7.11                              Terre Adélie and George V Lands  

Terre Adélie Land spans meridians 136º to 142º E, while George V Land extends from 142º E to 155º E (see Figures 7.9.1; and Figure 7.12.1). From west to east, key features or stations referred to in this section include:

·                         Dumont d'Urville Station         (66° 40' S, 140° 01' E, 0 to 42 m AMSL);

·                         Port Martin                              (66° 49' S, 141° 24' E);

·                         Cape Denison                          (67° 00' S, 142° 42' E);

·                         Commonwealth Bay    near    (67 º S, 143 º E).

7.11.1                            Dumont d'Urville Station

7.11.1.1                      Orography and the local environment

Dumont d'Urville Station (66° 40' S, 140°01' E, 0 to 42 m AMSL) occupies Île des Pétrels on the Adélie Coast, an ice–free, rocky island some 1 km wide by 1.5 km long with a central plateau of about 40 m altitude. The island, part of the Pointe Géologie archipelago discovered by Frenchman Dumont d'Urville in 1840, is less than 2 km from the coast and 5 km from a good access point to the plateau, Cape Prud'homme.

7.11.1.2                      Operational requirements and activities relevant to the forecasting process

The station is serviced by ship from Hobart, Tasmania, 2,700 km to the north–northeast, a relatively short six day crossing. The closest permanently staffed stations are Casey 1,400 km to the west and McMurdo 1,500 km to the southeast.

The current, typical population of Dumont d'Urville Station is 26 expeditioners over the eight‑month winter (14 for general support and 12 staffing the scientific laboratories) and up to 70 expeditioners over the four–month summer.

The scientific programmes conducted at Dumont d'Urville include: Earth sciences; atmospheric studies (katabatic winds, ozone, atmospheric chemistry, air–snow interactions); and biology. Most of these programmes are conducted in collaboration with foreign research organizations and are part of formal international programmes.

7.11.1.3                      Data sources and services provided

No specific information on forecasting has been obtained.

7.11.1.4                      Important weather phenomena and forecasting techniques used at the location

General overview

Due to local effects, the climate of Dumont d'Urville is relatively temperate and that's why the members of the French polar expeditions call the base the "Antarctic Riviera". That is also probably why many bird colonies have taken up residence in the archipelago. Parish and Wendler (1991) have shown, using a three–dimensional model, that the extreme katabatic wind regime as observed at Cape Denison and Port Martin is due to the confluence of cold airflow from the interior when approaching these zones of the Adélie Land coast. As Dumont d'Urville is not downwind of a confluence zone but rather a divergence one, an intense katabatic wind regime would not be expected there.

The meteorological station at Dumont d'Urville has been operating since 1956 and a long climatological data series is available to establish a reliable climatology of the region of Dumont d'Urville. Table 7.11.1.4.1 (in Appendix 2) shows the mean and standard deviation by month of pressure, temperature, wind speed and day–temperature range for Dumont d'Urville, while Table 7.11.1.4.2, (in Appendix 2), shows the maximum and minimum values of air pressure, air temperature and wind speed at the station. Table 7.11.1.4.3 (in Appendix 2) shows the mean and standard deviation of pressure, temperature, wind speed and diurnal temperature range.

Surface wind and the pressure field

The surface wind is the major climatic Adélie coast phenomenon. The meteorological station of Dumont d'Urville holds the record for the windiest month and the record for the highest surface wind speed (90 m s–1 (~175 kt)).

The wind speed is strong all the year, varying from 8 to 12 m s–1 (~16–23 kt), but one observes an annual cycle with two maxima during the transition seasons and two minima, one in summer and one in winter. This cycle results from the reinforcement of the depressions that force the katabatic wind during the inter–seasons. As for the temperature or the pressure, in relation to the semi–annual oscillation, the variability of the wind is stronger during the winter and the transition months. The rapid reduction in the average and the variability of the wind at the beginning of October indicates the beginning of the summer. There are several interesting aspects of the wind at Dumont d'Urville:

·                         Occurrence of strong katabatic winds: Since the local climate depends largely on the katabatic winds, it is interesting to study the katabatic duration and the frequency of the events. For this study episodes were counted during which the wind speed remained higher than 20 m s–1 (~40 kt). The results confirm the annual cycle of the katabatic winds with two maximum relative during the transition seasons, a minimum from October to January and a reduction relating to the middle of the winter in July.

·                         The diurnal cycle of wind: The formation of a layer of cold air on the surface due to outgoing long–wave radiation is an essential element so that a katabatic wind occurs. It is interesting to study the dependence of the force of the wind compared to the diurnal cycle of the radiation by separating the cases in which the fraction of insolation was higher than 70%. The result suggests that the force of the wind depends on the diurnal cycle of the radiation during all the year but with a strong reduction in the middle of the winter when the solar radiation is a minimum.

·                         Wind direction: The rose wind (not presented) of Dumont d'Urville shows the dominant direction of the katabatic winds that blow with a remarkable persistence in a limited sector from 120 to 140º. One notes some cases, however, where the wind blows in an opposite direction of 300º, i.e. from ocean towards the continent. These cases, which occur mainly in spring and in summer, correspond to situations of sea breezes (Pettré et al., 1993). Indeed, in spring and especially in summer, the ocean is free of ice but remains relatively cold while the continent can be heated by solar radiation creating the gradient of temperature at the origin of the breeze. One could also think that these winds directed towards the continent result from the passage of depressions in the area. Passing lows can also be responsible for initiating katabatic winds that are always directed towards the ocean.

As mentioned the Adélie coast is well known to be one of the areas of the Antarctic where katabatic winds blow most extremely, most frequently, and with the most persistence and so the forecasting of strong katabatic events is important. Because of their force and their very rapid onset, katabatic winds constitute a permanent danger for all the human activities that are performed far from the base, in particular the flying or marine activities. For several years, great efforts have been expended to understand the physical mechanisms at the origin of these winds and to improve their forecasting.

The forcing of the katabatic winds on a local scale, analysed for the area of Dumont d' Urville and modelled at the mesoscale for area of Terra Nova Bay, is now well known. On the other hand, although the effect of the synoptic field is undeniable, it remains difficult to quantify. Bromwich (1989) obtained contradictory results in the area of Terra Nova Bay. Parish et al. (1993) showed that the external atmospheric conditions modulate the intensity of the katabatic winds in the area of Dumont d' Urville and that the strong katabatic winds were associated with a flow at the top of the katabatic layer implying a south–north pressure gradient. In a study of two cases, Phillpot (1991) showed that the wind at the surface, in particular in Dumont d'Urville and Dôme C, was consistent with the contour pattern at 500 hPa.

Observations showed that the principal triggering factor for katabatic storms is the passage of a depression in the area. The depressions generally move from the west around the Antarctic creating in the coastal area a pressure gradient that induces the katabatic wind. The greatest frequency of cases of strong wind during the inter–seasons can be explained by the greatest number of stronger depressions for these periods. However, it will be noted that, because of the katabatic effect, the direction of the wind is relatively independent of the position of the depression.

The katabatic storms can persist several days. Taking into account the volume of air transported northwards towards the periphery of the Antarctic this supposes the formation of an important cold air reservoir and/or a meridional circulation ensuring a supply of air from above the ice surface.

A recent study had the goal of attempting to forecast the Adélie coast katabatic winds by using the data available in real at Dumont d'Urville, that is to say the data from the AWS of Dôme C and D–10 and of the daily analysis at 00 UTC. The construction of an index was based on the combination of three factors relating to the forcing of the winds:

·                         the slope of the 500–hPa surface;

·                         the quantity of cold air available on the plateau to supply the katabatic flow;

·                         the force of the wind at Dôme C like indicator of the penetration of oceanic disturbances inside the continent.

The results of trialling the forecast method over two years showed that the technique is able to predict the changes of situation, but not capture all cases of strong wind. The forecaster has thus to use other information, especially satellite imagery to precisely locate the lows, in conjunction with the index to improve his/her forecast.

A study on the external gradient of pressure evaluated according to the slope of the 500 hPa suggests that the cases of strong wind are associated with a parallel increase in the zonal pressure gradient and a reduction of the meridional gradient. One can thus suppose that the forecast would be improved if one had the data from an analysis or prognostic chart to evaluate the slope of the 500–hPa surface in the zonal direction. The comparative study of the monthly averages of wind and temperature at Dumont d' Urville and Dôme C from 1993 to 1995 showed that the wind at Dôme C was a good indicator.

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

The average temperature (Table 7.11.1.4.1 (in Appendix 2)) over the year shows three quite distinct phases, that is to say a cooling from –0.5°C to –15°C in autumn from February to May, a weak cooling from –15°C to –17°C during the long winter from June to September, then a very fast warming from –15°C to –5°C in spring over October and November. During the short summer in December and January the average temperatures do not exceed 0°C. One can make the following observations:

·                         the spring warming is faster than the autumnal cooling. This can be explained by the formation of the sea ice that takes longer than the melt. Indeed for sea water to freeze it must decrease to a temperature of –1.9°C (depending on the salinity), but the water cooled on the surface has its density increase and sinks and is replaced on the surface by warmer water that lengthens the time necessary for the formation of the sea ice.

·                         the end of the winter is colder than the beginning. The continent cools earlier than mid–latitude areas and consequently the sea ice reaches its maximum extent at the end of the winter when the mid–latitudes are coldest. At the same time, the circumpolar trough, whose depressions bring the warm air towards the Antarctic, moves away from the coasts, from where cooling takes place.

·                         one observes a slight warming in the middle of the winter. This phenomenon, called "coreless winter“ (van Loon, 1967). At Dumont d'Urville this phenomenon generally occurs in an obvious way.

·                         the variability of the temperature is much stronger in winter. In the Antarctic warm air can come only from mid–latitudes with the depressions of the circumpolar vortex. But the depressions force the katabatic winds that transport very cold air from the centre of the Antarctic towards its periphery. The winter variability of temperature thus represents the equinoctial reinforcement of the polar vortex and its katabatic feedback;

·                         in summer, at the beginning of the autumn and the end of spring variability is lower. At these periods the cyclonic activity is weak since the meridian variation in temperature in the middle troposphere is reduced. On the other hand the katabatic winds, even if they are less frequent, continue bring cold air, which explains the variability observed.

·                         the annual average temperature shows a tendency for a statistically significant warming of 0.025 °C over 37 years of record (1960–96) that is less than the result obtained by Jones (1990) who evaluated the tendency for warming of the Antarctic as 1°C over the century.

Icing

No specific information on forecasting has been obtained.

Turbulence

No specific information on forecasting has been obtained.

Hydraulic jumps

Apart from the severe downslope wind common to Adélie Land, the most spectacular phenomenon occurring during the katabatic periods is the "Loewe's phenomenon". This type of phenomenon has been described by Valtat (1960) for Dumont d'Urville.

As mentioned in Section 6.6.13 above the Interaction–Atmosphère–Glace–Océan experiment aimed at producing detailed documentation of the vertical structure of the katabatic layer, and of its evolution along the slope. Two French and one American team took simultaneous measurements of vertical profiles of atmospheric properties at three points (D57, D47 and D10) distributed over 200 km along the slope from the plateau, at about 2,000 m height, toward the coast near the Dumont d'Urville Station.

During the IAGO–experiment several Loewe's phenomena were observed as the terminal phase of several katabatic events. On 3 December 1985, a surprisingly large surface‑pressure change (almost 6 hPa) through a jump was measured, differing very strongly from the predicted value (about 2 hPa) derived from the hydraulic theory.

Sea ice

No specific information on forecasting has been obtained.

Wind waves and swell

No specific information on forecasting has been obtained.

7.11.2                            Port Martin, Commonwealth Bay and Cape Denison  

George V Land was discovered and explored by the Australasian Antarctic Expedition (AAE), under Sir Douglas Mawson, in 1911–14. More recently couples have been wintering over in the Commonwealth Bay area: the first being Don and Margie McIntyre in 1995. Although there is only a limited, albeit famous, record of the meteorology of the Commonwealth Bay area the section is included due to its historical significance and the renewed activity in the area. Moreover, the relatively recent installation of AWSs in the area means that data acquisition is increasing.

7.11.2.1                      Orography and the local environment

The name Commonwealth Bay may be used in a wide sense to denote the bay entered between Point Alden and Cape Grey (66º 52´ S, 143º 20´ E), ~55 km to the east, but the AAE used the name in a more restricted application to denote that part of the bay between Cape Hunter and Mackellar Islands. Cape Denison (67º 00' S, 142 40' E, 2 m AMSL) is the rocky outcrop where the Australasian Antarctic Expedition established its main base (see Figure 7.11.2.1.1).

Figure 7.11.2.1.1             Sketch map of the Cape Denison area. The position of Mawson's Hut is shown as “Main Hut”. (Adapted from a map provided courtesy of the Australian Antarctic Division.)

About 1 km within the cape the land, which is very broken, rises to an elevation of about 600 m about 22 km inland. At the western end an inlet 200 m long forms a useful boat harbour for shallow draft vessels. Being very windy and an ablation area the ice is sparsely covered by occasional drift patches. A small number of melt pools form on Cape Denison during the summer. The largest, Long Lake, is about 50 m long. About 2 km offshore at the head of this inlet lay the Mackellar Islands that are covered by 20 m thick caps of frozen spray.

Further east the Mertz and Ninnis Glaciers (see Figure 7.9.1) are major features of the coast and discharge an immense volume of ice into the sea from the inland plateau. The seaward extension of the Ninnis glacier is known as Ninnis Glacier Tongue. Near the inner end of the tongue there is a large rocky mass named Dixson Island, which is embedded in the ice and obstructs the outflow of the Ninnis glacier.

From the east edge of Ninnis Glacier Tongue the coast trends east about 63 km to Cape Wild (65º 15´ S, 149º 05´ E). The bight formed between this stretch of coast and the east side of Ninnis Glacier is named Buckley Bay.

7.11.2.2                      Operational requirements and activities relevant to the forecasting process

The hut built at Commonwealth Bay in 1912 by Sir Douglas Mawson's Australasian Antarctic Expedition of 1911–14 was the main base of the expedition. Although recent interest in preserving Mawson's Hut has increased activity in the area the demand for forecasting services remains more in the area of potential rather than actual services. This is probably due to the being no aircraft involved in recent expeditions.

7.11.2.3                      Data sources and services provided

The USA has installed several AWS in the area, in particular at Port Martin and at Cape Denison (see Table 7.1.1 in Appendix 1).

7.11.2.4                      Important weather phenomena and forecasting techniques used at the location

General overview

As mentioned above the first long duration stay in the region of Adélie Land and west of King George V Land was that by members of the AAE 1911–14, directed by Douglas Mawson. During the winters 1912 and 1913 at Cape Denison, Mawson's expedition measured wind blowing from the Antarctic plateau with a strength and a regularity not observed in other parts of the globe. Thus, Mawson (1915) entitled the report of the expedition: "The home of the blizzard". He described the violence of the storms and their influence over the life and the work of the explorers.

About forty years later, the Expéditions Polaires Françaises maintained a meteorological station at Port Martin, located ~62 km west of Cape Denison from February 1950 to January 1952. The climate of the two stations proved quite similar. At Port Martin the mean wind speed for March 1951 was 29.1 m s–1 (~56.5 kt), while 48 days experienced mean wind speeds larger than 30 m s–1 (~60 kt) and 10 occurrences of winds of around 40 m s–1 (~80 kt) over four days in succession were observed during 1951.

After the destruction of the meteorological station of Port Martin by fire in 1952, the French explorers established a permanent station at Dumont d'Urville (see Section 7.11.1).

Surface wind and the pressure field

Tables 7.11.2.4.1 and 7.11.2.4.2 show mean–monthly wind speed and directions for the Port Martin and Cape Denison AWSs respectively, while Tables 7.11.2.4.3 and 7.11.2.4.4 show mean–monthly station–level pressure for these stations respectively (all of these tables are in Appendix 2).

H. Phillpot (Australian Bureau of Meteorology, 1991) examined a short series of observations from Commonwealth Bay taken during the period 22 January to 25 February 1978. 72% of the winds were from directions 160º to 185º and a marked diurnal speed variation was noted with a maximum evident between 0000 and 0300 local (UTC +10 hours) and a minimum between 1200 and 2000 LST. These data indicate a katabatic wind influence.

The following is believed to come from a 1947 account by E. Kidson regarding the 1911–14 expedition. "At Commonwealth Bay the south or south–southeast katabatic wind was blowing almost constantly, was nearly always strong, usually of gale–force, and frequently of hurricane force for considerable periods. The stronger the wind the greater, in general, was the turbulence and the mixing of the lower layers of the atmosphere. The smaller, consequently, was the surface inversion and the higher the surface air temperature. This effect on the surface temperature was so large relatively to the changes due to the advent of different air masses that it is practically impossible to isolate the latter at any time. With the approach of a depression the blizzard almost always increased while at varying intervals, usually a few hours, after what according to the analysis was the time of advent overhead of the cold air there would be a decrease. The increase in the wind during the approach of a depression the writer believes to be due to the arrival of warmer air over the ocean to the north and also above the katabatic wind, over the land. The contrast between the radiation equilibrium temperature of the inland ice and the temperature of the free air is thereby increased. This would tend to cause an increase in the katabatic wind. The warm air, also, would be more stable so that the katabatic wind would probably be shallower. It is true that the warm air would have a component of motion from the north that would oppose the blizzard but apparently at Shackleton this effect was entirely masked by that described above. The sea was practically always free of ice at Commonwealth Bay so that the horizontal temperature gradient on the coast always tended to be steep."

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.

Temperature and chill factor

No specific information on forecasting has been obtained, however, it may be inferred from Table 7.11.2.4.5 (in Appendix 2) that mid–summer temperatures at Cape Denison are close to zero while mid–winter temperatures are often around –20ºC. Tables 7.11.2.4.6 and 7.11.2.4.7 show mean–monthly temperatures for the Port Martin and Cape Denison AWSs respectively and are similar to the earlier data.

Icing

No specific information on forecasting has been obtained.

Turbulence

No specific information on forecasting has been obtained, however it is likely that with the persistently strong surface winds low level mechanical turbulence would occur.

Hydraulic jumps

No specific information on forecasting has been obtained.

Sea ice

From observations made by the Australasian Antarctic Expedition, it would appear that the Buckley Bay ice does not break out of this bay until late summer. There is a continually thick and highly concentrated pack ice zone east of ~150º E understood to be associated with an ocean current system influenced by two large submarine banks above 500 m depth near 150º E and 153º E and that reach north of 67º S. The chain of islets extending north of Cape Denison staves off icebergs and the pressure of pack ice coming down with the wind and current from the east.

Wind waves and swell

No specific information on forecasting has been obtained.