Streamlines of the mean annual near‐surface winds over the Antarctic continent suggest a confluent channeling of the drainage flows off the ice sheets and onto the Ross Ice Shelf. A persistent cyclonic circulation to the north of the ice shelf supports a large‐scale pressure field that reinforces the continental drainage flows. Owing to these two processes, an enhanced low‐level airflow is present along the southern and western sections of the Ross Ice Shelf. The resulting air stream, known as the Ross Ice Shelf air stream (RAS), is one of the persistent and prominent low‐level wind features seen in the Antarctic. Real‐time mesoscale simulations of the Antarctic atmosphere and high southern latitudes using a modified version of the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Modeling System have been ongoing since the 1999–2000 austral field season. Model results from the 1‐year period November 2001 to October 2002 have been analyzed to investigate the mean structure and modulation of the Ross Ice Shelf air stream. Analyses of model results show the low‐level air stream over the western Ross Ice Shelf has a wind speed maxima that is linked to the steep topography to the west. Individual cases of strong wind events appear to contain a significant barrier wind component that arises from cold air damming against the Transantarctic Mountains. Cyclones that frequently form in the Ross Sea are shown to establish conditions that promote barrier wind dynamics and thus significantly modulate the intensity of the RAS.
This paper demonstrates how self-organizing maps (SOMs) can be used to evaluate the large-scale environment, in particular the synoptic circulation associated with widespread temperature extremes. The paper provides details on how SOMs are created, how they can be used to understand extreme events, and lessons learned in applying this methodology for extremes analysis. Using a SOM can be helpful in understanding the underlying physical processes that control extreme events, and how the extremes and the processes that control them may change in time or differ across space. Examples of widespread daily temperature extremes in 4 regions: 2 each in Alaska and in northern Canada during winter (December, January, and February) for 1989−2007 are presented to illustrate the application of the methodology. For the regions studied, the size of the domain over which the synoptic circulation was defined -in particular using a smaller domain focused on particular regions -and a greater number of classes to represent the archetypical synoptic patterns for the regions, give the best relationship between synoptic circulation and extremes. The results are most robust for the Alaskan domains and less so for the Canadian domains, leading to the conclusion that further study is warranted to better understand extremes in the Canadian regions.
The performance of the Weather Research and Forecasting (WRF) model was evaluated for month-long simulations over a large pan-Arctic model domain. The evaluation of seven different WRF (version 3.1) configurations for four months (January, April, July, and October 2007) indicated that WRF produces reasonable simulations of the Arctic atmosphere. Ranking of the model error statistics, calculated relative to the NCEP/Department of Energy Global Reanalysis 2 (NCEP-2), for sea level pressure, 500- and 300-hPa geopotential height, 2-m air temperature, and precipitation identified the model configurations that consistently produced the best pan-Arctic simulations. For all WRF configurations considered, large errors in circulation are evident in the North Pacific. The errors in the North Pacific are manifested as an overly weak and westward-shifted Aleutian low and overly strong subtropical Pacific high simulated by WRF. These circulation errors are nearly barotropic, with a slight increase in magnitude with height, and they vary slightly in magnitude and position as the WRF physics options and domain size are changed. It is concluded that the circulation errors are likely due to errors in the treatment of the model-top boundary. The use of a higher model top (10 hPa rather than 50 hPa) or spectral nudging of wavenumbers 1–3 in the top half of the model domain results in significantly reduced circulation biases. Simulations with WRF version 3.2 also show reduced errors.
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