Analyses and numerical modeling of a polar low over the Japan Sea on 19 December 2003

A meso-α-scale polar low was observed over the Japan Sea on 19 December 2003. It initialed around 11 UTC over the northwestern part of the Japan Sea within a synoptic-scale parent low under the influence of baroclinic environment and disappeared over the eastern edge of Japan Islands with a lifetime of about 20 h. It is of interest that this polar low had “concentric eye-walls” and “warm core” structure at its mature stage. The evolutionary process and spatial structure of this polar low were investigated by using almost all available observational data, including the Geostationary Operational Environmental Satellite (GOES)-9, the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery, the Final Analyses (FNL) data issued by National Centers for Environmental Prediction (NCEP), the surface observational data and the 9-station sounding data of Japan Islands. In order to study its development mechanism, a 24-h numerical simulation using the version 4.4 of the Regional Atmospheric Modeling System (RAMS) starting from 12 UTC 19 December 2003 with an 8 km × 8 km resolution was performed. It is shown that the RAMS model reproduced the main features of the polar low reasonably well. The vorticity budget analyses indicate that the stretching term is the major contributor for the vorticity increase of the polar low. The baroclinic background seems to play significant role for the initial development of this polar low. However, the effect of the diabatic heating for its later development is also significant.