A numerical investigation of polar lows

A quasi‐geostrophic numerical model is employed to find normal mode solutions for unstable disturbances with low static stability near the earth's surface. Three occasions on which polar lows developed are analysed to provide the necessary data for the model. The normal mode solutions which are obtained are, therefore, those which have the highest growth rates under conditions in which polar‐air depressions have developed. Since the model is two‐dimensional, being a cross‐section through the mean flow, the perturbation amplitude and energy conversion rates of the growing disturbances in this plane are described for each case. It is shown that horizontal variations in static stability are an important factor in the forecasting problem and some criteria for polar‐low development are suggested.