Utilization of Automatic Weather Station Data for Forecasting High Wind Speeds at Pegasus Runway, Antarctica

Transcription

1 VOLUME 15 WEATHER AND FORECASTING APRIL 2000 Utilization of Automatic Weather Station Data for Forecasting High Wind Speeds at Pegasus Runway, Antarctica R. E. HOLMES AND C. R. STEARNS Space Science and Engineering Center, University of Wisconsin Madison, Madison, Wisconsin G. A. WEIDNER AND L. M. KELLER Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison, Madison, Wisconsin (Manuscript received 14 December 1998, in final form 20 September 1999) ABSTRACT Reduced visibility due to blowing snow can severely hinder aircraft operations in the Antarctic. Wind speeds in excess of approximately 7 13 m s 1 can result in blowing snow. The ability to forecast high wind speed events can improve the safety and efficiency of aircraft activities. The placement of automatic weather stations to the south (upstream) of the Pegasus Runway, and other airfields near McMurdo Station, Antarctica, can provide the forecaster the information needed to make short-term (3 6 h) forecasts of high wind speeds, defined in this study to be greater than 15 m s 1. Automatic weather station (AWS) data were investigated for the period of 1 January 1991 through 31 December 1996, and 109 events were found that had high wind speeds at the Pegasus North AWS site. Data from other selected AWS sites were examined for precursors to these high wind speed events. A temperature increase was generally observed at most sites before such an event commenced. Increases in the temperature difference between the Pegasus North AWS and the Minna Bluff AWS and increasing pressure differences between other AWS sites were also common features present before the wind speed began to increase at the Pegasus North site. Many times, changes in one or more of these parameters occurred hours before the wind began to increase at the Pegasus North site. Monitoring of these parameters can lead to an improved 3 6-h forecast of these high wind speed events at Pegasus Runway, Antarctica. 1. Introduction The National Science Foundation s Office of Polar Programs operates the United States Antarctic Program s (USAP) year-round stations in the Antarctic at Anvers Island (Palmer Station), Ross Island (McMurdo Station), and at the South Pole (Amundsen Scott Station). While Palmer Station is generally serviced via USAP research vessels from South America, McMurdo Station is partially supplied by aircraft from Christchurch, New Zealand, and Amundsen Scott Station is supplied entirely by aircraft flights from McMurdo Station. Most of the personnel at McMurdo and Amundsen Scott Stations travel by air to and from New Zealand. The aircraft used for flights between Christchurch and McMurdo Station are the ski-equipped LC-130, and the wheeled C-130, C-141, and C-5. In the area of McMurdo Station, the ski-equipped LC-130 can land on ice, compacted snow, or, in extreme circumstances such as a Corresponding author address: Robert Holmes, Space Science and Engineering Center, 1225 West Dayton St., Madison, WI rbrbrn@ssec.wisc.edu whiteout, the vast snow field of the Ross Ice Shelf, while the wheeled aircraft can only land on runways constructed on ice. The reliance on an ice runway for wheeled aircraft increases the importance of a good weather forecast for these flights because after passing the point of safe return, the only landing sites that can accommodate wheeled aircraft are the two ice runways near McMurdo Station. Figure 1 shows the three landing sites, Williams Field ski-way, Pegasus blue-ice runway, and the sea-ice runway, in the vicinity of McMurdo Station. The sea-ice runway is located on the annual sea ice west of the southernmost point of Hut Point Peninsula although its location annually varies. The sea-ice runway is used by both ski-equipped and wheeled aircraft. Its season of use begins in August and continues until early December, when the sea ice becomes unsafe for aircraft operations. At this time, all aircraft operations shift to Williams Field and all flights, whether between Mc- Murdo Station and New Zealand, Amundsen Scott Station, or remote field camps, are made exclusively by the ski-equipped LC-130. The Pegasus blue-ice runway is used for flights by wheeled aircraft late in the austral summer field season, 2000 American Meteorological Society 137

2 138 WEATHER AND FORECASTING VOLUME 15 TABLE 1. Locations of AWS units used in support of this study. Site Lat Long Pegasus North Minna Bluff Elaine Willie Field Ferrell Lettau Gill Linda Schwerdtfeger Marilyn S S S S S S S S S S E E E E E W W E E E Elevation (m) FIG. 1. Location map of the Pegasus runway site (adapted from Blaisdell et al. 1995). approximately late January into March. During most of the austral summer, the runway is covered with snow to maintain the integrity of the ice surface, and then the snow is removed prior to the commencement of air operations. The use of the Pegasus runway also allows the ski-equipped LC-130 to operate on wheels and not on skis, which allows for as much as a 50% increase in the payload. The first automatic weather station (AWS) unit near all three runway facilities was installed near the wrecked aircraft named Pegasus in January It provided atmospheric pressure, air temperature, and wind speed and direction data until the unit was removed in November The data record over this time period showed that the Pegasus AWS site monthly mean air temperature was approximately 1 C warmer than the monthly mean temperature at Ferrell AWS site, approximately 88 km to the east and at approximately the same latitude (Table 1). The monthly average wind speed was 1 2 m s 1 less at Pegasus AWS site than that at Ferrell AWS site, but the maximum monthly wind speed had a tendency to be 8 10 m s 1 greater at Pegasus AWS site than that at Ferrell AWS site. The wind direction associated with the maximum monthly wind speed at Pegasus was from the direction of Minna Bluff, which lies 55 km to the south of the Pegasus AWS site. Data for 1989 for Ferrell and Pegasus AWS sites are given by Stearns and Weidner (1990). Plans were made to install AWS units at the north and south ends of the Pegasus Runway, on top of Minna Bluff, at Williams Field, and on the Ross Ice Shelf east of Minna Bluff based on the information initially provided by the Pegasus AWS unit. The purpose of the AWS site on top of Minna Bluff, at an elevation of 920 m, was to detect the strong winds flowing over Minna Bluff and continuing toward the Pegasus Runway. The AWS site east of Minna Bluff (Linda AWS site) was installed to investigate the airflow near the east end of Minna Bluff. The AWS units on each end of the Pegasus Runway were installed to observe any differences in the wind speed, wind direction, atmospheric pressure, and air temperature along the runway. The AWS unit on the west end of the Williams Field ski-way was installed to provide a nearby AWS test site and to obtain meteorological data for the entire year. Previously, meteorological data were collected only when Williams Field was in operation. The Pegasus North AWS site was installed in January 1990 at the north end of the now established Pegasus runway. The AWS sites at Minna Bluff, Linda, and Pegasus South were established in January The Pegasus South AWS unit also measures a vertical temperature profile in the ice. Stearns and Weidner (1991, 1992) reported that while the wind directions at both the Pegasus North and South AWS sites were most commonly from the east-northeast, winds with speeds greater than 8 m s 1 had a strong tendency to blow from the south. Several AWS units have also been deployed near Mc- Murdo and to the south (upstream) of the Ross Island area to aid in operational forecasting. Fleming (1983) utilized data from many of these units to forecast fog at the Williams Field runway. He found that during the period from 1 November 1982 to 13 January 1983 there was a strong correlation between a wind from the southeast quadrant ( ) at Ferrell AWS site and dense

3 APRIL 2000 HOLMES ET AL. 139 The data for this study were obtained from the AWS sites, whose geographic location and elevation are found in Table 1; Fig. 4 shows the locations for these AWS sites. Data from 1991 through 1996 were chosen for this study. Unfortunately, not all of the units were operating at all times. Missing data can be the result of any number of events that occur while the AWS unit is unattended. FIG. 2. Layout of the AWS unit used in the Antarctic. The installed AWS unit has a 3-m tower with a horizontal boom supporting the antenna, aerovane, air temperature thermometer, upper themopile, and the relative humidity sensor. The electronics enclosure is mounted at the midpoint of the tower. The gel cell batteries are placed at the tower base. The 10-W solar panel faces north. fog at Williams Field. Sinkula (1993) and Holmes (1994) have done previous studies on the use of AWS data for forecasting high wind speeds in the vicinity of Ross Island. Other examples of the use of AWS data as a forecasting aid are shown by Holmes et al. (1993) and Stearns and Sinkula (1994). This study focuses on the development of empirical rules based on AWS data to forecast high wind speed events at the Pegasus Runway. 2. Data acquisition The development and deployment of automatic weather stations have made possible the collection of near-surface meteorological data in remote areas of Antarctica. The automatic weather station is a surface meteorological data collection unit capable of operating throughout the year without any intervention. Each AWS unit measures air temperature, horizontal wind speed, and wind direction at a nominal height of 3 m. Atmospheric pressure is measured at the electronics enclosure (Fig. 2). Some AWS units also measure the relative humidity at 3 m and the air temperature difference between 1.0 and 3.9 m. The height at which each parameter is measured varies with time because of the accumulation or ablation of snow. The AWS units have been deployed in Antarctica since 1980 by the U.S. Antarctic Program (Stearns et al. 1993). As of 25 May 1998, 54 AWS units are operating in Antarctica, enhancing meteorological research (Fig. 3). The units have operated successfully at temperatures as low as 80 C and at wind speeds as high as 50 m s 1. The units are serviced as needed each austral summer. Stearns et al. (1994) and Holmes and Stearns (1995) describe the activities during two previous field seasons. 3. Statement of the problem Because of its remote location, air travel in the Antarctic can be dangerous. A downed aircraft could lead to loss of life not only from the crash itself, but from exposure or because of the lack of nearby medical facilities. Therefore, safety is a high priority for those involved in air transportation. Most of the personnel working for USAP are transported to the continent by air, using one of three runway facilities near McMurdo Station. The sea-ice runway is located just to the west of McMurdo on the annual sea ice and can accommodate conventional (wheeled) aircraft. The annual ice begins to deteriorate in early December, at which time air operations shift to Williams Field (Fig. 1). Williams Field, a compacted snow facility, is located approximately 15 km to the east-southeast of McMurdo Station and is used primarily by skiequipped aircraft. The use of skis limits the amount of cargo and passengers that can be carried on each airplane because friction with the snow surface limits the ability to reach takeoff speed. More fuel must be used because the skis create more drag in the air. These factors led to a search for a runway that could accommodate wheeled aircraft throughout the austral summer. During the summer of , the site known as Pegasus was chosen for the development of such a runway. The U.S. Antarctic Program opened the Pegasus runway on 6 February The runway is located about 12 km south of McMurdo and 16 km west of Williams Field. It lies just to the east of a loosely defined region between the area of net snow and ice accumulation and ablation (Fig. 1). Therefore, the ice surface of the Pegasus runway has a light snow cover that is removed before runway operation. The weather is ever changing near the Pegasus runway. Visibility is probably the most important forecast element at the Pegasus runway, and poor visibility can be caused by fog and/or blowing snow. Wind speeds in excess of 13 m s 1 can cause blowing snow in the austral summer, and during the winter, speeds in excess of 7 m s 1 can result in reduced visibilities from blowing snow (Bromwich 1988). The difference can be attributed to surface melting and greater adhesion in the summer. Abrupt changes in wind speed and direction can also have an impact on safety by affecting the lift of the aircraft, thereby hindering its ability to land or take off. Therefore, the ability to accurately forecast the wind would enhance the operational safety at Pegasus.

4 140 WEATHER AND FORECASTING VOLUME 15 FIG. 3. Antarctic automatic weather station locations as of 25 May Locations are identified by site name. The variations in the wind at Pegasus can be attributed to many factors and therefore can be difficult to forecast. The passage of a synoptic-scale disturbance can affect the wind, as may the local topography. Airflow between White and Black Islands can be accelerated by a jet effect (Savage and Stearns 1985). In fact, maximum wind speeds at Pegasus North tend to be greater than those at nearby sites (Keller et al. 1996, 1997). The onset of a barrier wind parallel to the Transantarctic Mountains or katabatic flow can also change the wind conditions at Pegasus North. Changes in static stability can promote or prevent vertical mixing, and mesoscale cyclogenesis may also play an important role. Typically, changes in wind speed and direction can be attributed to one or more of these factors and because most of these factors are on a relatively small spatial scale, global numerical models are of limited use. Examination of surface pressure analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) revealed no specific larger-scale synoptic features that can be used as a signature of high wind speed events. Observations of wind speed at Pegasus North from 1 January 1991 through 31 December 1996 were scanned for high wind speed events. These events were defined as those where the observed wind speed became greater than 15 m s 1 and was maintained for at least 1 h. One hundred nine such events were found. Data preceding these events at other AWS sites were investigated for the purpose of developing empirical rules for forecasting such events. 4. Observations and results A temperature increase with southerly winds seems to be common at many of the AWS sites before and during high wind speed events at Pegasus North. This may be due to turbulent mixing of an elevated, neutrally buoyant katabatic airstream with the underlying layer of air, which initially has a strong surface inversion (Bromwich et al. 1992). The temperature increase was usually most notable at the Pegasus North site itself. Significant temperature increases were also observed at both Marilyn and Schwerdtfeger. On many occasions Pegasus North recorded the high-

5 APRIL 2000 HOLMES ET AL. 141 FIG. 4. Location map for the AWS sites used in this study. est wind speeds among the AWS sites used in this study during these events. This fact should not be surprising because, as previously stated, the local topography of Minna Bluff and White and Black Islands creates a jet effect in that area. There were two AWS sites, Linda and Minna Bluff, that generally had winds of similar speeds as Pegasus North. However, many times the wind speed at Minna Bluff was greater than that at Pegasus North. Other sites usually experienced wind speeds significantly less than that observed at Pegasus North. In all of the cases studied, the high wind speeds at Pegasus North site coincided with winds from the south. The wind direction was usually from the northeast before the wind speed began to increase. The direction of the wind shifted southerly before the wind speed exceeded 15 m s 1. a. Estimated stability and its relationship to Pegasus winds Automatic weather stations have increased the amount of Antarctic surface meteorological data, leading to a better understanding of surface conditions in the Antarctic. But the lack of upper-air observations still presents a problem for forecasters. McMurdo Station, Antarctica, releases radiosondes every 12 h during the summer months, and nearly every day during the winter, but those are the only vertical measurements taken in

6 142 WEATHER AND FORECASTING VOLUME 15 the area. Because of its elevation (920 m), Minna Bluff can provide some insight into changes of meteorological parameters with height. To investigate this possibility, the temperature difference was calculated from data recorded by Pegasus North AWS and that from Minna Bluff: (Pegasus North temperature) (Minna Bluff temperature). Using this definition, a negative result would indicate inversion conditions (temperature increasing with height) and statically stable air, and a temperature difference greater than 8.9 C may indicate statically unstable air. This increase in the temperature difference between these two AWS sites can often be attributed to an increase in the temperature at Pegasus North. However, the temperature at Minna Bluff also plays an important role. Many times the temperature at Minna Bluff increased coincidentally with that at Pegasus North, resulting in a temperature difference that would indicate stable conditions, even though there was a large temperature increase at Pegasus North. There were also cases in which there was a relatively small increase in the temperature at Pegasus North but a decrease in temperature at Minna Bluff, resulting in a temperature difference indicating statically unstable conditions. This stability estimate was chosen because the calculation can be made quickly by the forecaster using data that is readily available. It must be stated that, because the results are calculated from AWS units more than 60 km apart, this temperature difference is only an estimate of the conditions present over this general area. Nevertheless, the results have given some useful insights to the meteorology of the area. The effects of diurnal temperature changes, especially at Pegasus North, must also be taken into account. The diurnal temperature changes in the summer make the use of the Pegasus North Minna Bluff temperature difference as an indicator of static stability less reliable. The temperature difference method was similar to the approach taken by Sinclair (1988) in which he compared weather data observed at Scott Base, Antarctica, to that recorded at Castle Rock, which has an elevation of 300 m and overlooks Scott Base. He observed that during inversion conditions, the wind tended to follow local topography, resulting in a northeast wind at Scott Base. However, during neutral or unstable conditions, the surface flow closely followed that at 300 m. These findings are consistent with simple physical arguments based on buoyancy considerations, which indicates that this method provides a realistic estimate of local static stability. As surface winds decrease, turbulent mixing is reduced, resulting in a cooling of the surface layer from radiation loss and an increase in static stability. This increase in static stability further decouples the surface flow from the flow aloft. Eventually, the kinetic energy of the surface flow is insufficient to overcome the effects of the increasing negative buoyancy (Sinclair 1988), and the air is restrained to flow around the topography. However, a slight increase in the larger-scale flow may be sufficient to sweep away the cold layer, resulting in stronger, and usually southerly, winds at the surface. The mixing process associated with these stronger winds may contribute to the further warming of the air near the surface, further enhancing the instability of the lower atmosphere. With some evidence of a relationship between the stability and the surface wind decoupling, it would seem reasonable that changes in stability may give advanced warning to an increase in wind speed. To investigate this possibility, time series graphs were made of the Pegasus North Minna Bluff temperature difference (TD) and Pegasus North wind speed during the hours preceding and during the high wind speed events. Because of missing data at either Pegasus North or Minna Bluff, this hypothesis could not be tested for every high wind speed event, but the data were available for 87 of the events studied. An increase in the TD was a common feature preceding high wind speed events at Pegasus North. In all but eight cases that had the data necessary for computing the TD, an increase to levels in excess of 8.9 C was observed during the 24 h prior to the increase in wind speed. The amount of time between when the TD became greater than 8.9 C and the increase in wind speed to 15 m s 1 ranged from 6.7 to 33.8 h, with the average being nearly 6 h. The range within one standard deviation of the mean was 2.0 to 13.6 h. The frequency distribution and various statistical data are shown in Fig. 5a. An example of this relationship can be seen during the high wind speed event of 15 April 1992 (Fig. 6). The wind at Pegasus North increased to 10 m s 1 at 0700 UTC, and then underwent a gradual decrease. At 1800 UTC, the wind speed increased abruptly from approximately 7 m s 1 to greater than 16 m s 1 in less than 1 h. Wind speeds remained generally at or above 15ms 1 until 0230 UTC 16 April. The TD was less than 0 C km 1 throughout the day of 14 April. During this period of temperature inversion, the wind speed at Pegasus North remained less than 4 ms 1. Beginning at 0000 UTC 15 April, the TD began FIG. 5. Frequency distributions for the amount of time that the trend of the AWS parameter was complete before the wind speed at Pegasus North increased to 15 m s 1 and associated statistics. The vertical dashed line denotes the 0-min line. Frequency distributions are in 60-min class widths and are for (a) Pegasus North Minna Bluff temperature difference, (b) Marilyn Schwerdtfeger pressure difference, (c) Elaine Marilyn pressure difference, (d) Marilyn temperature, (e) Marilyn wind direction, (f) Schwerdtfeger temperature, and (g) Schwerdtfeger wind direction.

7 APRIL 2000 HOLMES ET AL. 143

8 144 WEATHER AND FORECASTING VOLUME 15 FIG. 6. Pegasus North wind speed and Pegasus North Minna Bluff temperature difference vs time, Apr Pegasus North wind speed is denoted by solid circles. Open squares represent the Pegasus North Minna Bluff temperature difference. FIG. 7. Pegasus North wind speed and Pegasus North Minna Bluff temperature difference vs time, Feb Pegasus North wind speed is denoted by solid circles. Open squares represent the Pegasus North Minna Bluff temperature difference. to increase and continued to do so for the next 19 h. The TD increased to above 8.9 C by 1700 UTC, approximately 1 h before the abrupt increase in the Pegasus North wind speed. The TD remained above 8.9 C until 0600 UTC 16 April, shortly after the wind speed had decreased below 10 m s 1. The TD behaved in a similar manner prior to the high wind speeds observed at Pegasus North on 27 February 1993 (Fig. 7). The TD increased from 2.7 to 8.8 C and then quickly decreased back to 2.3 C between 0000 and 1600 UTC 26 February, without an appreciable increase in wind speed. A second increase in TD commenced shortly after 1600 UTC, when the TD increased to greater than 10.9 C by 0800 UTC 27 February. The TD increased to 8.9 C by 0720 UTC, approximately 1 h before the wind speed at Pegasus North increased to speeds greater than 15 m s 1. The TD remained above 8.9 C until 1600 UTC 28 February, nearly 10 h after wind speeds had decreased below 10 m s 1. b. The barrier wind and horizontal pressure gradients As mentioned previously, a barrier wind (Schwerdtfeger 1984) may contribute to the predominately southerly flow over the western portions of the Ross Ice Shelf. A synoptic-scale feature, such as a low pressure disturbance over the Ross Sea, can force cold, stable air against the Transantarctic Mountains. As this air becomes dammed up by the Transantarctic Mountains, a west east pressure gradient is formed and a southerly flow along the western edge of the Ross Ice Shelf results. By examining AWS data from sites located near the Transantarctic Mountains, evidence of this phenomenon and its relationship to high wind speed events at Pegasus North was investigated. Marilyn and Schwerdtfeger lie roughly along 80 S, which also places these units along a line nearly perpendicular to the Transantarctic Mountains (refer to Fig. 4). Marilyn is closest to the mountain chain, located approximately 100 km to the east of the Transantarctic Mountains. The Schwerdtfeger site is 95 km to the east of the Marilyn site. The pressure differences were determined between Marilyn and Schwerdtfeger for the periods prior to the onset of high wind speed events at the Pegasus North site. The pressure at Schwerdtfeger was subtracted from the pressure at Marilyn, and the result was then plotted as a time series. Station pressure was used instead of sea level pressure because it is the trend of this difference that is important, and station pressure is readily available to the forecaster. Of the 109 high wind speed events studied, the Marilyn Schwerdtfeger pressure differences were available for 76 of the cases. The Marilyn Schwerdtfeger pressure difference increased prior to the high wind speeds observed at Pegasus in 61 of those cases. The time between when the Marilyn Schwerdtfeger pressure difference began to increase and the wind speed at Pegasus North increased to greater than 15 m s 1 varied from 0.2 h to as much as 57 h and averaged just over 19 h. The increase of the pressure difference ended between 39.8 h before and 17.7 h after the winds increased at Pegasus, with the average time being about 5.5 h before the wind event. The range within one standard deviation of the mean was 15.0 h before and 4.0 h after the wind event (Fig. 5b). An example of this phenomenon can be found on 17 September During most of the day of 16 September, the Marilyn Schwerdtfeger pressure difference fluctuated between 0.5 and 1.0 hpa, but at 1500 UTC it began to increase (Fig. 8). By 1700 UTC it had increased to 3.0 hpa. The wind speed at Pegasus North began to increase at 1900 UTC, quickly reaching speeds of near 16 m s 1. The pressure difference remained steady at about 3.0 hpa until 1900 UTC, after which it underwent a second increase to 4.0 hpa by 2100 UTC. The pressure difference remained near 4.0 hpa until 0130 UTC 18 September when it then began to decrease, diminishing to near 0.0 hpa by 0600 UTC. The wind speed at Pegasus North remained relatively strong throughout the day of 18 September.

9 APRIL 2000 HOLMES ET AL. 145 FIG. 8. Pegasus North wind speed and Marilyn Schwerdtfeger pressure difference vs time, Sep Pegasus North wind speed is denoted by solid circles. Open squares represent the Marilyn Schwerdtfeger pressure difference. FIG. 9. Pegasus North wind speed and Elaine Marilyn pressure difference vs time, Jun Pegasus North wind speed is denoted by solid circles. Open squares represent the Elaine Marilyn pressure difference. A northward-directed horizontal pressure gradient force would also indicate an environment suitable for observing southerly winds at Pegasus North. Initially, such a pressure regime would result in easterly winds on the Ross Ice Shelf. But as the air encountered the Transantarctic Mountains, a barrier wind scenario would initiate a southerly wind in the region near and adjacent to the Transantarctic Mountains. Elaine and Marilyn lie in nearly a south-southeast to north-northwest line and are roughly parallel to the Transantarctic Mountains, with Elaine positioned 381 km to the south-southeast of Marilyn (refer to Fig. 4). The formation of a northward-directed horizontal pressure gradient force can be observed using these two AWS sites. Again, pressure differences between these two sites were calculated by subtracting the air pressure at Marilyn from that at Elaine. A positive result would indicate that the station pressure at Elaine was higher than that at Marilyn, hence a northward-directed horizontal pressure gradient force. The results of this simple calculation were then plotted as a time series and compared to the wind speed observed at Pegasus North. Of the 109 high wind speed events studied, the Elaine Marilyn pressure differences were available for 66 of the cases. The Elaine Marilyn pressure difference increased prior to the high wind speed in 57 of those cases. The amount of time between when the pressure differences began to increase and when the wind speed increased at Pegasus North also varied widely from case to case. The time between when the Elaine Marilyn pressure difference began to increase and the wind speed at Pegasus North increased to greater than 15 m s 1 varied from 6 h to as much as 57 h and averaged just over 20 h. The increase of the pressure difference ended between 46.5 h before and 9 h after the winds increased at Pegasus, with the average time being about 5.7 h before the wind event. The range within one standard deviation from the mean was 13.7 h before and 2.4 h after the event (Fig. 5c). An example of this relationship can be seen in Fig. 9. At 0550 UTC 12 June 1996, the Elaine Marilyn pressure difference began to increase, rising from 3.1 to 3.2 hpa by 0250 UTC 13 June. The wind speed at Pegasus North began to increase at 0900 UTC 13 June, approximately 27 h after the increase of pressure difference began. The wind speed had increased to 15 m s 1 by 1020 UTC 13 June, 7.5 h after the pressure difference had reached its maximum. The wind speed continued to increase, even though the pressure difference decreased, and reached a maximum of 22.1 m s 1 at 1700 UTC, approximately 14 h after the maximum pressure difference was reached. c. AWS data from selected sites 1) MARILYN SITE As mentioned earlier, a temperature increase was a common phenomenon observed at many of the AWS sites before an increase in the wind speed at Pegasus North. In all but six cases the Marilyn site temperature increased during the 18 h preceding the high wind speeds at Pegasus North. The amount of temperature increase ranged from 3.5 to 16.0 C, and the average increase was 9.7 C. Sometimes this was a gradual increase, of the order of C h 1, and other times the increase was more rapid. The largest rate of temperature increase observed was 5.5 C h 1. The temperature usually remained relatively high throughout the early portion of the event and then began to decrease. A decrease in wind speed at Pegasus North usually began shortly thereafter. Also, there seemed to be no correlation between the magnitude of the rate of temperature increase at Marilyn site and the magnitude of the wind speed at Pegasus North. Six of the cases studied were accompanied by a temperature decrease that occurred soon after the initial increase. There seemed to be no correlation between those with an overall increase in temperature or those with a temperature increase soon followed by a similar decrease and the magnitude of the wind speed at Pegasus North. The range within one standard deviation of the mean between the time when the temperature increase was complete and the wind

10 146 WEATHER AND FORECASTING VOLUME 15 speed increased to 15 m s 1 at Pegasus North was 2.1 to 16.5 h. The frequency distribution and various statistical data are shown in Fig. 5d. The winds at Marilyn site had a tendency to veer, defined in the Southern Hemisphere as a counterclockwise rotation with respect to time, prior to an increase in wind speed at Pegasus North. The wind direction generally veered from a westerly wind to a southerly wind. This veering of the wind direction may be due to a change in forcing mechanisms, from possible katabatic flow down the glaciers of the nearby Transantarctic Mountains, to one of a mesoscale or synoptic-scale disturbance. Also at times, the veering was followed by the temperature increase, most likely due to warm air advection, but at other times the temperature increase began hours before the winds began to veer. During most of the events, this veering occurred rather quickly, averaging 3.5 h, but ranged from 0.5 to 27 h. The amount of veering ranged from 40 to 145, but was generally between 60 and 110, and averaged 84. The average amount of time between when the veering was complete and the wind speed at Pegasus had increased to 15 m s 1 was about 7.1 h, and the range within one standard deviation of the mean was 2.7 to 16.8 h (Fig. 5e). 2) SCHWERDTFEGER SITE Data for 75 of the 109 cases were available from Schwerdtfeger site. A temperature increase was observed at Schwerdtfeger site prior to the increase in wind speeds at Pegasus North in 68 of these cases. Usually, the temperature increase occurred in less than 18 h and was complete before the wind speed at Pegasus North had increased to 15 m s 1. In those cases in which the temperature increase occurred over a longer time period, the increase tended to be complete after the wind speed at Pegasus North had increased to 15 m s 1 or greater. Again, at times this temperature increase may be due to warm air advection associated with a shift to southerly winds, but frequently, the temperature increase occurs long before any change in the wind direction. The magnitude of the temperature increases ranged from 5 to 39 C and began from 52.7 h before and 1.5 h after the wind speed at Pegasus North had increased to 15 m s 1. The average temperature increase was 13.3 C, and the average duration of the increase was 12.5 h. The average period of time between the completion of the increase and when the wind speed at Pegasus North increased to greater than 15 m s 1 was 6.1 h, and the range within one standard deviation of the mean was from 4.0 to 16.1 h (Fig. 5f). The winds at Schwerdtfeger site also had a tendency to veer prior to an increase in wind speed at Pegasus North. Veering winds were observed in 54 of the 62 cases in which data was available. The wind direction generally veered from a westerly wind to a southerly wind, again possibly due to a change in the forcing mechanisms. During most of the events, this veering occurred over a short period of time, averaging 4.2 h, but six of the cases had veering winds that occurred over a period of more than 10 h. The magnitude of the veering ranged from 30 to 180, but was generally between 60 and 120, and averaged 84. The average period of time between the completion of the veering and when the wind speed at Pegasus North increased to greater than 15 m s 1 was nearly 7.5 h, and the range within one standard deviation of the mean was 1.1 to 16.0 h (Fig. 5g). 5. Case study, 19 March 1991 The general synoptic pattern for this case had a low moving slowly southeastward into the Ross Sea and the northeast corner of the Ross Ice Shelf. Sea level pressure on the southern end of the ice shelf began to rise during 18 December 1991 while pressure on the northern end of the ice shelf fell slightly due to the approach of the low. By 19 March the sea level pressure had risen dramatically over the ice shelf especially along the Transantarctic Mountains. This pressure rise and nearly stationary low over the Ross Sea had caused a tight pressure gradient across the ice shelf and especially in the Pegasus runway area (Fig. 10). The weather conditions at Pegasus North were fairly tranquil on 18 March The wind speed was less than 8.0 m s 1 for most of the day, but the wind speed began to slowly increase at 2000 UTC and continued to increase the next day (Fig. 11a). The wind speed reached 15 m s 1 by 0630 UTC 19 March and then increased much more rapidly. The speed increased to 23.4 m s 1 in 1.5 h and reached a maximum of 25.7 m s 1 at 0830 UTC. Wind speeds remained greater than 15.0 m s 1 until 1230 UTC and then decreased rapidly. There was also a marked temperature increase observed at Pegasus North prior to the increase in wind speed (Fig. 11a). The temperature began to rise from 30.4 C at 1110 UTC on 18 March and reached 20.0 C at 2130 UTC that same day. The temperature remained near 20 C throughout the event and then decreased rapidly. A shift in wind direction occurred during the event. The wind direction was between 60 and 120 during much of the day on 18 March (Fig. 11b). The wind began to shift from 50 at 1930 UTC to 150 at 2230 UTC. As the wind speed continued to increase, the wind direction continued to shift and was 195 during the period of the highest wind speeds. As the wind speed began to decrease, the direction then returned to about 60. Temperatures at Marilyn were decreasing throughout the day on 18 March 1991 (Fig. 11c). At 0010 UTC 19 March the temperature increased from 39.1 to 29.4 C in 4 h and then began to decrease. The maximum temperature observed at Marilyn occurred 2.5 h before the wind speed at Pegasus North increased to 15

11 APRIL 2000 HOLMES ET AL. 147 FIG. 10. ECMWF surface analyses for (a) 0000 UTC 18 Mar 1991, (b) 1200 UTC 18 Mar 1991, (c) 0000 UTC 19 Mar 1991, and (d) 1200 UTC 19 Mar Light solid lines are isobars (mb) plotted at an interval of 4 mb. The black dot denotes the location of the Pegasus runway. ms 1, and nearly 4 h before the maximum wind speed was observed at Pegasus North. The change in wind direction exhibited the veering observed during many of these events. At 1600 UTC 18 March, the wind began to veer from approximately 280 to 180 in 5 h (Fig. 11d). The wind direction remained between 180 and 220 throughout most of the event, and then shifted abruptly to 325 shortly after the wind speed at Pegasus North decreased. The temperature at Schwerdtfeger increased from 41.9 to 27.0 C in 14 h, beginning at 1310 UTC 18 March (Fig. 11e). The temperature remained near 29.0 C for 4 h and then began to decrease. The temperature increase occurred as the wind direction began to veer from north-northwest (330 ) to south (180 ) in about 6 h, beginning at 1300 UTC 18 March (Fig. 11f). The wind direction remained southerly throughout the event and then backed to north-northwesterly afterward. The Marilyn Schwerdtfeger pressure difference began to increase from 3.5 hpa at 1920 UTC 18 March to 1.6 hpa at 0650 UTC 19 March (Fig. 11g). The pressure difference then decreased to less than 4.0 hpa by 1600 UTC 19 March. The maximum wind speed observed at Pegasus North occurred nearly 2 h after the

12 148 WEATHER AND FORECASTING VOLUME 15 FIG. 11. Time series of AWS data for 18 and 19 Mar Pegasus North wind speed is denoted by solid circles on all graphs. Open squares represent (a) Pegasus North temperature, (b) Pegasus North wind direction, (c) Marilyn temperature, (d) Marilyn wind direction, (e) Schwerdtfeger temperature, (f) Schwerdtfeger wind direction, (g) Marilyn Schwerdtfeger pressure difference, and (h) Pegasus North Minna Bluff temperature difference.

13 APRIL 2000 HOLMES ET AL. 149 Marilyn Schwerdtfeger pressure difference reached its maximum. Unfortunately, the data necessary for determining the Elaine Marilyn pressure difference were unavailable for this case. The temperature difference, as calculated from Pegasus North and Minna Bluff temperatures, also increased during this event. Between 0530 and 2240 UTC 18 March, the TD increased from 3.9 to 11.2 C (Fig. 11h). The TD remained between 10.0 and 10.9 C for the next 6 h before exhibiting some fluctuation and then increasing further to 16.2 C at 0900 UTC 19 March. Soon after, the TD decreased to less than 0.0 C by 1600 UTC 19 March. In this particular case, the Pegasus North Minna Bluff TD began to increase nearly a day before the wind speed at Pegasus North reached the threshold of 15 m s 1 and increased to statically unstable levels at approximately the same time as the Pegasus North wind speed began to increase. The wind direction at both Schwerdtfeger and Marilyn had veered from 330 to 180 in approximately 12 and 9.5 h, respectively, before the wind speed at Pegasus North reached threshold levels. The temperature at Schwerdtfeger began to increase approximately 7 h before the wind speed at Pegasus North, while the temperature at Marilyn increased coincidentally with the wind speed at Pegasus North. 6. Success rates We have shown that on a large majority of occasions, certain changes occur in various meteorological parameters obtained from AWS data before the wind speed at Pegasus North AWS site increases to 15 m s 1. For this information to be useful as a forecasting aid, the reverse must also be evident. We must answer the question, How often do these changes occur and no increase in the Pegasus North wind speed follows? To answer this question, the data were scanned for changes in the parameters discussed previously. Specifically, changes were considered that occurred in 18 h or less and were as follows: a TD increase from near 0.0 C to at least 8.9 C; an increase of the Marilyn Schwerdtfeger pressure difference of at least 2.5 hpa, and an increase of the Elaine Marilyn pressure difference of at least 6.0 hpa; a shift in the wind direction at both Marilyn and Schwerdtfeger sites from a westerly direction to a southerly direction; and an increase in the temperature at both Marilyn and Schwerdtfeger sites of at least 6 C. Results were tabulated for two scenarios. It was first determined how often a change in a parameter was observed and an increase in the Pegasus North wind speed to 15 m s 1 followed within a 24-h period. It was also determined how often a significant increase in the wind speed at Pegasus North occurred within the 24-h period. A significant increase in the wind speed at Pegasus North was defined as at least a doubling of the wind speed that ultimately resulted in a wind speed of at least 6 m s 1. For each parameter, a success rate was determined FIG. 12. Success rates of AWS parameters for both a significant wind speed increase and a wind speed increase in excess of 15 m s 1 at Pegasus North AWS site. as a percentage of the wind speed increases that occurred after the defined parameter was met. The results obtained were categorized in both a yearround tabulation as well as a seasonal grouping, and both varied among the individual parameters. The TD underwent the defined increase 299 times during the 6-yr time period, and an increase in the Pegasus North wind speed to levels above 15 m s 1 occurred after 81 of the TD increases, resulting in a success rate of 27.1%. Also, the wind speed at Pegasus North underwent a significant increase in 244 of those cases, a success rate of 81.6%. Year-round success rates for each parameter are shown in Fig. 12. Other parameters had success rates near or slightly below the rates calculated for the TD, but the temperature increase at both Schwerdtfeger and Marilyn AWS sites had success rates much lower for both the Pegasus North wind speed increase to 15 m s 1 and the significant wind speed cases. Many of the temperature increases at both Marilyn and Schwerdtfeger are most likely attributed to the diurnal cycle and therefore are not indicative of a meteorological event that could cause a substantial increase in the surface wind speed at Pegasus North. The largest seasonal variation in the success rate occurred with the TD (Fig. 13). The success rate for the TD in the summer months (December, January, and February) of 61.5% was significantly less than the rate of 96.3% in the winter months (June, July, and August). Conversely, the variations in the success rate were much smaller for both the two wind direction parameters and the pressure difference parameters. Also, the success rates for both the Marilyn and Schwerdtfeger temperature parameters are relatively smaller than the other parameters in each season. 7. Conclusions The primary purpose for the deployment of the AWS units near Ross Island and on the Ross Ice Shelf was

14 150 WEATHER AND FORECASTING VOLUME 15 FIG. 13. Seasonal success rates of AWS parameters for a significant wind speed increase at Pegasus North AWS site. to support operational forecasting. The purpose of this study was to derive empirical rules based on automatic weather station data for forecasting high wind speed events at Pegasus Runway, Antarctica. The results of the work presented here give several guidelines to improve both the accuracy and lead time of high wind speed and, hence, visibility forecasts. While these guidelines lack robustness, they do demonstrate the merit of the use of AWS data for this type of forecast. The 109 cases examined and the case study discussed have shown that the data from AWS sites can be used for specific forecasting problems. A substantial increase in temperature was observed at most of the AWS sites before a high wind speed event at Pegasus North. The temperature increases were usually greatest at Schwerdtfeger, Marilyn, and Pegasus North, and those at Schwerdtfeger and Marilyn sites were generally complete at least 3 h before the wind speed increased at the Pegasus North site. Additionally, a shift in the wind direction at both Marilyn and Schwerdtfeger sites can lead to a forecast for high wind speeds at Pegasus North. The wind directions at these two AWS sites had a tendency to veer from a westerly direction to a southerly direction. This veering usually occurred 3 6 h before wind speeds at Pegasus North increased. The station pressure differences computed between the stations near the Transantarctic Mountains also provided data that could lead to a forecast of high wind speeds at Pegasus North. The Marilyn Schwerdtfeger and Elaine Marilyn pressure differences proved useful predictors of high wind speeds at Pegasus. The temperature difference, as calculated between Minna Bluff and Pegasus North sites, also proved a useful indicator. The time series of TD versus wind speed and wind direction revealed that when the TD was unstable (greater than 8.9 C), the winds at Pegasus North tended to blow from the south, and that wind speeds at Pegasus North greater than 10 m s 1 generally occurred only when the TD was unstable. It was shown that an increase in the TD precedes a high wind speed event at Pegasus North by several hours. This was evident in nearly every case examined and is probably the most reliable predictor obtained in this study. Several empirical guidelines for forecasting high wind speeds at Pegasus were developed from this study. No single guideline is adequate to forecast a high wind speed event at Pegasus North. Both the magnitudes of the individual parameters as well as the duration of the changes of those parameters can vary greatly from one event to another. Many times these changes occur abruptly, while at other times, the change may occur over a period of 24 h or more, with the majority of these changes occurring in less than 18 h. By monitoring the trends of the parameters outlined in this study and using them in conjunction with the other data available at McMurdo Station, better forecasts of these events can be made. The following will generally precede the onset of high wind speeds at Pegasus by up to 6hifthey occur over a period of not more than 18 h: 1) an increase in the TD (estimated from the temperatures at Minna Bluff and Pegasus North) from inversion conditions (less than 0.0 C) to statically unstable conditions (greater than 8.9 C); 2) pressure differences (a) an increase in the Marilyn Schwerdtfeger pressure difference, usually on the order of 2.5 hpa or greater, and (b) an increase in the Elaine Marilyn pressure difference on the order of 6.0 hpa or greater; and 3) a veering of the wind direction with time, from west or southwesterly to southerly at the Marilyn and/or Schwerdtfeger sites. Although the use of the Marilyn and Schwerdtfeger temperature increases may have some merit, the relatively poor success rate excludes these two parameters from inclusion as forecasting aids. These guidelines, used in conjunction with radiosonde data, synoptic charts, satellite data, and the forecaster s knowledge of the local climatology, should improve both the lead time and accuracy of high wind speed forecasts at Pegasus North. Acknowledgments. This research was funded by the National Science Foundation s Office of Polar Programs under Grant REFERENCES Blaisdell, G. L., V. D. Klokov, and D. Diemand, 1995: Compacted snow runway technology on the Ross Ice Shelf near McMurdo, Antarctica. Contributions to Antarctic Research IV, D. H. Elliot and G. L. Blaisdell, Eds., Antarctic Research Series, Vol. 67, Amer. Geophys. Union, Bromwich, D. H., 1988: Snowfall in high southern latitudes. Rev. Geophys., 26, , J. F. Carrasco, and C. R. Stearns, 1992: Satellite observations

15 APRIL 2000 HOLMES ET AL. 151 of katabatic wind propagation for great distances across the Ross Ice Shelf. Mon. Wea. Rev., 120, Fleming, D. A., 1983: Antarctic automatic weather stations as forecasting aids. Antarct. J. U.S., 18 (5), Holmes, R. E., 1994: An investigation into the use of automatic weather station data for the forecasting of high wind speed events at Pegasus Runway, Antarctica. M.S. thesis, Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison, 111 pp. [Available from Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison, 1225 W. Dayton St., Madison, WI ], and C. R. Stearns, 1995: Antarctic automatic weather stations: Austral summer Antarct. J. U.S., 30 (5), , G. A. Weidner, and C. R. Stearns, 1993: Antarctic weather forecasting: Antarct. J. U.S., 28 (5), 337. Keller, L. M., G. A. Weidner, C. R. Stearns, M. T. Whittaker, and R. E. Holmes, 1996: Antarctic automatic weather station data for the calendar year Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison, 465 pp. [Available from L. M. Keller, Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison, 1225 West Dayton St., Madison, WI ],,,, and, 1997: Antarctic automatic weather station data for the calendar year Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison, 539 pp. [Available from L. M. Keller, Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison, 1225 West Dayton St., Madison, WI ] Savage, M. L., and C. R. Stearns, 1985: Climate in the vicinity of Ross Island, Antarctica. Antarct. J. U.S., 20 (1), 1 9. Schwerdtfeger, W., 1984: Weather and Climate of Antarctica. Elsevier, 261 pp. Sinclair, M. R., 1988: Local topographic influence on low-level wind at Scott Base, Antarctica. N. Z. J. Geol. Geophys., 31, Sinkula, B. B., 1993: Application of AWS data for Antarctic operational forecasting and climate research. M.S. thesis, Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison, 117 pp. [Available from Department of Atmospheric and Oceanic Sciences, University of Wisconsin, 1225 West Dayton St., Madison, WI ] Stearns, C. R., and G. A. Weidner, 1990: Wind speed events and wind direction at Pegasus site during Antarct. J. U.S., 25 (5), , and, 1991: Wind speed, wind direction, and air temperature at Pegasus North during Antarct. J. U.S., 26 (5), , and, 1992: Wind speed, wind direction, and air temperature at Pegasus North during Antarct. J. U.S., 27 (5), , and B. B. Sinkula, 1994: Antarctic weather forecasting: Antarct. J. U.S., 29 (5), 284., L. M. Keller, G. A. Weidner, and M. Sievers, 1993: Monthly mean climatic data for Antarctic automatic weather stations. Antarctic Meteorology and Climatology: Studies Based on Automatic Weather Stations, D. H. Bromwich and C. R. Stearns, Eds., Antarctic Research Series, Vol. 61, Amer. Geophys. Union, 1 22., G. A. Weidner, and R. E. Holmes, 1994: Antarctic automatic weather stations: Austral summer Antarct. J. U.S., 29 (5),