Research aircraft measurements of a polar low over the Norwegian Sea

Abstract

On 27 February 1984, the Arctic Cyclone Expedition carried out the first research aircraft measurements within a polar low. The low developed over the Norwegian Sea south of Jan Mayen in response to the baroclinic forcing by an eastward propagating upper-level synoptic-scale short wave. Observations from the NOAA WP-3D research aircraft documented the three-dimensional distribution of wind, temperature, moisture, and precipitation within the low. The polar low had a warm inner core and maximum surface winds of ~ 35 m s-1. Heavy meso-convective precipitation was encountered within a frontal-like, mesoscale, baroclinic shear zone that spiraled into the low center from its southwestern quadrant. Vorticity and divergence values within the front reached 25 × 10-4 s-1 and 13 × 10-4 s-1, respectively, where the frontal width narrowed to 10 km near the sea surface. Radar reflectivities exceeded 40 dBZ within the meso-convective precipitation band and were confined to low levels (below 3 km). The maximum total heat flux (sensible plus latent) from the sea surface into the atmosphere was 1000 W m-2, comparable with that observed for mature tropical cyclones. Satellite cloud images revealed that this polar low was the most intense development in a family outbreak of 5 polar lows that formed as an east-west vortex chain between Iceland and the north coast of Norway over a 48 h period.