7.3.10                            The Larsen Ice Shelf  

7.3.10.1                      Orography and the local environment

The Larsen Ice Shelf is on the eastern side of the Antarctic Peninsula and is an extensive area of floating (see Figure 7.3.1 and Figure 7.2.1.1.1). Since the Peninsula extends to more than 2 km in height for much of its length the ice shelf is well sheltered from the many depressions to the west of the barrier. During the 1990s there has been a disintegration of the northernmost parts of the shelf with the Larsen–A and part of the Larsen–B sections collapsing (see Section 2.7.3.2).

7.3.10.2                      Operational requirements and activities relevant to the forecasting process

There are no staffed stations on the Larsen Ice Shelf so the only forecasting requirement is for flights to the area in connection with research activities or those for maintenance of the AWS at 66º 58´ S, 60º 33´ W. With the AWS providing the only in situ observations for the area forecasts are made based on NWP fields and satellite imagery.

7.3.10.3                      Data sources and services provided

As noted above, the only in situ observations for the Larsen Ice Shelf are from the AWS at 66º 58´ S, 60º 33´ W. The provision of good forecasts is therefore reliant on NWP output and satellite imagery.

7.3.10.4                      Important weather phenomena and forecasting techniques used at the location

General overview

The climate of the Larsen Ice Shelf is much more continental than that on the western side of the Peninsula with temperatures several degrees colder than comparable latitudes to the west. The lower temperatures are partly a result of the frequent southerly barrier winds that occur because of the blocking effect of the Peninsula.

Since many of the depressions approaching the Peninsula are blocked by this substantial barrier there are fewer depressions found here than on the western side. However, some lows do cross the Peninsula and a number of lee cyclogenesis events take place on the Larsen and over the ocean area to the east. Although few depressions cross the Peninsula, the fronts associated with many of the lows that become quasi–stationary on the western side do pass down the Peninsula and affect the Larsen.

Surface wind and the pressure field

Mean–monthly wind speeds and directions at the Larsen Shelf AWS are shown in Table 7.3.10.4.1 (in Appendix 2) while mean–monthly station–level pressures at this AWS are shown in Table 7.3.10.4.2 (in Appendix 2).

The barrier winds are an important phenomena affecting the eastern side of the Peninsula (Schwerdtfeger, 1984). The barrier winds arise as a result of the piling up of cold low–level air, created over the Ronne and Filchner Ice Shelves and its advection towards the Peninsula on the climatological easterly winds that affect Antarctic coastal areas. The resulting thermal and pressure gradients create a surface wind parallel, in a generally southerly sense, to the mountain barrier of the Peninsula. Since the NWP models do not currently have a very good representation of the Peninsula the model pressure and wind fields should be used with care in this area. The surface wind direction in the area can often be determined from the cloud motion observed in sequences of satellite images if low clouds can be seen.

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

Climatologically there is less cloud over the Larsen than to the west of the orographic barrier, however, there can often be extensive high–level cloud streaming off the top of the Peninsula, which can obscure the surface. The channel 3 images are particularly useful over the ice shelf in detecting water cloud over ice surfaces and can at times detect lower cloud beneath cirrus. There is often a coastal lead along the edge of the ice shelf that can be covered in cloud.

Visibility: snow and fog

Visibility is usually very good on the ice shelf but fog can occur, especially close to the coastal lead that can provide a source of moisture. As elsewhere across the Peninsula, 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 on the ice shelf is usually better than in other parts of the Peninsula because of the smaller amount of cloud occurring. However, contrast can be poor if extensive low cloud cover is present. Contrast is predicted using satellite imagery.

Horizontal definition

Horizontal definition is very important on the ice shelf because of the featureless nature of the surface, causing problems to aircraft operations at times. Since the horizontal definition is affected strongly by the cloud cover satellite imagery is extremely important in predicting this quantity. The presence of clear water leads off the ice shelf can be an important element in enhancing the horizontal definition in these areas.

Precipitation

Mean annual precipitation on the Larsen ice shelf is about 0.5 m water equivalent per year, roughly half that found on the western side of the Peninsula. Virtually all the precipitation falls in the form of snow, with rain being very rare indeed. The frequent southerly barrier winds bring little precipitation and the heaviest snowfalls occur when relatively mild, moist air masses approach from the north or northwest. Such conditions can arise when a depression becomes quasi–stationary in the Drake Passage or north of the Weddell Sea.

Temperature and chill factor

Temperatures on the eastern side of the Peninsula are much colder than those on the western side since the area is effectively isolated from the maritime air masses brought southwards by the climatological low–pressure system over the Amundsen/Bellingshausen Sea. However, warmer air can reach the region when a low becomes slow–moving north of the Weddell Sea and air is advected around this system. Temperatures are predicted using the AWS observations and a knowledge of the air–mass origins. Mean–monthly temperatures at the Larsen Ice Shelf AWS are shown in Table 7.3.10.4.3 (in Appendix 2).

Icing

Satellite imagery indicates that cloud composed of water droplets (often supercooled) is quite common on the Larsen Ice Shelf so airframe icing can be expected fairly frequently. However, because of the low temperatures and hence generally low water content of the cloud most icing will be light.

Turbulence

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

Hydraulic jumps

Hydraulic jumps do not occur on the bulk Larsen Ice Shelf but may take place at the western side, close to the mountains down the spine of the Peninsula.

Sea ice

The western Weddell Sea is the largest area of multi–year sea ice in the Antarctic and presents problems in navigation for even the most powerful ships. As discussed earlier, a lead is often present along the edge of the ice shelf because of the prevailing westerly flow. The location and movement of the sea ice is usually predicted from sequences of high‑resolution satellite imagery.

Wind waves and swell

Not relevant for this area.