Baroclinic instability as a mechanism for the serial development of polar lows: a case study

Abstract

Satellite infrared images of a series or train of four polar lows are presented, and the synoptic environment of the lows is documented with the help of operational analyses prepared by the European Centre for Medium Range Weather Forecasts (ECMWF). The analyses reveal that the disturbances formed in a shallow baroclinic zone of small static stability close to the region of maximum 1000–500 mb thermal wind and that the flow was of the reversed shear type.

The observed characteristics of the disturbances were in partial agreement with those obtained from a linear, quasi-geostrophic, dry baroclinic model. The wavelength of maximum instability in the model of approximately 500 km was in satisfactory agreement with the observed wavelength. The computed phase speed was somewhat larger than the observed speed, a result attributed to the effect of organized deep convection in retarding the actual motion. The large growth rate of the 500 km wave was qualitatively consistent with the rapid growth of the systems as seen on the satellite images. It is argued, however, that baroclinic instabilty alone was not capable of accounting for the rapidity of the developments and that some other mechanism, presumably latent heat release in organized deep convection, also played an important part in the growth.