7.7                                   Mac. Robertson Land  

Mac. Robertson Land spans 60º to 73º E (see Figure 7.6.1)). From west to east, key features or stations/bases referred to in this section include:

·                         Mawson                                               (67° 36´ S, 62° 53´ E, 16 m AMSL);

·                         The Prince Charles Mountains;

·                         Soyuz                                       (70° 35¢ S, 68° 47¢ E, 336 m AMSL);

·                         The Amery Ice Shelf;

·                         The Lambert Glacier.

7.7.1                                Mawson Station  

7.7.1.1                          Orography and the local environment

Mawson is near 67° 36´ S, 62° 53´ E. Although the coastline is generally oriented east/west near Mawson, the station itself is on a northwest–facing section of coastline on the Mawson coast. It is situated at the foot of the continental ice plateau, with mountain ranges to the south (see Figures 7.6.1, 7.7.1.1.1, and 7.8.1).

7.7.1.2                          Operational requirements and activities relevant to the forecasting process

From the 2004–05 Austral summer–season CASA 212 aircraft will be used to ferry personnel to Mawson from Davis and return. At present, S76 helicopters are used to ferry personnel to Mawson from Davis and return during the Austral summer. Operations at Mawson are currently supported from Davis (see Section 7.8.4). As in the past, should fix–winged aircraft operate into Mawson then a blue–ice airstrip at the nearby Rumdoodle site (67º 48´ S, 62º 50´ E, 498 m AMSL) may be used (see Figure 7.7.1.1.1).

7.7.1.3                          Data sources and services provided

The only service available at Mawson at present is APT satellite reception, although grid point data from some global models can easily be sent to a forecaster at Mawson via e–mail. Access to the World Wide Web allows the user to browse a wide set of products from various forecasting centres, including centres in Australia and the USA that are dedicated to NWP in support of Antarctic weather forecasting operations.

7.7.1.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Table 7.7.1.4.1 (in Appendix 2) gives a summary of mean–monthly values of certain weather elements at Mawson.

Mawson is affected by katabatic winds from the ice slopes to the south. These katabatic winds are associated with a strong diurnal variation of wind speed, with a morning maximum. Temperatures above zero are common in summer, often leading to the break–out of fast ice from along the Mawson coast in January. Winter temperatures are more typically near –15 to –25°C. Mawson is affected by mobile depressions, and, while many depressions appear to form to the north near the Antarctic convergence south of Kerguelen Island, intense depressions often reach the base.

Frontal precipitation is generally in the form of snow generated by warm fronts, occasionally travelling westwards along the Antarctic coastline as the parent depression moves slowly eastwards. Snow commonly falls from stratocumulus or nimbostratus.

Table 7.7.1.4.2 (in Appendix 2) is taken from work by D. Shepherd (personal communication) and summarises the suitability of Mawson 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, Mawson enjoys a relatively low percentage of weather that might be adverse to aviation. December (only 6 % adverse conditions (excluding white–out)) is the best month and the July–August period the worst (17 % adverse conditions).

Surface wind and the pressure field

Figure 7.7.1.4.1 (in Appendix 2) shows wind roses for Mawson for each season. It may be seen from this figure that for Mawson the prevailing winds are from the southeast. About 15% of Mawson's winds is over 15 m s^–1 (~30 kt) for summer, rising to around 27% for winter. The mean wind speed is 11 m s^–1 (~22 kt) or more for most months and still around 9 m s^–1 (18 kt) for December and January. There is little diurnal variation in mean speed through late autumn and winter, but for other months there is a maximum near 0000 UTC and a minimum near 1200 UTC. The January range is from about 11.9 m s^–1 (~23 kt) to 6.7 m s^–1 (~l3 kt) for those times.

Significant winds at Mawson are also mostly southeasterly in the case of katabatic flows or easterly when strong to gale–force synoptically forced winds occur. (Gale–force winds rarely have westerly components at Mawson, with a strong preference for easterlies.) Katabatic flows may be nearly southerly, but rarely have a westerly component. Sea breezes occur during summer, generally northwesterly. 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 or eastwards towards or along the Mawson 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 Mawson. It is observed (Phillpot 1997, p 98) that gale–force winds at Mawson are associated with 500–hPa ridges crossing the coast between Mawson and Davis, especially when the ridge causes north to northeast flow at 500 hPa over Mawson (see Phillpot's Figure 4.24d – Phillpot’s composite for gales at Mawson). It appears the northerlies are needed in order to ‘steer’ the synoptic scale lows close to the coastline, however short blizzards have been caused by low–pressure systems moving eastwards. (As the Antarctic convergence moves closer to the coast near Casey, it would be expected that synoptic scale depressions would more frequently affect the coast.)

Considerable variation is found over the Mawson coast in wind speed. Field party reports suggest that the strong katabatics of Mawson Station are not commonly experienced to the west near the Stillwell Hills, even in exposed areas. However, to the east the strong katabatic zone extends beyond Scullin Monolith.

In terms of aircraft operations it is useful to not only consider the potential absolute wind strength but the cross–wind component may also be of concern. Cross–wind component thresholds for safe landing/takeoffs are aircraft dependent. Table 7.7.1.4.3 (in Appendix 2) shows the percentage frequency of occasions when the wind normal to the prevailing 130º mean wind direction at Mawson exceeds 7.7 m s^–1 (~15 kt). Such occurrences are not common at Mawson, ranging from 8% in August to 3% in December.

The above data relate to Mawson itself where perhaps only rotary–winged aircraft could land. However, there is an area of blue ice at the nearby Rumdoodle site (see Figure 7.7.1.1.1) where fixed–winged aircraft can, and have, landed. Observations made in February and March 1960 indicate that average winds at this site are about one half those at Mawson, there being shelter from the south and east due to the Masson range. P. Targett (personal communication) has processed approximately three months of wind data collected during the period 6 September 1993 to 11 Dec 1993 at the "Rumdoodle Turn Off" (RTO) site located at 67º 47´ S, 62º 42´ E, which is somewhat closer to Mawson than Rumdoodle itself. Targett has compared the Mawson winds for the same period and reports that the average 10 m elevation wind speed for Mawson was 8.9 m s^–1 for the period, compared to the RTO anemograph recording, at 3 metres, of 11.1 m s^–1. Advice from Allison (personal communication) suggests that the RTO data could be increased by between about 1.1 (for snow surface) and 1.3 times (for blue ice) to adjust from 3 m to 10 m elevation.

So, using a height adjustment factor of 1.2 (assuming a part blue ice/part snow surface), for the approximate three month period the winds at RTO were about (11.1*1.2)/8.9=1.5 times greater than at Mawson. This is in contrast to Streten's (1961) finding for the more inland Rumdoodle site. This ratio compares with the 1.89 factor reported by Shaw (1960) at Mount Henderson, however, Mather and Miller (1966) state that the Mount Henderson site was at a "high rock nunatak, and the winds sweeping over the ridge could well be much stronger than those on the adjacent plateau". It should also be noted that the RTO site is between the Massom and David Ranges and so may be protected from the general southeast katabatic flow to some extent.  

To assess the likely risk to aircraft it should be noted that Targett's 1993 data indicated a 10 m mean wind speed at the RTO site of around 13.3 m s^–1 (~26 kt) for spring/early summer. The strongest 10 minute averaged 10 m wind inferred from Targett's data set was 36 m s^–1 (70 kt) on 10 and 20 October. An inference regarding the likely strongest gust at RTO was made as follows, taking the month of December as an example. The strongest gust on record at Mawson for December is 53 m s^–1 (~103 kt) in 1966. A "guesstimate" as to the ratio of gust to mean speed at Mawson would be about 1.2 to 1.3. Taking a factor of 1.25 gives a mean speed at Mawson at the time of the gust as 42 m s^–1 (~82 kt). Scaling this by 1.5 gives an estimated mean wind speed at RTO of 64 m s^–1 (~124 kt). RTO is slightly smoother than Mawson so the gustiness factor should be less, let us say, 1.2. Thus the extreme gust at RTO in December is likely to be in the order of 64×1.2=81 m s^–1 (~148 kt). (A second way of estimating the peak gust might be to simply scale the gusts according to the ratio of mean wind speeds, thus the 53 m s^–1 scaled by 1.5 would give 79 m s^–1 (153 kt).  It should be stressed that this would likely be an extreme event, perhaps a one in 30 to 50 year event. On the other hand, from Targett's 1993 data, peak gusts of around 28 to 42 m s^–1 (~50 to 75 kt) might be expected at RTO at least once per month during spring and early summer.

Upper wind, temperature and humidity

These fields are usually taken from GASP numerical products, with cross checking of satellite images.

Clouds

As with most continental areas, stratocumulus or stratus are the most common low clouds, with cirrostratus the most common high cloud. As may be seen from Table 7.7.1.4.4 (in Appendix 2) low cloud of major significance to aircraft landing/takeoff at Mawson 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.

Stratocumulus is most commonly brought over the base by maritime streams, but satellite pictures need careful analysis to detect the occasions when low to mid level stratiform clouds, especially upper–level deformation clouds, are swept along the coast by depressions to the north along the coast near Mawson. Satellite imagery, especially when looped between pictures with the same projection, is the major tool for detecting and forecasting cloud, as no land–based observations are available close to Mawson.

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

Visibility: blowing snow and fog

Visibility is generally good, at least during summer, other than with snow. Tables 7.7.1.4.5 and 7.7.1.4.6 (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.

Fogs are rare during summer, but may occur with light northerly winds 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 1,000 m or more and reducing visibility to a few kilometres at times.

Surface contrast including white–out

White–out is significant for aviation along the Mawson coast, however, it is often possible to navigate eastwards towards Cape Darnley by reference to the coastline, due to the frequent occurrence of open water about 100 km east of Mawson. Flight to the west probably requires fairly clear skies unless the fast ice breaks out. Flight to the south obviously requires clear skies as it passes over the plateau.

For Mawson itself, the local rocky terrain, if substantially ice or snow free during the summer melt, affords some relief. However, as may be seen from Table 7.7.1.4.7 (in Appendix 2) the potential for white–out at Mawson, at least in some sectors, is greater than 50 per cent for much of the year.

Horizontal definition

The presence of extensive ice and snow make the loss of both horizon and surface definition common near Mawson, especially at any blue–ice airstrip inland, mainly due to overcast cloud at any level and falling snow reducing visibility.

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 Mawson, often followed by strong winds. Snow falls most frequently from stratocumulus or stratus if it is at least 200 m thick.

Temperature and chill factor

Temperature is not particularly important in this area, provided that wind speeds are minimal. Chill factor is significant for fieldwork, as although rain is unlikely workers may be wet if undertaking work in small boats. Wind chill is 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.

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 Mawson leads to only light icing. There is little experience of icing amongst pilots because helicopter operations through cloud have not been approved in the past. It is possible for icing to occur in clear air when the air is close to saturation and air temperature is near zero, although this is not common.

Turbulence

Severe mechanical turbulence is common in the lee of elevated terrain to the south of Mawson.

Hydraulic jumps

Hydraulic jumps have been reported near Mawson on occasions both published and anecdotal. Presumably the steep terrain to the south of the station is conducive to their formation, however a study of their frequency and climatology is not available.

Sea ice

Sea ice commonly forms during April between Mawson and the offshore islands becoming thick enough to support vehicular travel in May, although routine checks of thickness are made before using the ice. The ice extends some 50 to 100 km offshore by September, with shipping usually unable to reach Mawson in that month. Grounded icebergs stabilise the fast ice northeast of Mawson, while an indentation usually forms in the fast ice to the northwest, with a polynya allowing shipping to approach within approximately 50 km of Mawson – if able to penetrate the sea ice further north. In most years, this fast ice breaks out in January, however occasionally it remains for the full summer, causing breeding failures amongst penguins on the islands near Mawson. In the years when the sea ice breaks out, small boating activities become possible for the period January to mid March.

Wind waves and swell

The tendency towards southeasterly flow, especially in strong winds, means that fetch is rarely long enough for significant wave development, however rough seas can obviously be generated under storm force wind conditions. During the presence of extensive pack ice to the north, little swell reaches Mawson.

7.7.2                                Prince Charles Mountains (including Soyuz Base)  

7.7.2.1                          Orography and the local environment

The Prince Charles Mountains (PCMs) consist of a number of massifs aligned along the Lambert Glacier, extending from the Athos, Porthos and Aramis Ranges in the northwest, to Mount Mather in the southwest to Mount Borland in the south and along the Mawson Escarpment (see Figure 7.6.1 and Figure 7.8.1. Most of the mountains are submerged below continental ice, with glaciers flowing between them, tributaries to the Lambert Glacier. Figure 7.7.2.1.1 shows a portion of the northern part of the PCMs: the Lambert Glacier flows northwards immediately east of the area shown in this figure. The prevailing winds exhibit a drainage pattern, with east to northeasterlies on the eastern side of the Lambert Glacier and southwesterlies on the western side, although near the larger mountains themselves more complex flows may occur. Being such a significant feature is likely that the Lambert basin would have a significant effect on winds throughout the lower troposphere.

7.7.2.2                          Operational requirements and activities relevant to the forecasting process

The Russian Soyuz Station (70° 35¢ S, 68° 47¢ E, 336 m AMSL) was opened in December, 1982 and closed in February 1989 and was located on the eastern shore of Beaver Lake, (Figure 7.7.2.1.1) 260 km away from the coastline of Prydz Bay. The purpose of the base was to support geological work.

For Australian science too, geological and glaciological work has predominated in the Prince Charles Mountains. Thus the main requirement to date has been support of scientific parties travelling to and within the PCMs. Because of the extensive ice and snow cover, travel between massifs is mostly limited to visual flight rules to avoid white–out conditions. Fuel is often placed into depots in the PCMs/Amery Ice shelf at Sansom Island, Beaver Lake and near the northern end of the Mawson Escarpment. Shelters are also in place at Sansom Island and Beaver Lake; hence operations in the past have tended to focus on these locations. Previous summer base camps have operated at: Moore Pyramid (70°18' S, 65°06' E, 1,460 m AMSL) from 1972–74 (eg: Woods, 1971); Mount Cresswell 72° 44' S, 64° 23' E, 1,161 m AMSL) from 1971–74; and Dovers (70° 14' S, 65° 51' E, 1,099 m AMSL), from 1988‑92 (eg: Bromham, 1989)–for locations see Figures 7.6.1 and 7.7.2.1.1.

7.7.2.3                          Data sources and services provided

Forecasters are no longer positioned in the field in this location but are based at Davis. Satellite observations are the main source of information for this area, although upstream AWS observations are becoming available surrounding the Lambert/Amery. No regular surface observations are available.

7.7.2.4                          Important weather phenomena and forecasting techniques used at the location

General overview

The PCMs are frequently affected by mesoscale low–pressure systems moving southwards into the Lambert Basin. Such features do not appear to be intense, however snowfall often accompanies them and the cloud systems tend to persist over several days. Surface winds are strongly affected by the drainage flow about the basin. Synoptic systems also affect wind speed, especially in the northern PCMs but not wind direction.

Activities during 1994–95 and 1995–96 ANARE summer operations demonstrated that mid–November to mid–February is the optimal season for field parties and helicopter operations as high winds and low temperatures hindered operations in the earlier and later months, especially in elevated areas. During late February to early March 1996 very little work could be achieved at Mount Kirkby (elevation 1,500m) due to wind, blowing snow, low temperatures or poor visibility. In early and late parts of the 1994–95 season helicopters suffered mechanically from the low temperatures (–25°C or lower).

Data are available for the 1997–98 summer parties visiting the Mawson escarpment and Mount Menzies. These parties consisted of field training officers and geologists, who stayed for about a month in the two locations. Their observations were made generally near 00 UTC and 15 UTC near their camps. Although untrained in meteorology, they were professionals in their respective fields and their observations represent the best data available, so the following comments draw upon their observations.

At the Soyuz base the climate was found to be favourable for work in summer. The mean temperature of the warmest month (January) was –3.1⁰C, although the extreme temperatures can vary from +3.5⁰C in January to –23⁰C in March during the summer. The monthly mean wind speed varied from 5 m s^–1 (~ 10 kt) in December to 9 m s^–1 (~18 kt) in February and in March. The maximum wind speed reached 20–25 m s^–1 (~40 to 50kt) with gust up to 30 m s^–1 (~60 kt). December and January had the clearest summer weather and the minimum of snow–storm days.

Surface wind and the pressure field

The main influence on wind is the continental Antarctic drainage flow, often experienced as a katabatic outflow. This is probably strongest during autumn, winter and spring, however few records are available to support that suggestion. There are anecdotal reports of gale to storm force katabatics in both the northern and southern PCMs in April (observed by aircrew in 1998). AWSs located at higher elevations about the Lambert Basin show highly persistent winds, representing a cyclonic circulation about the basin at near–surface levels dominated by the slope of the ice.

The 1997–98 Mawson Escarpment party reported winds averaging about 4 m s^–1 (8 kt), mostly easterly. These winds were probably affected by choice of camp site (for many of the observations (26 over 22 days) the direction 110º was reported). Wind speeds (estimated with a hand held anemometer) were not over about 13 m s^–1 (25 kt). Surface pressure was fairly constant, averaging 984 hPa with a standard deviation of 5 hPa, however, variations in site elevations makes this figure unreliable.

The 1997–98 Mount Menzies party reported winds of no more than 5 m s^–1 (~10 kt), however their reports were strongly site–dependent. They were camped on the northern side of the mountain and sheltered from the prevailing winds, thought to be southerly.

Observations at Beaver Lake (October/November 1995 and February/March 1996)) and Mount Kirkby (February/March 1996) did not show the diurnal variation typical of summer–katabatic flow, that is, pronounced winds in the morning then an easing in wind strength in the afternoon. The strongest wind usually occurred with strong synoptic pressure gradients. At Beaver Lake the wind direction is south to southwesterly due to the orientation of Pagodrama Gorge, just upstream. There was no useful correlation between wind speed at Mount Kirkby and Beaver Lake. From February 1996 the six AWSs around the Lambert Basin could be used to approximate Mount Kirkby and Beaver Lake winds. Overcast low cloud, snow and fog depressed wind speed by 5 to 10 knots on average compared with no cloud or cirrus only. Gusts at Mount Kirkby were of the order of 1.5 to 2 times the average wind, with severe mechanical turbulence and rotors being common in strong winds. Winds at both sites and the AWS strengthened (but maintained direction) when the notional inland high on mean sea level charts intensified, or pressure gradients increased due to synoptic systems, or significant fronts approached from the east.

Upper wind, temperature and humidity

These fields are usually taken from GASP numerical products, with cross checking of satellite images. No local observations are currently available; although it is believed that upper winds were recorded from Dovers in the northern PCMS during the late 1980s. In the absence of data, numerical products are the only guides.

Clouds

Cirrus can be advected over the PCMs from inland, most likely associated with synoptic systems outside the field of the locally available satellite imagery. It has also been observed in association with oceanic lows near longitude 55° E. Altostratus can also form inland near the PCMs, probably from upper troughs over the area. Banner cloud, lee waves and rotor cloud all form due to the mountains. Banners and wave clouds can be quite extensive and persistent. Convective cumulus occurs in summer over bare rock. Layers of low or mid–level cloud may reduce or clear during summer days then reform in the evening.

Moisture, once inserted into the PCMs by northerly airflows, tends to persist for several days. There appears to be a slight correlation between 500–hPa northerlies (suggested by ECMWF composite 500–hPa gradients) and subsequent cloud increases. Total cloud averaged over the observations taken by both the 1997–98 Mawson Escarpment and Mount Menzies parties was about 3 to 4 oktas. There was poor correlation between pressure and cloud. Accordingly, an attempt is made to nowcast cloud movement from satellite pictures using image loops. Prediction of cloud formation and dissipation is difficult, and relies on numerical products and the slender correlation between northerly gradients in the upper levels and moisture advection into the continent.

Cloud can be exceedingly difficult to detect especially in winter. Extensive stratus at very low level may be impossible to detect. The forecaster can use low sun angles in visual images to detect cloud, however stratus may cast no shadow.

The reader is also referred to relevant comments in Section 7.8.1.4.

Visibility: snow and fog

Fog has been reported on numerous occasions during the summer over the Amery Ice Shelf and occasionally in the southern PCMs. The summer maximum appears to be associated with extensive melt pools over the Lambert/Amery system providing a source of moisture for steam fog, often advected by light afternoon winds into the northern and southern PCMs.

Little experience exists of conditions during the spring, autumn or winter, so it would seem prudent to assume that ice crystal hazes are likely during those seasons as temperatures fall below –20ºC, however strong katabatics may prevent them from forming.

Surface contrast including white–out

White–out is highly significant for aviation within the PCMs. Most of the mountains are separated by extensive ice sheets and of course the glacier is fairly featureless from an aircraft. Flight from the northern to the southern PCMs and to the more isolated peaks usually requires fairly clear skies. Operations within the various massifs can, however, usually continue under overcast skies. The Mawson Escarpment party in January 1998 reported poor surface definition on three occasions during the month, associated with overcast, while the Mount Menzies part reported poor surface definition on five occasions.

Careful notice must be taken of all cloud types. Even apparently thin stratocumulus (difficult to detect on satellite pictures unless AVHRR channel 3 images are available) can cause white–out in this region. The coastline or mountains in the interior are often clearly visible through cloud that produces very poor surface definition and white–out. Provided there are breaks in the stratocumulus cloud, however, or a layer of altocumulus cloud is sufficiently high, a setting sun can generate shadows from sastrugi to provide acceptable surface definition.

Horizon definition

As with most Antarctic locations, horizon definition is easily lost under falling snow. The Mawson Escarpment reported poor horizon in at least one direction on nine occasions in January 1998. The Mount Menzies party reported poor horizon on four occasions. Generally, horizon definition was poor in only part of the horizon.

Precipitation

Nearly all precipitation is in the form of snow. It is possible that rain may occur at times during summer, as temperatures can reach well above 0°C.

Temperature

Average temperatures for the month of January reported by the two field parties are shown in Table 7.7.2.4.1 (in Appendix 2).

Icing

No reports have been reported, however it would be expected that with temperatures near zero during summer icing would be an occasional hazard.

Turbulence

Mechanical turbulence has been anecdotally reported, associated with terrain, as expected.

Hydraulic jumps

Under clear sky conditions it is often observed that surface temperatures (as evidenced by satellite images) are lower along the eastern side of the Lambert Glacier/Amery Ice shelf than to the immediate east on the continental ice sheets or to the west over the western Lambert/Amery. This may be due to the formation of a hydraulic/katabatic 'jump' on the sloping ice sheets to the east, allowing very light winds in the lee of the jump and associated radiational cooling of the eastern side of the glacier.

Sea ice

Not relevant to this area.