Polar lows affecting Denmark

Abstract

18 cases of polar lows (sub-synoptic cold air vortices) over Denmark in the 10 polar low seasons (November to April) from 1980–90 have been analysed. The different properties of the lows are discussed and examples are shown illustrating the variety of the lows ranging from relatively strong, rapidly developing, baroclinic disturbances down to weak lows of the horizontal scale of 100 km forming out of a small group of cumulo-nimbus clouds. They generally form in an almost moist adiabatic atmosphere with pre-existing large-scale positive vorticity, and it is suggested that this can explain why polar lows can have a horizontal scale down to 100 km and at the same time be stable systems persisting for several days. The strongest lows are found to have a strong baroclinic influence, but it is shown by using the model proposed by Craig and Cho (1988), that for most of the lows both baroclinic and convective processes can be of importance.