7.4                                   Ronne and Filchner Ice Shelves  

This section deals with the Ronne and Filchner Ice Shelves and, excluding the Antarctic Peninsula covers the area between about 85º W and 35º W (see Figure 7.3.1). From west to east, some of the features and bases referred to in this section include:

·                         Haag Nunataks           near (77º S, 78º W);

·                         Ronne Shelf Depot       near (75º 30´ S, 57º 25´ W);

·                         Berkner Island             near (80º S, 50º W).

7.4.1                                Haag Nunataks 

7.4.1.1                          Orography and the local environment

Haag Nunataks are located on the Fowler Peninsula at the base of the Antarctic Peninsula and have an elevation in excess of 1,000 (see Figure 7.3.1). The British Antarctic Survey maintains a small unstaffed fuel depot here at approximately 77º S, 78º W. The area is also visited by glaciological field parties, who can be input by aircraft at this location.

7.4.1.2                          Operational requirements and activities relevant to the forecasting process

Forecasts are required for occasional summer visits by aircraft to collect/input field parties.

7.4.1.3                          Data sources and services provided

There are no staffed stations or AWSs anywhere near Haag. So all analyses and forecasts are prepared using satellite data and model output.

7.4.1.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Haag is located well south of the circumpolar trough so few very deep lows affect the area, although a number of less active systems cross the base of the Antarctic Peninsula from the Bellingshausen Sea. A number of cold fronts with persistent cloud have been observed to affect the area.

Surface wind and the pressure field

The predominant wind direction is southwest to southeast, based on limited field party reports. Field reports also indicate that periods of up to two weeks of extensive cloud and high winds can occur at this location.

Upper wind, temperature and humidity

Since no in situ upper–air measurements are available anywhere near Haag the main upper–level parameters are estimated and forecast using model fields, adjusted in the light of satellite imagery.

Clouds

Cloud associated with old fronts will often penetrate as far south as Haag and may be quite persistent. There are reports that extensive low cloud may develop over the Evans Ice Stream, but it does not always affect Haag.

Visibility: fog

Little is known about the frequency of fog at this location.

Surface contrast including white–out

The usual conditions for the occurrence of poor surface contrast over an interior site apply here.

Horizontal definition

During periods of good visibility the Ellsworth Mountains are visible on the horizon from Haag Nunataks, but with extensive cloud cover the featureless nature of the top of the nunatak can make detection of a horizon difficult.

Precipitation

There is not a great deal of information available about the types of precipitation that occur at Haag but moderate of heavy precipitation is probably rare because of the low temperatures and the lack of active weather systems. Climatologically it is thought that about 30 cm water equivalent of precipitation falls at this location each year. Precipitation is forecast using a combination of model output and satellite imagery.

Temperature and chill factor

Annual mean surface temperature at Haag, deduced from 10 m snow temperatures, is about ‑30ºC.

Icing

Severe airframe icing has been reported on the Ronne Ice Shelf so moderate to severe icing can probably also occur in exceptional circumstances at Haag. The AVHRR channel–3 imagery can indicate when clouds composed of supercooled water droplets are present.

Turbulence

Turbulence is predicted using the model upper–level winds and from noting the locations of the jet streams.

Hydraulic jumps

Hydraulic jumps have not been reported near Haag Nunataks.

Sea ice

Not relevant for this area.

Wind waves and swell

Not relevant for this area.

7.4.2                                Shelf Depot  

7.4.2.1                          Orography and the local environment

The Ronne Shelf Depot is an unstaffed British fuel depot on the edge of the Ronne Ice Shelf at 75.30º S, 57.41º W. British aircraft do not visit the shelf very often, but it is a useful depot and lay–up location for aircraft flying to/from Halley and Berkner Island in case they encounter severe weather.

7.4.2.2                          Operational requirements and activities relevant to the forecasting process

Forecasts are required for the Shelf Depot area when aircraft are flying past on the way to/from Berkner Island and Halley.

7.4.2.3                          Data sources and services provided

There are no staffed stations anywhere in the vicinity of shelf depot, although there is an AWS called ‘Limbert’ located on the edge of the shelf at 60º W, which can provide some in situ data. Otherwise forecasts are prepared based on NWP output and satellite imagery.

7.4.2.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Shelf Depot is located well south of the circumpolar trough and is therefore affected be few major depressions. However, some lows can track across the base of the Antarctic Peninsula and affect the area. In addition, a number of synoptic and mesoscale lows can develop on the shelf because of the general convergent nature of the low level wind field on the shelf, coupled with the baroclinicity that exists between continental air and more maritime air masses over the Weddell Sea.

Surface wind and the pressure field

Mean–monthly wind speeds and directions at the Limbert AWS are shown in Table 7.4.2.4.1 (in Appendix 2) while mean–monthly station–level pressures at this AWS are shown in Table 7.4.2.4.2 (in Appendix 2).

As described above, the katabatic flow down from the plateau onto the ice shelf gives general convergent conditions with an outflow into the Weddell Sea. There is therefore generally southerly surface flow at Shelf Depot. This is also consistent with the veering of the wind in this general location from a coastal easterly to a southerly barrier wind up the eastern side of the Peninsula.

Upper wind, temperature and humidity

Since no in situ upper–air measurements are available on the Ronne Ice Shelf the main upper–level parameters are estimated and forecast using model fields, adjusted in the light of satellite imagery.

Clouds

The Ronne Ice Shelf is often clear of low cloud, but large areas of shallow low stratus or fog are not uncommon. This cloud/fog is usually composed of water droplets and hence can be picked up on the AVHRR channel 3 imagery quite readily.

Visibility: fog

When a gap of ice–free ocean opens up between the edge of the ice shelf and the sea ice, sea smoke may occur and even develop into cumulus cloud. This is also an area for the development of fog if winds are light.

Surface contrast including white–out

Surface contrast can be poor on the featureless ice shelf when there is extensive cloud.

Horizontal definition

The lack of orographic features can make the determination of a horizon very difficult if stratiform cloud is present. Although the coastal lead just off the edge of the shelf can be very useful in finding a good horizon.

Precipitation

Moderate of heavy precipitation is very rare at the shelf depot because of the small number of deep, active depressions that cross the location. However, slight precipitation occurs on many occasions either as clear sky precipitation or from non–frontal cloud. All the precipitation falls as snow. About 30 cm water equivalent of precipitation falls at this location each year. Precipitation is forecast using a combination of model output and satellite imagery.

Temperature and chill factor

Annual mean surface temperature at Shelf Depot, deduced from 10 m snow temperatures, is about –20ºC. During the winter temperatures can be very low on the shelf as cold air drains from higher ground. Mean–monthly temperatures at the Limbert AWS are shown in Table 7.4.2.4.3 (in Appendix 2) and confirm these inferences.

Icing

Severe airframe icing has occurred in low cloud/fog over the Ronne in the past, with a pilot reporting severe ice build up on descent through the cloud in just 3 minutes with temperatures of –20ºC.

Turbulence

Turbulence is predicted using the model upper–level winds and from noting the locations of the jet streams.

Hydraulic jumps

Hydraulic jumps have not been reported near Shelf Depot.

Sea ice

A coast lead near the edge of the ice shelf is a common occurrence throughout the year. As discussed above this can have implications for the weather at Shelf Depot.

Wind waves and swell

Not relevant for this area.

7.4.3                                Berkner Island 

7.4.3.1                          Orography and the local environment

Berkner Island is located on the eastern side of the Ronne Ice Shelf (see Figure 7.3.1) and consists of an ice–covered island with two domes. South Dome (Thyssen Höhe) has an elevation of around 886 m (~2,900 ft), while North Dome (Reinwarth Höhe) is lower at 700 m (~2,300 ft) elevation.

7.4.3.2                          Operational requirements and activities relevant to the forecasting process

There are no staffed stations on Berkner Island itself. However, the area is important since there are major ice core drilling projects taking place on the domes of the island and accurate forecasts are required for the flights to deploy and collect the drilling parties at this location.

7.4.3.3                          Data sources and services provided

No in situ observations are available for Berkner Island so forecasts are prepared based on NWP output and satellite imagery. 

7.4.3.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Berkner Island is located close to the edge of the Ronne/Filchner Ice Shelf at the southern end of the Weddell Sea. Generally the area is sheltered from most of the major storms in the circumpolar trough by the high orographic barrier of the Antarctic Peninsula. However, a number of lows develop as lee cyclones to the east of the Peninsula and these can move southeastwards towards Berkner Island. In addition, when the upper–level steering flow is from the north moisture–laden lows can move over the area from the South Atlantic giving some of the largest precipitation events experienced at this location.

With the convergent, low level flow experienced on the shelf, because of the katabatic winds from the surrounding higher ground, a number of synoptic and mesoscale lows develop around Berkner Island. Many of the mesoscale systems tend to be found on the ice shelf and not on the island itself. Reijmer et al. (1999) give an account of the annual cycle of meteorological variables and the surface energy balance on Berkner Island. Table 7.4.3.4.1 (in Appendix 2) lists some meteorological averages taken at Thyssen Höhe, the south dome of Berkner Island.

Surface wind and the pressure field

Climatologically the edge of the ice shelf is within the easterly flow regime that is found around much of the continent. However, offshore flow is common on many occasions, as can be seen via the coastal polynya that is often present over the southern Weddell Sea. During the winter months in particular there is a strong surface inversion on the shelf and there will be katabatic flow down from the top of Berkner Island onto the ice shelf.

The many low–pressure systems around the island make the surface wind field more variable than on the high interior plateau.

Upper wind, temperature and humidity

These fields are usually taken directly from the model fields and used to predict the winds at the aircraft flight levels if required. Adjustments to the winds can be made in the light of the satellite imagery.

Clouds

Satellite imagery shows that cloud cover is rather variable around Berkner Island with periods of extensive, non–frontal cloud or cloud associated with low–pressure systems, alternating with cloud–free interludes. Throughout the year there is more cloud over the northern part of the island compared to the southern end. This is particularly true during the summer months when there is a plentiful supply of moisture from the coastal polynya at the edge of the ice shelf. Climatologically, the International Satellite Cloud Climatology Project suggests that there is about 25% (45%) cloud cover over the southern (northern) part of the island during the summer and 45% (60%) in winter. Satellite imagery indicates that the high, central part of the island is often cloud–free when there is extensive low cloud on the ice shelf.

Visibility: snow and fog

Visibility is generally good on the ice shelf but fog can occur, especially close to the coastal lead during periods of southerly flow. However, as indicated above, the fog/low cloud can often ring the island with the top being clear. As elsewhere, precipitation is a major factor in reducing visibility, although moderate or heavy precipitation events are fairly rare in this area.

Surface contrast including white–out

The surface contrast is rather variable around Berkner Island and, as with other locations depends on the amount of cloud present. Contrast will usually be better on the top of the island because of the tendency for this area to be cloud–free. Contrast is predicted using satellite imagery.

Horizontal definition

Horizontal definition is again very dependent on the amount of cloud present and will generally be better on the top of the island. Just north of the island the coastal lead can be very useful in finding a good horizon.

Precipitation

Ice core data indicates that the mean annual accumulation on Berkner Island is 15 cm water equivalent per year over South Dome and 21 cm on North Dome. (Reijmer et al. (1999) indicate that over the three years of their study the mean annual accumulation on Berkner Island was 18 cm water equivalent per year over South Dome). All the precipitation falls as snow. The island receives some clear sky precipitation as well as precipitation from low–pressure systems and non–frontal cloud. Precipitation is forecast using a combination of model output and satellite imagery.

Temperature and chill factor

Annual mean surface temperatures on Berkner Island, deduced from 10 m snow temperatures, are –26.1ºC on South Dome and –24.1ºC on North Dome. (Reijmer et al. (1999) indicate that over the three years of their study the mean annual temperature on Berkner Island’s south dome was about –24.0ºC) The near surface temperatures are strongly dependent on the cloud cover, which can be predicted in the short term by the use of satellite imagery. When quasi–stationary deep lows are present in the Weddell Sea warm air can be drawn down the eastern side of the Weddell Sea affecting Berkner Island. The warmest temperatures are found when such a synoptic pattern persists for several days.

Icing

Icing can be a problem around Berkner Island because of the occurrence of cloud with supercooled water droplets. Icing is forecast using satellite imagery (especially channel 3 of AVHRR) and a knowledge of air temperatures determined from a model. When relatively warm air intrudes into the area icing can be moderate or severe because of the higher water content of the cloud.

Turbulence

Turbulence is predicted using the model upper–level winds and from noting the locations of the jet streams.

Hydraulic jumps

Hydraulic jumps have not been reported around Berkner Island.

Sea ice

Not relevant for Berkner Island itself. However, the sea ice off the Ronne Ice Shelf and especially the presence of a coastal lead can have an impact on the weather over the island, as described above.

Wind waves and swell

Not relevant for this area.