Antarctic Meteorology, a study with Automatic Weather Stations

This thesis chiefly addresses a) the use of Automatic Weather Stations (AWS) in determining the near-surface climate and heat budget of Antarctica and, specifically, Dronning Maud Land (DML), and b) the determination of source regions of Antarctic moisture with the aid of a trajectory model and an atmospheric model. The primary motivation behind this interest is the drilling of two ice cores in the Antarctic ice sheet within the framework of the European Project for Ice Coring in Antarctica (EPICA). A thorough knowledge of the meteorological conditions will increase our understanding of the processes that influence the surface mass balance and heat budget.

In Chapter 1, ground-based observations of broadband, narrowband, and bidirectional reflectance are used to study the albedo of blue ice and snow. During summer, surface albedo plays an important role in the amount of heat exchanged between the surface and the atmosphere. The main objective of the study is to improve the methods used to derive surface albedo from satellite measurements and arrive at a better understanding of the processes influencing the magnitude of the albedo.

Chapters 2, 3 and 4 describe the data obtained from ten AWS in Antarctica and how they were used to determine the local surface energy budget. The AWS were placed on two transects perpendicular to the coastline in DML and one on Berkner Island. As expected, mainly the strength of the katabatically forced flow, in combination with the geostrophic flow, determines the near-surface conditions at these locations. The katabatic flow varies in strength depending on the magnitude of surface slope and temperature inversion, and is not active on Berkner Island, a station on a topographic dome. In DML, the strength of the katabatic flow varies, resulting in maximum wind speeds and potential temperatures at the sites with the steepest slopes, at the edge of the Antarctic plateau. The annual mean wind speed varies between 4 ms-1, near the coast and on the plateau, to 7 ms-1, on the edge of the plateau. The annual mean potential temperature varies between -18°C and -1°C. The annual mean temperature ranges from -16°C in the coastal area, where occasional melt occurs, to -46°C on the Antarctic plateau. Owing to the low temperatures, the specific humidity is very low. Accumulation is highest in the coastal regions and decreases with increasing elevation and distance from the coast, from ~400 mm water equivalent per year (w.e. yr-1) near the coast to ~30 mmw.e.yr-1 on the plateau.

The AWS data, together with a model based on Monin-Obukhov similarity theory, are used to calculate the surface energy budget for the measuring period. The strength of the katabatic flow largely determines not only the near-surface meteorological conditions but also the surface energy budget. The annual average energy gain at the surface from the downward sensible heat flux varies between ~3 Wm-2 to 25 Wm-2, with the highest values at the sites with the highest wind speeds and potential temperatures. The negative net radiative flux largely balances the sensible heat flux and ranges from ~2 Wm-2 to 28 Wm-2. The average latent heat flux is generally small and negative (~-1 Wm-2), indicating mass loss through sublimation. The annual subsurface fluxes are small (~-0.2 Wm-2).

In Chapters 4 and 5, moisture sources of snow falling at five deep-drilling locations in Antarctica (Byrd, DML05, Dome C, Dome F and Vostok) are defined, based on five-day backward air parcel trajectories calculated from data of the European Centre for Medium Range Weather Forecasts. Based on model precipitation, a distinction is made between cases with and without snowfall at the point of arrival. Of the snowfall trajectories ending at DML05 in 1998, 40 - 80% originate in the Atlantic Ocean between 40 and 60°S, within four days before arrival. Evaporation along these trajectories is largest during the first half. A case study for May 1998 shows that during snowfall exceptionally high temperatures and wind speeds prevail in the atmospheric boundary layer. The trajectories from the ECMWF Re-analysis Project (ERA-15) cover a 15-year period and show that the oceans closest to the five drilling sites contribute most of the moisture. The latitude band contributing most (~30%) of the total annual precipitation is at 50 - 60°S, the area just north of the sea ice edge. The calculated trajectories show seasonal dependency, resulting in a seasonal cycle in the moisture sources, which is further enhanced by a seasonal cycle in the amount of precipitation.