The polar low as a baroclinic disturbance



This paper describes the three dimensional airflow and precipitation within small ‘polar’ depressions which sometimes cross Britain during northerly outbreaks and which in winter can be responsible for heavy snowfalls. The data used in the study consist of Doppler and conventional radar information, together with routine synoptic data and sequential radiosonde ascents from the radar station. Three dimensional airflow was derived from the radiosonde data assuming that wet bulb potential temperature was conserved. Horizontal and vertical air velocities were also derived from the Doppler radar measurements.

Previous knowledge of polar lows is meagre; they are generally thought to be shallow features resulting from enhanced convection within cold air flowing over a warm sea. However, the well-developed polar low which is the main subject of this paper is shown to have been an essentially baroclinic disturbance. Although enhanced small-scale convection occurred in one sector, the main area of widespread precipitation associated with the polar low was produced not by small-scale convective overturning but rather by slantwise convection within a narrow tongue of air ascending steadily at about 10 cm s−1.

The speed of travel and short wavelength (900 km) of the polar low in this study are consistent with its having formed in a region of enhanced baroclinicity within the polar air below 850 mb rather than in the major baroclinic zone bounding the polar air mass. Considerable low-level baroclinicity within the polar air is also shown to have been present during the formation of other intense polar lows.