A wintertime polar low over the eastern weddell sea (Antarctica): A study with AVHRR, TOVS, SSM/I and conventional data

The baroclinic development of an intense meso-scale cyclone (MC) over the Weddell Sea near the Antarctic coast close to the German Georg-von-Neumayer Station (GvN) during the period 26-28 April 1989 was studied by means of satellite and conventional data. ECMWF analyses, radiosonde data and surface observations were used together with AVHRR data, TOVS and SSM/I retrievals for the description of the synoptic and subsynoptic environment associated with the development of the MC. The MC had a diameter of about 500 km, a lifetime of about 40 hours and reached the intensity of a polar low. Wind speeds up to 19 m/s (with gusts up to 24 m/s) were recorded at GvN as the MC approached on 27 April and remained quasi-stationary for about 24 hours. Its development took place in baroclinic conditions of strong low-level cold air advection close to the sea ice front. The genesis of the MC seemed to be triggered by a 500 hPa short wave trough and a resemblance to a baroclinic development at a boundary layer front was noticed. Low-level thickness fields from TOVS data reflected the baroclinic structure of the MC, but gradients were relatively weak. Wind speed retrievals from SSM/I data did not allow a full analysis of the wind field structure for this case, as they were limited to ice-free ocean. They showed the incipient MC lying in an area with increased wind speed and high surface fluxes of sensible heat. In the mature stage, a maximum in the SSM/I wind speed field was found in the northwestern part of the MC. A meso-scale analysis with a limited area assimilation system for the mature stage showed only weak support of the low-level MC by upper level cyclonic vorticity advection.