George A. Weidner

Antarctic Automatic Weather Station Program: 30 Years of Polar Observation

Antarctica boasts one of the worlds harshest environments. Since the earliest expeditions, a major challenge has been to characterize the surface meteorology around the continent. In 1980, the University of Wisconsin—Madison (UW-Madison) took over the U.S. Antarctic Program (USAP) Automatic Weather Station (AWS) program. Since then, the UW-Madison AWS network has aided in the understanding of unique Antarctic weather and climate. This paper summarizes the development of the UW-Madison AWS network, issues related to instrumentation and data quality, and some of the ways these observations have and continue to benefit scientific investigations and operational meteorology.

The Influence of Blowing Snow and Precipitation on Snow Depth Change across the Ross Ice Shelf and Ross Sea Regions of Antarctica

Measuring snowfall in the polar regions is an issue met with many complications. Across the Antarctic, ground-based precipitation measurements are only available from a sparse network of manned stations or field studies. Measurements from satellites promise to fill in gaps in time and space but are still in the early stages of development and require surface measurements for proper validation. Currently, measurements of accumulation from automated reporting stations are the only available means of tracking snow depth change over a broad area of the continent. The challenge remains in determining the cause of depth change by partitioning the impacts of blowing snow and precipitation. While a methodology for separating these two factors has yet to be developed, by comparing accumulation measurements with meteorological measurements, an assessment of whether these terms were a factor in snow depth change during an event can be made. This paper describes a field study undertaken between January 2005 and October 2006 designed to identify the influences of precipitation and horizontal snow transport on surface accumulation. Seven acoustic depth gauges were deployed at automatic weather stations (AWS) across the Ross Ice Shelf and Ross Sea regions of Antarctica to measure net accumulation changes. From these measurements, episodic events were identified and were compared with data from the AWS to determine the primary cause of depth change—precipitation or horizontal snow transport. Information regarding the local impacts of these two terms, as well as climatological information regarding snow depth change across this region, is also provided.

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

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 differen...

Atmospheric circulation around the Greenland Crest

The Greenland Ice Sheet Program 2 (GISP2) required meteorological support for the layout of the field camp, ice core site, landing strip, snow sampling sites, and air sampling sites. An automatic weather station installed in May 1987 provided the initial data for the support of several components of the program using the 2 year wind speed and direction distributions. The automatic weather station (AWS) network was expanded to six sites with four sites located approximately 100 km from the highest point in Greenland in the cardinal directions and sites located at GISP2 and at the air sampling site. The wind direction and speed wind roses for August and September 1994 show that there is considerable variation in the wind field around the Greenland Crest. The constancy (ratio of the vector wind speed to the scalar wind speed) of the monthly wind vectors was lowest at the southern site and increased as one moved clockwise around the crest to the eastern site. Vorticity and divergence were determined from triangles formed by three AWS sites. The mean horizontal divergence for August and September 1994 was 1.4 x 10 -5 s -1 , and the vorticity was -3.7 x 10 -5 s -1 . Apparently, the flow around the Greenland Crest is most influenced by topography when the 500 hPa contours indicate a pressure ridge over central Greenland. The surface winds showed anticyclonic flow, positive horizontal divergence, and negative vertical vorticity. In the mean, this configuration indicates the presence of an inversion wind, especially at the sites located on the slopes to the west and east of the crest.

Antarctic Satellite Meteorology: Applications for Weather Forecasting

For over 30 years, weather forecasting for the Antarctic continent and adjacent Southern Ocean has relied on weather satellites. Significant advancements in forecasting skill have come via the weather satellite. The advent of the high-resolution picture transmission (HRPT) system in the 1980s and 1990s allowed real-time weather forecasting to become a reality. Small-scale features such as mesocyclones and polar lows could be tracked and larger-scale features such as katabatic winds could be detected using the infrared channel. Currently, HRPT is received at most of the manned Antarctic stations. In the late 1990s the University of Wisconsin composites, which combined all available polar and geostationary satellite imagery, allowed a near-real-time hemispheric view of the Southern Ocean and Antarctic continent. The newest generation of satellites carries improved vertical sounders, special sensors for microwave imaging, and the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. In spite of the advances in sensors, shortcomings still impede the forecaster. Gaps in satellite data coverage hinder operations at certain times of the day. The development and implementation of software to derive products and visualize information quickly has lagged. The lack of high-performance communications links at many of the manned stations reduces the amount of information that is available to the forecasters. Future applications of weather satellite data for Antarctic forecasting include better retrievals of temperature and moisture and more derived products for fog, cloud detection, and cloud drift winds. Upgrades in technology at Antarctic stations would allow regional numerical prediction models to be run on station and use all the current and future satellite data that may be available.

Topographic influence on surface winds

Using data collected during 1975–1976 from a meteorological network operating in the vicinity of the Columbia Generating Site approximately 8 km south of Portage, Wis., the influence of the Baraboo Hills on the surface wind field is determined. Half-hour means of wind speed and direction measured at 9 m at three sites were used to compute divergence and vorticity using Bellamys method. The data were grouped into 18 sectors each 20 deg wide and averages computed for each quantity. Results indicate that for wind directions perpendicular to the eastern edge of the Baraboo Hills, the surface (9m) wind field is significantly perturbed up to 4 km from the bluffs. The largest convergence of 2.1 × 10−4 s−1 occurs with 160 deg wind direction and the largest divergence of 1.2 × 10−4 s−1 with 290 deg wind direction. The maximum anticyclonic vorticity was 1.6 × 10−4 s−1 at 210 deg and the maximum cyclonic vorticity was 1.6 × 10−4 s−1 at 330 deg.

A Diagnostic Study of the Influence of a Gravity Wave upon Regional Weather

Abstract On 20 May 1977 a periodic pressure disturbance over the upper Midwest was detected by synoptic and mesoscale microbarograph arrays. Analysis indicates that a pressure wave with an amplitude of approximately 1.0 mb, a phase speed of approximately 14 m s−1, a period of between 1.5 and 3 h, and a horizontal wavelength of approximately 100 km moved through Illinois, Iowa and Wisconsin from the south within the warm sector of an occluded cyclone. We interpret them pressure data as the signature of an internal atmospheric gravity wave. At several locations convective precipitation occurred coincident with the advancing ridges of the pressure wave, suggesting that the gravity wave influence convection. Openings in the cloud cover correlated with the location of the gravity wave. Simple modeling confirms the destabilization potential of the gravity wave.

Alexander Tall Tower! A Study of the Boundary Layer on the Ross Ice Shelf, Antarctica

AbstractBecause of the harsh weather conditions on the Antarctic continent, year-round observations of the low-level boundary layer must be obtained via automated data acquisition systems. Alexander Tall Tower! is an automatic weather station on the Ross Ice Shelf in Antarctica and has been operational since February 2011. At 30 m tall, this station has six levels of instruments to collect environmental data, including temperature, wind speed and direction, relative humidity, and pressure. Data are collected at 30-, 15-, 7.5-, 4-, 2-, and 1-m levels above the snow surface. This study identifies short-term trends and provides an improved description of the lowest portion of the boundary layer over this portion of the Ross Ice Shelf for the February 2011–January 2014 period. Observations indicate two separate initiations of the winter season occur annually, caused by synoptic-scale anomalies. Sensible and latent heat flux estimates are computed using Monin–Obukhov similarity theory and vertical profiles of ...