7.3.8                                Fossil Bluff, King George VI Sound  

7.3.8.1                          Orography and the local environment

The northern part of King George VI sound lies between the high orography of the Antarctic Peninsula and that of the mountains of Alexander Island (see Figure 7.3.1 and Figure 7.2.1.1.1). It is therefore in a relatively sheltered position with winds being predominantly from the north or south. The southern end of the Sound curves round to the west and is more exposed to weather systems arriving from the west. The Sound is largely ice–covered with the ice shelf being up to 200 m (~ 650 ft) thick. However, there are some ice–free areas and pools of open water on the ice surface during the summer.

7.3.8.2                          Operational requirements and activities relevant to the forecasting process

The British Antarctic Survey maintains a summer only forward field station and fuel depot at Fossil Bluff (71.3º S, 68.3º W) as an aid to the southward transport of fuel and field parties into the interior of the continent. During the summer there are always at least two staff on the station including, whenever possible, a meteorological observer who maintains the surface observing programme.

Forecasts for Fossil Bluff are provided by the forecaster at Rothera when aircraft are passing through the station.

7.3.8.3                          Data sources and services provided

The Fossil Bluff surface meteorological observations taken at 12, 18 and 00 UTC are sent by HF radio to Rothera for use in the analysis and forecasting process. No forecasts are available from Fossil Bluff.

7.3.8.4                          Important weather phenomena and forecasting techniques used at the location

General overview

With its location on the western side of the Antarctic Peninsula, King George VI Sound/Fossil Bluff has a relatively mild climate and comes under the influence of the many synoptic–scale lows in the circumpolar trough. Often depressions to the west of Rothera will move east–southeast to become slow–moving near Alexander Island where they fill steadily, although areas of snow/showers can still be generated by these old lows, even when in their death throes and leave a lot of cloud to the west of the Peninsula. On occasions lows can move southwards down the western side of the Peninsula, producing 25–30 m s^–1 (~50–60 kt) winds at Fossil Bluff, but these events are rare and the relatively sheltered position of Fossil Bluff to the east of the mountains of Alexander Island means that few low–pressure centres actually move over the station itself.

With the large amount of water present during the summer, fog can be a problem for the flying operations.

The narrow sound is not well represented in the current generation of NWP models so model output needs to be used with care and satellite imagery is an important tool in the analysis/forecasting process.

Surface wind and the pressure field

Because of the effects of orography, the winds at Fossil Bluff tend to be from the north or south and are therefore not representative of the large–scale flow produced by the synoptic–scale pressure field.

The prevailing northerly winds usually bring mild, moist air southwards, with southerly winds tending to bring dry, cold air northwards. However, returning maritime air on a southerly or returning continental air on a northerly can occur. Returning continental air from the north can bring improving conditions southwards down the western side of the Peninsula to Fossil Bluff.

The wind speed as measured at the accommodation hut may be different to that at the skiway. The wind is often a north north–westerly at the hut (where most of the observations are made) due to the flow down the Eros glacier. The direction is often more northerly and stronger at the skiway.

Upper wind, temperature and humidity

With no radiosonde data available for the region, upper–air conditions are predicted at Rothera using the fields from the UKMO model. These tend to be reasonable, although errors can occur in the strength and position of the jets.

Clouds

The distribution of low cloud is very variable due to the complex orography, with areas of quite low stratus 60–150 m (~200–500 ft) in some places whilst it may be completely clear in others. Sometimes with extensive stratus over the Sound the skiway may be in the clear as it is approximately 200 ft above the Sound. Stratus seems to be more extensive in the middle to late summer, which is probably due to the formation of melt pools in the Sound. When the Sound is frozen there is less available moisture, so stratus has to be advect in, rather than form in situ.

Fronts arriving from the north can cause a deterioration of conditions in terms of cloud lowering to the surface and visibility falling to fog limits.

Channel 3 (3.7 mm) imagery is particularly valuable during the summer for the detection and tracking of low cloud/fog in the Sound.

Visibility: blowing snow and fog

As in the rest of the Peninsula, visibility in the Sound is usually good, although fog does occur and can be a problem for flying activities. The formation of fog is aided by the large areas of water on the ice shelf, coupled with the areas of exposed ice. Fog over the Sound tends to be rather reluctant to clear in light winds, particularly if temperatures are around zero or above and melt pools have developed.

Surface contrast including white–out

Surface contrast is important at Fossil Bluff because of the large amount of flying activity in the area. During the summer, observations of contrast are available for the skiway and the flags at its edge help during landing. Contrast often improves as the summer progresses with the development of large melt pools that take on a blue or turquoise colour.

Horizontal definition

Since the Sound is a floating ice shelf horizontal definition can be problem when aircraft are attempting to land. However, the observers at Fossil Bluff provide frequent reports of horizontal definition to the aircraft, especially when conditions are changing rapidly.

Precipitation

The Bluff is usually well protected from precipitation, and snow stake data suggests that the Sound generally has low precipitation. Large depressions near western Alexander Island often fail to produce any precipitation at the Bluff, although the northern half of the Sound may receive precipitation and low cloud. Some of the heaviest and most prolonged precipitation events have been associated with mesoscale features. Slow–moving fronts can also produce precipitation. A spell of northerlies with a slow–moving front to the west will advect low cloud and precipitation into Fossil Bluff. Precipitation is usually in the form of snow or sleet, although rain has been reported.

Although large depressions west of Alexander Island often do not produce any significant precipitation at the Bluff, snow often occurs when the depression is filling and a relatively light pressure gradient develops with snow falling from shallow stratocumulus cloud.

Temperature and chill factor

Temperatures at Fossil Bluff are usually several degrees colder than at Rothera, but nevertheless get a few degrees above freezing on a number of days during the summer. The exposed rock surfaces on the sides of the Sound absorb a lot of radiation during the summer, resulting in extensive melting of the snow and ‘rivers’ running down many of the glaciers.

Icing

With the large amounts of water droplet cloud in the Sound, icing of airframes can be a problem, although when it occurs it is usually light. However, moderate or even heavy icing can occur on occasions. One severe icing event occurred between Sky–Hi and Fossil Bluff in the 1996/97 season when a Twin Otter reported quickly picking up 4–5 cm of clear ice as it descended towards the Fossil Bluff Skiway. The thickness chart showed a cold air mass (514 dm) over the Peninsula and there was a surface temperature of –14ºC. The thickness chart for 12 UTC 11 February 1997 indicated the invasion of a warm air mass from the northwest. This air mass moved over the colder and denser air in the lower levels introduced multi–layered clouds with precipitation. This precipitation formed in the warm air and fell as liquid droplets through the colder air below, so becoming supercooled. When these droplets collided with the aircraft (or surface) they turned immediately to clear ice.

Turbulence

As in other parts of the Peninsula, turbulence is predicted by analysis of the model upper–level winds and especially from the locations of the jet streams. However, it is questionable whether we get true jet streams in this area and most turbulence can be regarded as being orographically driven. Mountain waves are not unusual in this area.

Hydraulic jumps

Hydraulic jumps have not been reported in the Sound.

Sea ice

Sea ice is only found at the two ends of the Sound and its location is determined from the HRPT imagery available at Rothera.

Wind waves and swell

Not relevant at this location.