Observing and modeling the Antarctic planetary boundary layer
Abstract
The Alexander Tall Tower (ATT) is a 30-m high instrumented automatic weather station remotely sited about 150 km south of McMurdo Station on the flat Ross Ice Shelf. Two years of observed wind, temperature, and relative humidity profiles were compared with short-term forecasts from the Antarctic Mesoscale Prediction System (AMPS) that employs the polar version of the Weather Research and Forecasting (WRF) model. From this comparison, three issues with the planetary boundary layer (PBL) predictions of AMPS were found: (1) a dry bias that amplified with height and was larger when strong winds speeds blew from the direction of Byrd Glacier; (2) a positive wind speed bias, particularly during the colder months (March-August); and (3) a misrepresentation of the weaker and stronger surface inversions during winter. Subsequently, a two-week field campaign was conducted at the ATT site during summer using Small Unmanned Meteorological Observer (SUMO) aerial systems to collect data throughout the PBL above the 30-m high ATT. The comparison between SUMO observations and AMPS forecasts largely corroborated the results from the year-round study of the ATT results and suggested that the dry model bias is large through the depth of the PBL throughout the year above the height of the ATT. A large dry bias at these critical heights for aircraft operations also implies poor AMPS low cloud forecasts, a known long-standing issue in the model. What is the cause of the large dry bias in AMPS? It is important to emphasize that the katabatic winds that cross ATT come from a very large part of the East Antarctic ice sheet, and therefore the dry bias is a widespread problem. One possible explanation for the dry bias is that AMPS does not simulate blowing snow that sublimates into the PBL and moistens it. The impact of this effect is enhanced for the ATT location because blowing snow and blowing snow sublimation are a maximum over the plateau upwind of the ATT site according to satellite lidar observations. Another cause of the dry bias may be related to the excessively strong near-surface katabatic winds in AMPS, which results in too much subsidence into the PBL and enhanced drying. Overall, the dry bias in AMPS is symptomatic of problems with the simulation of atmospheric moisture over Antarctica found in many atmospheric models.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.A53L..02B
- Keywords:
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- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3359 Radiative processes;
- ATMOSPHERIC PROCESSES;
- 0750 Sea ice;
- CRYOSPHERE;
- 0758 Remote sensing;
- CRYOSPHERE