7.8                                   Princess Elizabeth Land and Wilhelm II Land  

Princess Elizabeth Land extends from 73º to 86º E and Wilhelm II Land spans 86º E to 91º E (see Figure 7.6.1 and Figure 7.9.1). From approximately west to east, key features or stations/bases referred to in this section include:

·                         Prydz Bay;

·                         The Amery Ice Shelf;

·                         Progress                                               (69º 22´ 50″ S, 76º 23´ 22″ E, 15.5 m AMSL);

·                         Druzhnaya IV                          (69° 44¢ S, 72° 42¢ E);

·                         Zhongshan                               (69º 22´ S, 76º 22´ E, 15 m AMSL);

·                         Law Base                                             (69º 25´ S, 76º 30´ E, 7 m AMSL);

·                         Davis                                       (68° 36´ S, 78° 00´ E, 22 m AMSL);

·                         The Vestfold Hills.

Most of the above places are shown in Figures 7.6.1, 7.8.1 or 7.8.1.1. Although Zhongshan, Progress II and Law Base are less than 3 km apart they are included separately due to the complementary information provided for each station. And due to the somewhat limited amount of information available to be synthesised due to the relatively recent permanence of activities in the area. These three sites are located in the eastern part of the Larsemann Hills, the general features of these hills being described in the section on Law Base, for convenience only.

7.8.1                                The Larsemann Hills and Law Base  

7.8.1.1       Orography and the local environment

The Larsemann Hills are located on the east of Prydz Bay and consist of four large peninsulas and more than 130 small islands (of heights up to about 60 m) with a seasonal ice–free area of approximately 200 km² (see Figure 7.8.1.1). The area became free of the Antarctic ice sheet about 100,000 years ago, and became an oasis on the edge of the ice sheet. The local coastline has many indentations with fjords forming deep inlets.

7.8.1.2       Operational requirements and activities relevant to the forecasting process

Law Base (69º 25´ S, 76º 30´ E, 77 m) has been and is still visited intermittently, and was occupied by the ANARE in the summers of 1986–87 and 1987–88. It is from the reports (Nairn, 1987, McCarthy, 1991) of the forecasters for these summers that the information below is provided. The general method of transportation to and from the area is by helicopter and so aviation forecasts are the main requirement

7.8.1.3       Data sources and services provided

Forecasts for Law Base would now be provide from forecasters located at Davis, or in their absence, at Casey, or Hobart, Australia.

7.8.1.4       Important weather phenomena and forecasting techniques used at the location  

General overview

The summer weather at Law Base seems to vary from year to year depending on how the atmospheric long–wave trough–ridge system is arranged. In 1987–87, for example, only one day in forty–one days of flying was lost due to inclement weather. However, in 1987–88, when the downstream long–wave ridge was further east of the base the low–level air–flow was more commonly onshore. Limited meteorological data are shown in Table 7.8.1.4.1 (in Appendix 2).

It is probably useful to briefly describe the two contrasting summer seasons noted above. In 1986–87 days were noted to be sunny until about the end of the first week in February when the skies remained overcast, the lakes froze, and snow fell several times. As noted, only one day saw flying halted and this was when the wind averaged 25 m s^–1 (~50 kt) and dense snow fell all day.

In 1987–88, one out of three days of flying operations was lost due to inclement weather. There were three major poor weather situations in that particular season:

·                         Frontal passage: on one occasion a deep low–pressure system was north of Mawson and moved slowly in a south–southeast direction and an associated front moved over the base. Large flaked snow from nimbostratus occurred ahead of the front, with calm winds. After the frontal passage cumuliform clouds with hail, granular snow and rapidly strengthening east to northeast winds occurred. Conditions improved after about 24 hours to allow resumption in flying.

·                         Westward moving troughs: westward moving troughs and associated snow bearing cloud bands (usually middle level at Law base) originated from major depressions in the vicinity of Casey. These cloud bands in many ways resembled those described by Callaghan and Betts (1987) but lacked the associated sharp pressure minima. The cloud bands were relatively easy to track on satellite images, and their passage over Davis gave a good indication of arrival time at, and duration over Law Base.

·                         Light northerly airflow: with the mean position of the 500hPa trough just to the west of Mawson, the resulting light northerly airstream, being maritime in origin, advected moisture over Prydz Bay. Within this moist air mass, small troughs and at times low–level mesocyclonic circulations formed just off the coast. Often these minor perturbations in the air–stream remained a few kilometres off the coast, but periodically they drifted onto the coast producing moderate snowfalls and disrupted flying operations. Pressure changes were only slight and the cloudy conditions dampened down the morning katabatic wind.

·                         It should be noted that both the westward moving and moist northerly troughs reached the Prince Charles Mountains on several occasions and would have had a serious effect if flying operations had been taking place there. Closed circulations were evident over the PCMs at least twice during the summer.

Surface wind and the pressure field

Indications are that on about three days out of four during the 1986–87 and 1987–88 seasons a katabatic wind of around 9–10 m s^–1 (~17–20 kt) peaked in the morning (local time) after about 0200 UTC (~7 AM local) with the time of onset varying between 1500 and 2200 UTC. The katabatic occasionally was strong enough to delay flying for a few hours. Afternoons were almost always calm (in the absence of major maritime lows) and an occasional northwesterly sea–breeze was noted. Table 7.8.1.4.2 (in Appendix 2) shows the wind frequency distribution for the 1996–87 summer.

Upper wind, temperature and humidity

Forecasters would now use upper–air observations from Davis to assist in nowcasting upper–air elements and would use NWP output for longer term trends.

Clouds

Satellite imagery and NWP output would be used in the forecasting of clouds. Even in 1987‑88 when the position of the long–waves allowed frequent on–shore flow low cloud was seldom a problem in the area of the Larsemann Hills, Bolingen Islands and Amanda Bay. Aircraft travelling northeast to the Rauer Islands then onto the Vestfold Hills tended to encounter significantly lowering cloud bases.

Visibility: snow

During the 1995–96 summer season westward moving fronts were more likely to produce snowfall over the Larsemann Hills than at Davis, and drifting snow was more common also. Reduced visibility in falling snow was the main deterrent to flying during the 1987–88 summer season.

Surface contrast including white–out

In the area of the Larsemann Hills, Bolingen Islands and Amanda Bay white–out does not seem to be a problem due to the exposed nature of the rock terrain. However, white–out was a more serious consideration when flights occurred over the continental ice or over the Amery Ice Shelf.

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.

Turbulence

Turbulence does not appear to have affected helicopter operations in the Larsemann Hills in surface wind speeds below about 10 m s^–1 (~20 kt).

Hydraulic jumps

No specific information on forecasting has been obtained except that in the summer of 1987–88, during the passage to the west of a relatively narrow cloud band event, helicopters flying from Davis to Law Base at about 300 m (~1,000ft) reported an hydraulic jump just west of the Sørsdal Glacier. The pilots described a distinct shear line, rotor–like clouds, a pressure surge and a sharply delineated falling snow edge. The phenomena were orientated almost east–west and appeared to extend seawards towards the Rauer Islands.

Sea ice

No specific information on forecasting has been obtained except the points noted in the relevant Section 7.8.3 on Zhongshan below should be relevant.

Wind waves and swell

Although not relevant at the stations of Law Base, Zhongshan or Progress themselves, re–supply and research vessels may moor relatively near to the sites. However, no specific information on forecasting marine conditions has been obtained.

7.8.2                                Progress Station and Druzhnaya–IV Base

7.8.2.1       Orography and the local environment

The Druzhnaya–IV field base (69° 44¢ S, 73° 45¢ E, 250–300 m AMSL) was opened on January 1, 1987 and closed on April 18, 1995 and was located on the edge of the Amery Ice Shelf on a nunatak called Landing Bluff overlooking Sandefjord Bay. This bay is effectively the southern–most portion of the much larger and adjacent Prydz Bay (see Figures 7.6.1 and 7.8.1. (In the latter Sandefjord Bay is just north of Sansom Islands). The main functions of activities at the Druzhnaya–IV summer base were the logistical support of the field station Soyuz, which was situated in the Prince Charles Mountains (see Section 7.7.2 above ), and the assistance in the development of the all–year Progress Station. The Ingrid Christensen shore in the vicinity of Druzhnaya–IV base is an area with irregular glacier relief. Southward the orography rises gradually, turning into continental ice–slopes.

Progress Station has had a couple of locations and was operating as a summer base at least as early as the summer of 1986–87. The station moved to Progress II (69º 22´ 50″ S, 76º 23´ 22″ E, elev. 15.5 m AMSL) on February 26, 1989 and is located at the eastern end of the Larsemann Hills on a rocky–sandy plateau with a relatively smooth surface (see Figures 7.6.1, 7.8.1.1 and 7.8.2.1.1).

7.8.2.2       Operational requirements and activities relevant to the forecasting process

The Druzhnaya–IV base functioned continuously during five summer seasons (1987–91), then it was temporarily closed on March 24, 1991, put into operation on February 6, 1994, and it functioned in summer seasons of 1994–95 then closed again.

The Progress Airstrip is situated 5.8 km to the south–southwest of Progress Station on continental ice.

7.8.2.3       Data sources and services provided

At Progress the standard meteorological measurements at the station were carried out in 1988–89, 1991, and also from April 1998 to April 1999. The relocation of the station to the coastline by 2.5 km has had insignificant influence on the meteorological observations.

7.8.2.4       Important weather phenomena and forecasting techniques used at the location

General overview

At the Druzhnaya–IV field base the climatic conditions are favorable for out–door activity. For the spring– summer period (the middle of November – the middle of March) the temperature varies from 0ºC to – 25ºC, but at the beginning and at the end of the period the temperature can fall to –30º and even lower, especially at night. Clear and cloudless weather takes place about 10 – 12 days a month.

Similarly at Progress, due to orographic features, the climatic conditions are less severe in comparison with the nearest coastal stations. The mean annual air temperature is –9.8ºC, the absolute maximum of +9.3ºC was registered in December 1989, the absolute minimum of –38ºC took place in April 1998. December is the warmest month; July is the coldest. The mean annual wind speed is 6.7 m s^–1 (~13 kt) and the prevailing wind direction is easterly.

The weather conditions at Progress and at the aerodrome site are very different.

Surface wind and the pressure field

At Progress the number of days with wind speed of 15 m s^–1 (~30 kt) or stronger is near 50 days per year. The maximum wind gust of 53 m s^–1 (~103 kt) was registered in July 1998.

In summer a diurnal variation of wind direction and wind speed is typical. As a rule, at night, an easterly wind is registered with speeds up to 10 m s^–1 (~20 kt) and stronger. In the afternoon the wind drops significantly and wind the direction can change to west or southwest.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained.

Clouds

No specific information on forecasting has been obtained.

Visibility 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 except at Progress it is noted that snow is the typical precipitation type. In summer the precipitation is ice grains, although sometimes there is rain. The number of snow–storm days is about 60 per year.

Temperature and chill factor

No specific information on forecasting has been obtained, although see the general overview above.

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

With respect to Druzhnaya base, the coastline of Sandefjord Bay has the following characteristics: the altitude of the Amery Ice Shelf does not exceed 6 m in this area; the sea depth does not exceed 100 m, and the thickness of the coastal fast ice is not more than 160–180 cm. The land–fast ice break–up takes place usually from the end of January to the first days of February.

The sea ice conditions in the area of Prydz Bay near Progress is controlled by the presence of Dalk Glacier, situated immediately to the east of the Larsemann Hills. This glacier produces large icebergs. There are a lot of icebergs: these and their fragments are found near the coast from this source and from further east. The sea ice situation is improved with westerly winds, but the frequency of westerly wind events is very insignificant in the area of the station. During the summer period the Bay is clear of ice except in the fjords where ice breakout does not occur in some seasons. According to observations in 1998 the beginning of ice formation was registered on March 14 with fast ice developing very soon thereafter.

The relevant part of Section 7.8.3 on Zhongshan should be also be consulted.

Wind waves and swell

No specific information on forecasting has been obtained.

7.8.3                                Zhongshan Station

7.8.3.1       Orography and the local environment

Zhongshan Station (69º 22´ S, 76º 22´ E, 15 m AMSL) was established by the Chinese National Antarctic Research Expeditions (CHINARE) in February 1989. The position of the station is located on Mirror Peninsula in the Larsemann Hills (see Figures 7.6.1, 7.8.1.1 and 7.8.3.1.1).

Located at a relatively high latitude for stations on the Antarctic coast, Zhongshan Station is influenced by Antarctic continental highs as well as intense oceanic depressions. The forecasting of gales is thus the principal task of the station's weather forecasters. On the other hand precipitation is relatively light in intensity, and generally has little or no impact on scientific activities of the station.

7.8.3.2       Operational requirements and activities relevant to the forecasting process

Routine meteorological observations have been made continuously since March 1989. The visibility, clouds and weather phenomena are observed visually according to the WMO standards.

7.8.3.3       Data sources and services provided

A weather service has been provided for the safety and effectiveness of voyages and Chinese research scientists at Zhongshan Station since 1989. The operational weather service consists of forecasts, which are prepared on site, for the station and its vicinity, and for the research ships Polar and Snowdragon. The Zhongshan weather forecasts are based on information from Antarctic Meteorological Centres, e.g. the AMC at Casey, and from Southern Hemisphere surface, upper–air and NWP products issued by the World Meteorological Centre, Melbourne, Australia. Other data include high–resolution imagery from polar orbiting weather satellites and analysis of station elements. Meteorological information for the operation of the research vessels is processes by ship–board meteorologists with the aid of facsimile synoptic charts, satellite data, marine and meteorological observations, and weather related ship routing data sent from Beijing for consideration. This section outlines the meteorology of the Zhongshan area, with technical support provided by the China Meteorological Administration.

7.8.3.4       Important weather phenomena and forecasting techniques used at the station.

General overview

Situated to the south of the Antarctic circumpolar low–pressure belt and to the north of the Antarctic continental high, the Larsemann Hills is under the effect of prevailing easterly flow all the year round, and influenced by the two systems in a systematic manner. In the presence of high pressure, clear weather with light winds and low temperature appears in sharp contrast to the low–pressure trough related weather that is markedly windy and snowy. Figure 7.8.3.4.1 (in Appendix 2)  shows that the frequency of snowfall days is higher in winter than in summer; the percentage of sunshine days is 50% in summer while overcast and cloudy days are dominant in winter, suggesting that severe winter days are more common than in summer.

Surface wind and the pressure field

It may be seen from Figure 7.8.3.4.2 (in Appendix 2) that the monthly mean pressure at Zhongshan and the monthly mean pressure in the 70–80° E sector of the circumpolar trough axis exhibit an oscillation with a half–year period. The trough is always centred to the north of the Larsemann Hills and is southward of its average position in February–March and August–September, and northward in other months, with its northernmost locality in June – July, a conclusion that was obtained by Streten (1986) in the context of the Davis Station dataset.

Severe weather is often observed in the Larsemann Hills, where gales and snowstorms can last a few days, the duration and intensity depending on the position of a low–pressure centre and the strength of pressure gradients. Related to the orographical role of Prydz Bay, a low may be steered in its eastward course to the north of the Larsemann Hills, then turns westward at low speed of movement, during which Zhongshan Station will experience a drop in air pressure; an air temperature and humidity rise; with these trends accompanied by a snowstorm. The intensity of some lows exceeds that of a tropical cyclone/hurricane with central pressures as low as 930 hPa and maximum winds of 50 m s^–1 (~100 kt). Synoptic processes, controlling the weather over Prydz Bay are so intricate that some have not been understood completely.

A site close to the ice cap is more affected by katabatic winds than a site more seaward of the continental ice. To determine whether or not the station is influenced by katabatic winds Figure 7.8.3.4.3 (in Appendix 2) was prepared to show the daily variation in wind speed for Zhongshan versus Mawson. It may be seen that an almost identical pattern occurs at the two stations except there is a difference in mean wind strength. By inference, at Zhongshan katabatic winds are dominant in January (summer) and October (spring), with few, if any, occurring in winter and autumn.

Zhongshan has annual mean wind speed of 7 m s^–1 (~14 kt), larger (smaller) when compared to Davis (Mawson) but its extreme maximum winds are larger than the other two, a phenomenon that is obviously attributed to the joint effects of the orography of Prydz Bay and weather systems. Inspection of the Zhongshan hourly winds on an annual basis shows that winds of greater than force 5 (on the order of 8 m s^–1 (~16 kt)) constitute 42% (see Figure 7.8.3.4.4 (in Appendix 2)) they are enhanced from summer to autumn and weakened from spring to summer. Winds of less than force 5 make up 80% of occasions. Winds greater than force 6 occur on 32 % of occasions in winter, suggesting that gales are frequent in that season.

Figure 7.8.3.4.5 (in Appendix 2) is a plot of monthly variation in the frequency of calm and strong winds/gales (>14 m s^–1 (~27 kt)) at the Zhongshan Station, indicating that calm conditions often occur between 1800 to 2400 hr (local time) and gales between 0700 and 1200 hr (local) in summer and autumn, which is typical of katabatic winds. The gale frequency is considerably higher in winter than in other seasons, with no diurnal variation. The frequency of gales and calm conditions is smaller in spring.

At Zhongshan Station easterly winds prevail all the year round (see Figure 7.8.3.4.6 (in Appendix 2)). As seen from the hourly wind direction record on an annual basis, east–northeast to east–southeast wind directions (67.5 – 112.3°) have a frequency of 72%, of which 31% is for the due east wind and 27% for east–southeast winds. Owing to the seasonal transition in intensity between polar depressions and anticyclones, the northerly (southerly) component of the winds is diminished (enhanced) from summer to autumn and vice versa from spring to summer.

Table 7.8.3.4.1 (in Appendix 2)) presents the means of the elements investigated at the station. One can see that the number of gale days are 171 (47% of the year) on an annual mean basis and exceeds 10 (days) on a monthly basis, except January; easterly winds prevail all the year round, with maxima of >40 m s^–1 (~78 kt) observed from March – October.

For Zhongshan, gales result dominantly from oceanic cyclones and Antarctic continental highs and from the effects of subtropical highs. The weather patterns may be separated into the following classes: (1) a single polar depression (2) a single frontal surface; (3) subtropical high blocking; and (4) katabatic wind. These classes are outlined below:

·                         Gales from a single polar depression: This type of gale at Zhongshan results from a single low moving eastward past the station. Typically a well–developed and deep depression travels north of the station on an eastward path at around latitude 65° S. The locality of the subtropical high is normal and there is no evidence of an outbreak of continental air due to the building of an Antarctic high. These single depressions can be classified into two kinds: lows that develop in longitudes 20–30° E and mature as they travel eastwards; and, secondly, lows that develop closer to the station between latitudes 45–50° S. Gales develop as the low approaches the station and are strongest when the cyclone is nearby. The gales diminish after the passage of the low–pressure centre.

·                         Gales related to a single frontal surface: The weather in question owes its origin to a depression, travelling along the fringe of the continent, with the associated cloud system moving faster than the low and detaching from it. Gales develop as the frontal cloud band approaches the station. At this time, the continental high is weak, small, and generally ill defined (see Figure 7.8.3.4.7 (in Appendix 2)). This type of event with a weak gale occurs at intervals of 2–3 days, on average, lasting 1–24 h per event.

·                         Gales from a blocking situation: This type of gale situation is characterized by northward displacement of an Antarctic continental ridge, often generating a high–pressure block by connecting with the ridge of a southward extending Indian Ocean subtropical high. The ridge blocks depressions on its western flank. The lows stay almost stationary, and deepen so that the pressure gradients increase giving rise to gales.

·                         Gales related to katabatic winds: The outbreak of cold air from an Antarctic continental high–pressure system is another cause of gales in the Zhongshan area. There are two typical scenarios. Firstly, a low–pressure system may be well north of high–pressure ridging over the station. Local weather conditions will be typically cloudless with a steady east–southeast wind. In this case the pressure gradient between the low and the ridge may increase sufficiently to cause gales at the station. Secondly, purely gravity driven winds are believed to affect the station. The Antarctic plateau air (cold and dense) moves northwards at increased speed over the ice cap and is deflected eastward under the influence of the Coriolis force. East to southeasterly gales occur as the cold air arrives at the station. Gales from the two cases do not differ greatly from each in character and as such they are considered to be of a "katabatic" wind type. For Zhongshan, katabatic winds occur frequently in summer (January) and spring (October) but are not common in winter and autumn.

Gale forecasting is more difficult at Zhongshan when compared to the other CHINARE station at Great Wall. Generally, the success rate is higher in forecasting the first three gale types based on the circulation features identified through the use of satellite imagery and synoptic charts. Using weather charts from the Antarctic Meteorological Centre at Casey and the World Meteorological Centre at Melbourne, then quite a good success rate is achieved in forecasting these types of gales.

The forecasting of strong gully–channelled katabatic winds is more difficult. They usually occur in stable conditions and with wind speeds often being in the range 17–25 m s^–1 (~33–48 kt), the characteristics of this type of wind is worthy of further research.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained.

Clouds

Prydz Bay and the surrounding area is a region of high–frequency genesis of mesoscale polar lows. On reaching Prydz Bay a frontal cloud–band may exhibit a variety of behaviours. At times the local orography might cause the band to become stationary or even retrogress. The cloud band might also slow down due to the blocking action of a south Indian Ocean subtropical ridge. Often, as soon as a disturbance occurs in the cloud system, a new low will form, detach from its parent, and develop rapidly, causing pressure gradients to strengthen sufficiently over Zhongshan for gales to develop. Upon the low leaving Prydz Bay, the gale reduces its severity swiftly. As seen from satellite imagery the new low is shown first as an enhanced cloud feature, which becomes inhomogeneously white, looking like a thick–wall cell. Cirrus filosus is shown in a divergent form, indicative of suitable upper–air conditions for development. This is followed by the development of the cloud system typical of depressions.

However, caution must be exercised in deciding on the potential for gales at Zhongshan from a single front. In some cases a few hours after the cyclone’s frontal cloud band moves south of Zhongshan (that is south of 67° S), the band will move back over the station and gales redevelop. This is attributed to the station locality and southeast flow from the Antarctic high.

If the cloud band far away from the main low experiences a small perturbation a new low will be generated over Prydz Bay, leading to gales from a single front. When a blocking high is east of the station and a low is to the west, gales are normally intense: the gale duration depending on the duration of the blocking. Once the blocking situation collapses, the low moves quickly away and the gale rapidly diminishes in severity. Sometimes blocking may persist for a few days so that the parent cyclone is stagnant, followed by steady genesis of new lows, or by complement or replacement of eastward travelling cyclones, thus bringing the station under the effect of gales for a number of days.

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

From Table 7.8.3.4.1 (in Appendix 2) it may be seen that the number of days per year of precipitation is 150 (41%) on average. However, because measurement of the amount of snowfall is impossible in southeast Antarctica, the Zhongshan Station, like others in that area, has no objective data available.

It is also relevant to note that the relative humidity is 57% on a yearly basis for the station while it is higher in the Larsemann Hills only when the temperature is above 0ºC, leading to a higher content of water in air in mid–summer. Additionally, during a snowstorm or blowing snow episode, the relative humidity is higher, too, sometimes in excess of 90% but absolute humidity remains low.

Temperature and chill factor

Figure 7.8.3.4.8 (in Appendix 2) shows the annual variations of 1989–95 monthly mean temperature at stations Zhongshan, Davis and Mawson, which, though differing in latitude and distance from the ice cap, show a roughly similar trend except individual months in winter. This indicates that the climate in the Larsemann Hills is similar to that of stations to the east and west, suggesting that they are under the effect of the same large–scale climate regime. As shown in Streten (1986), oases along the coasts of southeast Antarctic have very few effects on the monthly mean temperature.

Zhongshan has a mean annual temperature of –10ºC and a range of 16ºC in the monthly means, with January's mean temperature above 0ºC and the coldest month, September's mean temperature of –16.3ºC (see Table 7.8.3.4.1 (in Appendix 2)). Its temperature record shows a rapid drop from summer to autumn and fast rise from spring to summer, and quite steady variation between May and August, a pattern analogous to that of a “coreless” winter and a “brief” summer. In view of the fact that the station is frequently affected by warm and moist air from the north and, conversely, cold and dry air from the south, it has an extreme maximum of 10ºC and an extreme minimum of –40ºC.

The temperature displays small diurnal variation in summer and its daily temperature range is 3ºC, on average. The maximum temperature occurs in the afternoon when the winds are lightest. In contrast, little or no diurnal variation occurs during winter.

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

Figure 7.8.3.4.9 (in Appendix 2) delineates the annual variation of the northern limit of sea ice between 70–80° E. Similar to other sea areas around Antarctica, the sea ice grows steadily during March – September, with maximum growth in April – June, reaching maximum area in September – October; it experiences steady melting in November – December, and a minimum in extent in February. Based on 1973–92 observations, the ice extends as far north as 57°S (60°S) as its northernmost limit in the year with maximum (minimum) growth.

Details of the concentrations of summertime sea ice distribution along the coast north of the Larsemann Hills are poorly understood because the observations are of low resolution and even the microwave data have a resolution of only 25 km × 25 km. The Chinese Antarctic research ships have discovered through their voyages in the sea ice regions that ice–free sectors often emerge on the west side of the Prydz Bay and an ice dam occurs on the east side in December – January, and the ice is nearly disintegrated in February. In contrast, ice along the Larsemann Hills shore is always present in some of the years and moves away only when westerly winds are persistent. However, the westerlies have low frequency of occurrence, which is indicative that the ice distribution bears a close relation to wind direction and orography on a local basis.

The ice thickness is another parameter of great operational (and climatic) importance. Chinese meteorologists at Zhongshan Station made drilling measurement of ice depth at three sites 3–5 km distant from the coast in 1989–93. Figure 7.8.3.4.10 depicts the annual variation in the depth averaged over the sites, with a maximum depth of 1.5 m in October – November, showing no great difference as compared to the same latitude. Investigation of the inter–annual variation requires monitoring over a long period.

Wind waves and swell

No specific information on forecasting has been obtained.

7.8.4                                Davis Station

7.8.4.1       Orography and the local environment

Davis is one of four stations supported by the Australian Antarctic Division. It is near 68° 36´ S, 78° 00´ E, on a westward facing section of coastline on the shores of Prydz Bay. It is situated on the seaward side of the Vestfold hills, a roughly triangular area of ice free islands and peninsulas (see Figures 7.6.1, 7.8.1, and 7.8.4.1.1).

7.8.4.2       Operational requirements and activities relevant to the forecasting process

Davis is currently (2004) the hub for Australian flying operations into the Prince Charles Mountains and Mawson, and from the 2004–05 season two CASA 212 aircraft will replace the Twin Otter aircraft used in previous seasons for these operations. Helicopters will also continue to operate in support of local field work around Davis.

From the 2004–05 season aviation forecasting and weather watch for Davis flying operations will be provided from the Antarctic Meteorological Centre (AMC) at Casey.

7.8.4.3       Data sources and services provided

A basic APT facility is present, supplemented by HRPT images processed by Casey into JPG format for limited areas. The meteorological office at Davis also performs twice–daily radiosonde flights.

7.8.4.4       Important weather phenomena and forecasting techniques used at the location 

General overview

Table 7.8.4.4.1 (in Appendix 2) gives a summary of mean–monthly values of certain weather elements at Davis.

The influence of katabatic winds at Davis is limited by the presence of the Vestfold Hills to the east. Wind speeds are thus generally light when compared, for example, to Mawson or Zhongshan. Temperatures above zero are common during the summer, with extrema as high as 10°C, while winter temperatures are typically near –25°C, ranging down to extrema near –40°C. Davis is commonly affected by depressions passing north of the station during autumn, winter and spring, although gales are less common than at Mawson. Mesoscale lows commonly form in Prydz Bay, leading to near–gales and showers of snow. While synoptic scale depressions are well handled by numerical models, the mesoscale lows are rarely predicted.

Frontal precipitation is generally in the form of snow generated by warm fronts, often travelling westwards along the Antarctic coastline as the parent depression moves slowly eastwards. Snow showers are possible during summer as convection over the Vestfold hills occasionally produces large cumulus; however snow more commonly falls from stratocumulus.

Table 7.8.4.4.2 (in Appendix 2) is taken from work by Shepherd (personal communication) and summarises the suitability of Davis 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, Davis enjoys a relatively low percentage of weather that might be adverse to aviation. December and January (only 4% adverse conditions (excluding possible white–out)) are the best months, while the period March through to September experience uniformly around 13 to 14% adverse conditions.

During the summer of 1996 a field party worked on Hop Island in the Rauer Group, just to the south of the Sørsdal Glacier. Their measurements of wind speed and observations of clouds and precipitation indicated similar conditions as to those experienced at Davis. Temperatures tended to be up to 2ºC warmer than at Davis, but no reason for this is suggested.

Surface wind and the pressure field

Significant winds at Davis are mostly northeasterly, as can be seen from the many semi–permanent snowdrifts within the hills and from the wind rose shown in Figure 7.8.4.4.1 (in Appendix 2). Gale–force southerlies or southeasterlies are rare but can be caused by continental outflows during winter. Strong northwesterlies or westerlies are very rare, although moderate sea breeze westerly to southwesterly flow is common on summer afternoons.

Winds stronger than gale–force are generally associated with synoptic scale low–pressure systems moving southwards towards the coast. The northward deviation of the Antarctic convergence towards Kerguelen Island may be responsible for the tendency of most low–pressure systems to pass well north of Davis. Phillpot used composite 500–hPa data to suggest that gale–force winds at Davis are associated with 500–hPa ridges crossing the coast between Davis and Mirny, especially when the ridge caused north to northeast flow at 500 hPa over Davis (Phillpot, 1997, his figure 4.24c).

Considerable variation is found over the Vestfold hills in wind speed. Pilot reports suggest gale–force winds near the eastern border of the hills, where permanent ice fields rise into the Antarctic interior, while field reports suggest that these gales often extend well into the fjords near the plateau. These gales often occur with light winds observed at Davis. Evidence has been provided for the propagation of hydraulic jumps (Targett, 1998) from the plateau towards Davis as synoptic scale depressions pass north of Davis.

In terms of cross–wind component effects on aircraft, Table 7.8.4.4.3 (in Appendix 2) shows the percentage frequency of occurrence of wind components normal to the mean wind direction of 050º at Davis greater than 7.7 m s^–1 (~15 kt). It may be seen that the frequencies are all very low.

Upper wind, temperature and humidity

Forecast upper winds, temperatures and humidity are extracted where needed from numerical model data, with cross checking of satellite images.

Clouds

As with most continental areas, stratocumulus or stratus are the most common low cloud types, with cirrostratus the most common high cloud. Stratocumulus is most commonly brought over the base by maritime airstreams, but satellite pictures need careful analysis to detect the occasions when low to mid–level stratiform clouds are swept inland by depressions to the northeast, persisting to cross the coast near Davis. Southwesterlies often bring extensive low stratus (cloud base about 250 m (~800 ft)) in summer, moistened by convection over large ice–free sea areas, but this is usually shallow – in which case helicopters are able to fly underneath without difficulty. Satellite images, especially when looped between pictures with the same projection, is the major tool for detecting and forecasting cloud, as very few land‑based observations are available.

During summer, cumulonimbus clouds form on rare occasions, leading to short–term reductions in visibility under heavy snow showers.

Extensive low stratus was observed on a flight from Davis towards Mirny Station to the northeast, just after sunrise in April 1998. On the return leg, it was not seen, although the route was very similar, suggesting that nighttime cooling of the near–surface layer may have produced fog or low. On this occasion, no cloud was detectable from satellite imagery, as would be expected for low cloud over the ice plateau. Obviously, the forecaster must bear in mind the difficulty of detecting low cloud or fog over ice at all times.

As may be seen from Table 7.8.4.4.4 (in Appendix 2) low cloud of major significance to aircraft landing/takeoff at Davis has a very low frequency of occurrence. As discussed in the section on white–out below, the total cloud amount could lead to white–out problems if it were not for the relief afforded by the rocky terrain.

Visibility: blowing snow and dust; and fog

Visibility is generally good, at least during summer, other than with snow. Tables 7.8.4.4.5 and 7.8.4.4.6 (in Appendix 2) show, for example, frequencies of occurrence of poor visibility and of adverse weather types (all of which affect the visibility) respectively. 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 ten per cent.

Good horizons prevail over the Vestfold hills because of the areas of rock. Fogs are rare during summer, but may occur in transition seasons and winter, especially in moist maritime air masses that pass over tidal cracks in the sea ice. As daytime temperatures fall below ‑25°C, ice crystal haze is common, extending to 1000 m or more and reducing visibility to a few kilometres at times. It is wise to examine channel 3 AVHRR imagery for fogs

Visibility reduction by blowing snow is rare during summer because of the absence of snow upstream of Davis and over much of the Vestfold hills and adjacent plateau area (due to melting in summer temperatures, extending some distance inland from the edge of the plateau). Gales do, however, cause considerable blowing dust and sand at times, especially in summer. In fact, in the absence of snow cover, even only "strong" winds may allow flying grit or pebbles to cause considerable damage to vehicles, parked aircraft, and windows etc.

Near the ice plateau, blowing snow is still possible during summer in very strong wind events. After snow falls, of course, gales are able to cause blowing snow or even blizzards in any season.

Surface contrast including white–out

White–out is not usually significant for aviation in the Vestfold Hills, given the presence of areas of rock, however persons on foot, on skis or on motor vehicles travelling under extensive overcast do become disoriented because of their shorter distance focus.

At Davis itself, the local rocky terrain, if substantially ice or snow free during the summer melt, also affords relief. However, as may be seen from Table 7.8.4.4.7 (in Appendix 2) the potential for white–out at Davis, at least in some sectors, is 60% or greater throughout the year.

Horizontal definition

Horizon is more likely to be lost than surface definition in the Vestfolds, as areas of plateau to the northeast and southeast and sea ice to the west can merge with cloud to make the horizon indistinguishable. This is a fairly common event, especially in spring and autumn, although pilots can often continue flight along the coast with some difficulty by reference to rock areas.

Precipitation

Nearly all precipitation is in the form of snow, with rare occurrences of rain during the summer months. Ice crystals reach the surface on colder days of autumn on occasion. Snowfalls can be heavy as warm frontal cloud bands pass over Davis, often followed by strong winds. Snow falls most frequently from stratocumulus or stratus if it is at least 200 m thick. Coastal convergence, presumably associated with variation of friction, has occasionally led to a maximum of precipitation near the coast, with lighter falls inland over the Vestfolds. Mesoscale lows and lines of stratocumulus clouds forming over the water west of Davis may produce snowfall.

More commonly, favoured areas for snowfalls appear to be the Sørsdal Glacier to the south, the ice edge to the west when it exists and, during summer, over the Vestfolds where convection may play a part. The forecaster must be careful not to confuse "ice blink" (whiteness of ice reflecting against the lower surface of low clouds) or "water sky" (darkness of water shown up as lack of brightness on the under–surface of low clouds) with precipitation, however, especially during summer.

Heavy convective showers are rare, but can occur in summer or with strong southwesterly flows off the Lambert Glacier in autumn.

Temperature and chill factor

Temperature is not particularly important in the forecasting process, provided that wind speeds are minimal. Little success has been achieved in forecasting maxima using forecast thickness variations, however very low values of 1000–500–hPa thickness were associated with record low temperatures experienced at Davis in April 1998. Chill–factor is significant for fieldwork, as although rain is unlikely, workers may be wet if undertaking work in small boats. Wind chill is far more significant, especially under gale–force winds, and needs to be considered by all field workers.

Very low temperatures can lead to difficulties for helicopter operations, with certain aircraft being hard to start below –30°C. In such conditions, interior aircraft temperatures may also be too low for aircrew to function effectively. Very low temperatures can also lead to higher fuel consumption on base as electricity requirements soar.

Icing

Generally, the low water content of cloud over Davis leads to only light icing, although it appears likely that the occasional large cumulus could lead to moderate icing. There is a variation of experience of icing amongst pilots because helicopter operations through cloud have not been approved in the past. It is certainly possible for icing to occur in clear air when the air is close to saturation and air temperature is near zero.

Turbulence

Severe turbulence is common over the portions of the Vestfolds near the plateau during the mornings when katabatic flow is strongest. Moderate thermal turbulence is also common during summer when a large area of bare rock is exposed. CAT at high levels has not been reported due to the lack of aircraft in transit.

Hydraulic jumps

Hydraulic jumps are common to the east of Davis during the autumn, winter and spring (Lied 1964). On occasion (either the passage of a deep depression to the north of Davis or strong ridging over the Mawson coast) a katabatic jump may propagate as far as Davis.

Sea ice

Sea ice commonly forms during April between Davis and the offshore islands becoming thick enough to support vehicular travel in May, although careful checks of thickness are made before using the ice. The ice extends some 20 to 30 km offshore by September, with some difficulty being experienced by shipping in getting within 4 km of Davis. In Prydz Bay to the west, ice is generally a metre, or so, in thickness with up to 10/10 coverage during the winter as far as 64º S, although by January it tends to reduce to about 3/10. The sea ice between Davis and the offshore islands becomes unsafe for mechanised travel by early December and by the end of December the ice is usually closed to human access. Small boating activities become possible for the period January to mid–March.

Wind waves and swell

The tendency towards northeasterly flow, especially in strong winds, means that fetch is rarely long enough for significant wave development. Swell rarely penetrates the sea ice deeply enough to cause more than half a metre of swell at Davis. Standing on the sea ice, however, it is clear that a small swell is present occasionally, causing movement of adjacent ice slabs. Tidal effects are much more significant in cracking the ice near islands or submerged rocks.