First Results From SOP-1

Antarctic data available on the GTS

Similar numbers of surface SYNOP reports were received in real-time at Bracknell (5563) and Hobart (5381) via their GTS connections. However, many of these were different observations so that when they were supplemented with an additional 184 observations received directly from the Antarctic operators, we obtained a total of 6956 or 91% of the observations that could be collected, based on the normal reporting frequency of 6 or 12 hours.

Automatic weather station data

At present we hold 7717 observations from AWSs. The rapid processing of the offline, 3-hourly data from the US AWSs by the University of Wisconsin-Madison allowed us to assemble 86% of the possible data for these sites. Other data are available for the Finnish and Australian AWSs. These figures show that there are now more AWS observations from the Antarctic than from manned stations.

The main conclusions of the re-analysis exercise

Over the whole of the winter SOP no large (diameter of several thousand kilometres) depressions were found to be missed from the operational analyses.

The main errors in the analysis of depressions consisted of errors in the locations of systems rather than in the depths of the lows.

The satellite imagery revealed that many of the depressions over the Southern Ocean were much more complex than represented in the model analyses, with many systems having multiple centres, especially after the depressions became slow-moving.

The inclusion of non-GTS automatic weather station data resulted in some deepening or filling of lows in the coastal region.

The rain rate and cloud liquid water fields produced from the SSM/I data were particularly valuable in positioning the fronts and indicating how active they were.

A comparison of the US and Australian analyses

When the monthly mean surface pressure fields were compared, it was found that the differences were mostly less than 1 hPa, with some small areas above this value close to 70 S, near 20 E and 120 E. This was within the latitude where the circumpolar trough is deepest so that errors in positions of the depressions would be expected to give the largest differences. However, the largest difference between the monthly mean fields is over the Amundsen and Bellingshausen Seas between the longitudes of 120 and 150 W. In this sector of the Antarctic there are no research stations on the coast, although there is an AWS at Mt Siple (73.198 S, 127.052 W), which provides some surface data over the interior of the Antarctic. The production of operational analyses over the ocean area is therefore dependent on SATEM data..

Relatively data-rich areas, such as the Antarctic Peninsula, had only very small differences, indicating that the observations from stations in these areas are reaching the analysis centres in a timely fashion and are being used effectively.

The largest absolute differences between the monthly mean surface pressure fields are found in the Amundsen/Bellingshausen Sea area where the BOM field has values some 11 hPa below that of NCEP, with the greatest difference next to the coast. Examination of the daily difference fields for the month, indicate that this large difference was mainly the result of very large differences over the period 2-6 July 1994. Throughout this time the BOM analyses had lower pressure in this region that the NCEP fields, with the maximum difference being 42 hPa at 00 GMT 5 July. Determining what is ‘truth’ in such a data-sparse is not easy, although the satellite imagery does allow the locations of the main low pressure centres to be determined with some confidence because of the associated cloud bands. Examination of the imagery for this period would suggest that close to the location of maximum difference (70 S, 135 W) pressure was high, as no cloud was present and the leads could be seen in the sea ice.

Overall, the project found that the operational forecast systems were capable of resolving most of the synoptic-scale weather systems around the Antarctic, but that significant differences could occur in data-sparse regions, such as the Amundsen/Bellingshausen Seas.