changes. A peculiar feature is in the absence of a pronounced minimum in one of the winter months (the 'coreless" winter). The difference of the average temperatures of any two adjacent months is not greater than 2.0ºC. Frequently in the middle of winter, warm periods occur that are also clearly pronounced in the middle troposphere being due to heat advection from the north. Active cyclonic eddies reaching the station area are more often observed during the colder period than at the warmer time of the year.

Similar large–scale warming with record high air temperatures both near the surface and in the troposphere was observed at some stations of East Antarctica at the beginning of January 1974, while at the inland Vostok Station a new record of –13.6ºC was observed on January 5, which exceeded the previous maximum air temperature by 7.3ºC.

This process was preceded in late December 1973 by generation of a strong blocking ridge in the middle troposphere (at 500 hPa) and near the surface south of Tasmania. As a result of meridional cyclonic activity, there was a transfer of warm air masses along its western periphery. This in turn, contributed to the formation of an extensive surface anticyclone above East Antarctica near Dumont d’Urville Station. Intensifying, it slowly moved southwestward towards Vostok Station. At the time when the anticyclone centre was located east of the station, the transfer of warm and moist air masses from the north along its western periphery was most intense. With the anticyclone centre passing over Vostok and the establishment of southerly winds whose speed increased up to 18–22 m s^–1 (~35–43 kt), the temperature sharply dropped for two days.

The formation of ridges and heat sources in the upper troposphere above Antarctica begins more frequently in the Indian–Australian sector. This is primarily connected with a large frequency of occurrence of cyclones compared to other regions, governing intense meridional air exchange processes in the troposphere and lower stratosphere.

In April, the average monthly temperature at Vostok drops below –65ºC and remains at this level throughout the entire winter. In some years, the mean–monthly temperature in June–September can be below –70ºC. August is the coldest winter month (–68.6ºC) when the atmosphere cooling above the mainland achieves its maximum.

Transition from winter to summer is characterized by a steady and significant air temperature increase. The mean temperature of the first spring month (October) is 9.7ºC higher than in the last winter month (September). From September (–65.8ºC) to December (–32.9ºC), mean air temperature increases almost two–fold.

In the middle of summer (December–January) the temperature is highest not dropping below –36ºC on average, for over a month. The highest temperature is observed in late December–early January indicating a direct relation to the sun’s elevation. Abundant and uniform incident solar radiation determines not only relatively high temperatures in summer, but also an insignificant difference between mean temperatures of the summer months (0.2ºC)

The beginning of autumn is accompanied with dramatic cooling. The average temperature in February is 11.3ºC lower than January. The lowest mean–monthly temperatures in autumn are recorded in March (–61.8ºC) with the highest temperatures (–42.7ºC) in February.

The temperature regime of the transition season is characterized by sharp variations in the middle of each season. Mean–monthly temperatures in spring (October to November) increase and in autumn (February to March) decrease by the same value comprising 13.4ºC. Table 7.10.2.4.3 (in Appendix 2) shows mean–monthly temperatures for Vostok based on 13 years of record, but because of the short period of record available for this table the variations mentioned here are not evident in the data in the table.

The mean annual air temperature anomalies vary from year–to–year between 2.1ºC to –1.4ºC. The daily temperature variability is small with mean values comprising 4.5ºC in winter, 1.4ºC in summer and 2.6ºC in the intermediate seasons. This indicates the increased activity of inter–latitudinal exchange of air masses in winter and a significant role in these processes of advective heat transfer by cyclonic eddies moving along the meridional trajectories to the inner areas of the continent.

Icing

No specific information on forecasting has been obtained on airframe/ship superstructure icing but see the section on precipitation on hoar frost and ice needle formation.

Turbulence

No specific information on forecasting has been obtained.

Hydraulic jumps

No specific information on forecasting has been obtained.

Sea ice

Not relevant at Vostok.

Wind waves and swell

Not relevant at Vostok.

7.10.3                            Concordia (Dôme C) Station  

The French and Italian Antarctic programmes have agreed to cooperate in developing a research programme that includes the construction and operation of a scientific station named "Concordia" located at Dôme C, high on the Antarctic plateau, some 1,100 km inland from the French station of Dumont d'Urville and 1,200 km inland from the Italian station at Terra Nova Bay (see Section 7.12.2). The station at Dôme C was originally chosen for a drilling experiment because of the very large ice thickness the needs in paleoclimatology and is open to the worldwide scientific community for conducting scientific research.

After several exploratory seasons the station was officially opened for summer routine operation in December 1997 and is expected to be open year round from the 2003 Austral summer.

7.10.3.1                      Orography and the local environment

Dôme C (74° 30´ S, 123° 00´ E, 3,280 m AMSL) is located on the Antarctic plateau 1,080 km from the coast (see Figure 7.9.1 and Figure 6.6.13.1).

7.10.3.2                      Operational requirements and activities relevant to the forecasting process

Concordia can be reached by:

·                         tractor trains from Dumont d'Urville. About 2,000 tonnes of cargo will have been delivered by tractor trains during the construction phase then about 380 tonnes each year during the operational phase; and

·                         (ii) ski–equipped planes. Personnel and light, fragile or urgent equipment are delivered by ski–equipped planes from Terra Nova Bay or Dumont d'Urville.

Accommodation at Concordia will be provided for a typical population of 15 expeditioners over winter and 30 over summer.

Six areas of scientific research have been selected by the Concordia steering committee. The station will also carry out programmes in other research areas after the first winter of operation. The six initial research areas selected are: glaciology; atmospheric sciences (including the study of the annual variation of the evolution of the ozone hole in spring; the polar boundary layer; and the triggering of katabatic winds); astronomy and astrophysics; earth sciences; human biology and medicine, and technology.

7.10.3.3                      Data sources and services provided

Dôme C AWS has been replaced by Dôme C II AWS. The original Dôme C AWS stopped recording in January 1996. The new Dôme C AWS (Dôme C II or Dôme “Concordia”) (75° 07´ S, 123° 22´ E) started recording in December 1995. Dôme C is in line of sight of geostationary satellites and the future AUSSAT communications satellite, being at the same longitude as Dôme C, will cater for fast data transmission needs. Communications are also available through the Argos system.

7.10.3.4                      Important weather phenomena and forecasting techniques used at the location

General overview

Information not yet provided.

Surface wind and the pressure field

Tables 7.10.3.4.1 and 7.10.3.4.2 (in Appendix 2) show mean–monthly wind speed and directions for the Dôme C and Dome “Concordia” AWSs respectively. The wind speeds at these AWSs are very light. A comparison with other stations shows that Dôme C experiences the lowest wind speeds of any inland station in Antarctica. A mean value of about 3 m s^–1 (~6 kt) is observed with no pronounced annual cycle. This is unique, as a freely exposed station at a height of 3,280 m (~10,760 ft) on any other continent would have higher wind speeds than lower–lying stations. In winter (June), there is no diurnal variation in wind speed at Dôme C. The sun is, of course, below the horizon, and no systematic variation in surface heating can occur. However, in summer (December), higher wind speeds are observed in the afternoon. Assuming no gravitational flow for Dôme C, the stronger inversions at night hinder the transfer of momentum to the surface, whereas in the afternoon the inversions are weaker.

Tables 7.10.3.4.3 and 7.10.3.4.4 (in Appendix 2) show mean–monthly station–level pressures for the Dôme C and Dome “Concordia” AWSs respectively. As at Dumont d'Urville the annual course of the atmospheric pressure shows a well–pronounced semi–annual cycle. Minima are observed in autumn and spring, while the maxima are observed in summer and winter. The inter‑diurnal pressure variation gives an indication of the cyclonic activity. The highest values are found in winter, the lowest in the summer. This is in agreement with the idea that the increased horizontal latitudinal temperature gradient in the Antarctic atmosphere in winter increases cyclonic activity, even though the circumpolar trough is farthest away from the continent in winter.

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

No specific information on forecasting has been obtained.

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

In Table 7.10.3.4.5 (in Appendix 2) the monthly mean, minimum and maximum temperatures are presented for various periods of the year. Tables 7.10.3.4.6 and 7.10.3.4.7 (in Appendix 2) show mean–monthly temperatures for the Dôme C and Dome “Concordia” AWSs respectively The Dôme C area is very cold; the winter is kernlose or ("coreless"); that is, no systematic changes in temperature are observed during the winter months. An absolute minimum of –84.6°C has been recorded, and the average winter temperature hovers –60°C. Even in summer, Dôme C is cold, with the warmest monthly temperature around –30°C. The highest temperature ever measured was well below freezing point. Dôme C, of course, lies in the dry snow zone, where melting never occurs. In autumn (March), there is a high degree of diurnal temperature variation with the maximum occurring in the early afternoon. The variation is much larger for Dôme C (about 10°C) than for the coastal station D10 (about 2°C). The reason for this is not easy to find because it is the result of the coupled processes of heat exchange at the surface and conditions in the boundary layer. An explanation was first offered by Simpson, the well–known meteorologist on Scott’s expedition. The larger heat conductivity of snow and ice in the coastal areas suppresses extremes in temperature, while the drier and lower–density snow of inland areas has a poor heat conductivity and capacity, thereby enhancing extremes.

Icing

No specific information on forecasting has been obtained.

Turbulence

No specific information on forecasting has been obtained.

Hydraulic jumps

Do not occur at Dôme C

Sea ice

Not relevant at Dôme C

Wind waves and swell

Not relevant at Dôme C.