DAVID A. BRAATEN, Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045
Snow accumulation in a windswept area on the Ross Ice Shelf was investigated between November 1995 and November 1996 at a site adjacent to Ferrell automatic weather station (AWS) (78.02°S 170.80°E). The AWS provided this project with measured meteorological parameters such as wind speed, wind direction, temperature, and relative humidity (Holmes, Stearns, and Weidner 1993) at 10-minute intervals. The Ferrell AWS site has a wind regime that is frequently influenced by strong katabatic outflow winds from the Trans-antarctic Mountains. These winds make the site very suitable to study the role of winds on snow accumulation. More than 44 percent of the wind-speed observations exceed 5 meters per second (m s-1), which is approximately the threshold wind speed for snow transport by wind. This site also has a dominant wind-direction corridor from 210° and has little directional variability for wind speeds greater than 5 m s-1 as shown by the wind rose in figure 1.
Snow-surface height was measured with a time resolution of 1 hour using a SR50 acoustic-ranging sensor, and the data were stored by a CR10 datalogger on a SM192 data-storage module (Campbell Scientific, Inc.). The data were recovered from the field at the end of the study period. The SR50 has a field of view of approximately 22° and measures the distance to the closest object within this field of view (e.g., the top of a snow-surface feature) with an accuracy of ±10 millimeters.The sensor was initially positioned approximately 1.4 meters (m) above the snow surface, and snow accumulation was calculated as the original distance of the SR50 sensor to the surface minus subsequent measured distances. The data-recovery percentage during the deployment period was more than 99 percent.
After quality-control checks of the data, a daily mean snow accumulation was calculated, and these data for the 12-month deployment are shown in figure 2. The acoustic snow-depth gauge shows four main snow-surface height increases ranging from about 5 centimeters (cm) to more than 20 cm, all occurring before 1 June 1996. These positive snow-surface height changes may be associated with precipitation, wind-blown transport of snow into the target area, and/or the formation of a surface feature on the target area. Because the winds during these periods are generally greater than the threshold speed for wind-blown snow (approximately 5-6 m s -1 ), it is not known if the snow accumulation is due to precipitation, wind-blown snow, the formation of a surface feature, or some combination of two or more of these processes. The acoustic snow-depth gauge also shows two large decreases (-8 and -15 cm) in snow-surface height occurring over several weeks after a large snow-surface height increase. Negative changes to surface height could have been caused by wind erosion of surface snow grains, sublimation at the snow surface, and/or metamorphic changes and densification of the firn.
During the 1996-1997 antarctic field season, Ferrell AWS was visited to conduct pit sampling and to retrieve the SR50 acoustic-ranging sensor data. The two field team members were D. Braaten and S. Seunarine. Three snow pits were sampled at 6, 12, and 16 m from the SR50 acoustic-ranging sensor in the prevailing downwind direction. The primary goal of the snow-pit sampling was the recovery of glass microspheres dispersed throughout the study period by the microsphere dispersal system (MDS) (Braaten 1994, 1995), but this sampling also provided a detailed snow-density profile and allowed the position of visual statigraphy observed along the snow-pit wall to be measured. The MDS was collocated with the SR50 acoustic-ranging sensor. Snow density was measured with a 1-cm resolution using disposable cuvette sampling tubes. Cuvette tubes were pushed into the snow-pit wall to obtain the snow sample, and snow density was determined by measuring the snow volume and water volume in the cuvette after melting. Results from the three snow pits are shown in figure 3. The snow-density profiles all have similar features, but these features can vary in depth by several centimeters. In each of the snow pits, six thin crust or glaze layers were visually identified, but it is not clear if these layers all correspond to the same time period. From the locations of microspheres recovered from the snow pits and SR50 acoustic-ranging sensor data, research is ongoing to date to explain the crust layers and to characterize the variability of snow accumulation on this temporal and spatial scale.
Other tasks performed during the 1996-1997 antarctic field season were removal of the MDS and SR50 acoustic-ranging sensor from Ferrell AWS, installation of this equipment adjacent to Marilyn AWS (79.954°S 165.130°E), and servicing of the MDS and SR50 acoustic-ranging sensor system at AGO-2 (85.67°S 46.38°W) on the polar plateau during the annual automatic geophysical observatory service visit.
This research was supported by National Science Foundation grant OPP 94-17255.
Braaten, D.A. 1994. Instrumentation to quantify snow accumulation and transport dynamics at two locations on the Ross Ice Shelf. Antarctic Journal of the U.S. , 29(5), 86-87.
Braaten, D.A. 1995. Assessment of snow accumulation and transport dynamics using glass microspheres. Antarctic Journal of the U.S. , 30(5), 331-332.
Holmes, R.E., C.R. Stearns, and G.A. Weidner. 1993. Antarctic automatic weather stations: Austral summer 1992-1993. Antarctic Journal of the U.S. , 28(5), 296-299.