UPDATED INFORMATION ON*

ANTARCTIC NWP – AUSTRALIAN EFFORTS:

focussing on the use of ensemble techniques

for

The International Antarctic Weather Forecasting Handbook:

IPY 2007-08 Supplement

Neil Adams

Australian Bureau of Meteorology,

GPO Box 727 Hobart TAS 7001

E-mail: N.Adams@bom.gov.au

Submitted April 2008

Since the last edition of the IAWFH significant advancements have been made in Numerical Weather Prediction (NWP) with global models increasing in resolution and accuracy and new and improved regional models being employed to assist in Antarctic weather forecasting. The Australian Bureau of Meteorology, through the Tasmania and Antarctica Region has implemented a polar stereographic version of the Antarctic Limited Area Prediction System (ALAPS). The old ALAPS only covered East Antarctica and the surrounding southern Ocean and suffered from issues of converging meridians on the southern boundary at 80^oS. The new polarLAPS system covers the entire continent of Antarctica and much of the surrounding ocean. It employs the same physics as the older ALAPS and the same horizontal resolution of 27.5 km, but with a more regular grid. Figure 1 (NWP ‑ Australia) shows the domain employed by polarLAPS. The model has no assimilation system so essentially downscales from the initial conditions information, which are provided by the NCEP-GFS model. Two instances of the model are run twice a day at 0000 and 1200 UTC. The first run is a simple downscaling run where the NCEP-GFS data are interpolated onto the polarLAPS grid. The second run calculates analysis increments using the latest NCEP-GFS analysis and the +12 hour forecast from the previous NCEP-GFS run, coincident with the current analysis. These increments are then added to the +12 hour forecast from the previous polarLAPS run to generate a starting analysis for a new polarLAPS run. This second polarLAPS run on occasions provides some improvements to the model run, but more importantly provides a second model member for the single station Antarctic Ensemble System (AES). The AES is a poor-man's ensemble system, employing model output from three global models, the two polarLAPS models and the two models currently run within the United States Antarctic Meso-scale Prediction System (AMPS). The original AMPS system employs the MM5 code from Pennsylvania State University, with polar modifications added from the AMPS team. The new AMPS system employs the Weather Research and Forecasting (WRF) model, with similar polar modifications employed. At present both the MM5 and WRF versions of AMPS continue to run with both being employed within the AES. Figure 2 (NWP ‑ Australia) shows a site map of all locations around Antarctica where single station ensembles are generated. To increase the number of ensemble members both the current and previous model runs are used in generating each time series plot. Figure 3 (NWP ‑ Australia) shows an example ensemble forecast for Mawson station. Each plot includes wind direction, wind speed, Surface Level Pressure (SLP), temperature, percentage cloud cover and precipitation. Each model is plotted in a different colour with the previous model run plotted using a faded colour and thinner line. Where real-time observations are available for a site they are plotted on the ensemble in black, with wind gust information plotted in grey, and the charts updated routinely to make available the latest observations. These observations are also used to bias correct the surface level pressure and temperature forecasts. The AES charts have proved extremely useful for single station forecasts and are made available to international Antarctic forecasters through the Australian Bureau of Meteorology's external web site at:

   http://www.bom.gov.au/bmrc/mdev/expt/Antarctic/Antarctica.shtml

Australian forecasters also have access to a system where by ensemble forecasts can be generated, on the fly, for any site within the group model domain.