Weddell Sea case

(Günther Heinemann, MIUB)

A wintertime mesocyclone (MC) event occurring during the Winter Weddell Sea Program field phase 1986 (WWSP86) was further evaluated by digital AVHRR data and numerical simulations using the meso-scale model NORLAM. The MC considered developed during 30 July 1986 near the sea ice front of the northern Weddell Sea. During its initial and development phases it moved eastwards parallel to the sea ice front. During the first 24 h, baroclinic instability and diabatic heating were found to be of great importance for the intensification. During its mature stage (the following 24 h), a pronounced frontal system formed and baroclinic instability can be regarded as the main process for its further development. The MC moved over the sea ice during its mature stage, so the forcing by surface energy fluxes was less important. A cyclonic upper-level PV anomaly was found to be present during the mature stage. The MC had a diameter of about 700 km, a lifetime of more than 48 hours and reached the intensity of a polar low (measured near-surface wind speeds of up to 26 m/s). The model results revealed good agreement with observations (radiosondes and synoptic observations) in the case of the "full physics" run, but in the case of the "no cloud physics" run this was very poor. A paper on this case is in print (Heinemann, 1998).