A wintertime mesocyclone (MC) event occurring during the Winter Weddell Sea Program field phase 1986 is studied by digital advanced very high-resolution radiometer data and numerical simulations using the mesoscale Norwegian Limited Area Model. The MC considered developed during July 30, 1986, near the sea ice front of the northern Weddell Sea. During its initial and developmental phases it moved eastward parallel to the sea ice front. During the first 24 hours, baroclinic instability and diabatic heating were found to be of great importance for the intensification. During its mature stage (the following 24 hours), 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 potential vorticity anomaly was found to be present during the mature stage. The MC had a diameter of about 700 km and 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. ¿ 1998 American Geophysical Union