A warm core in a polar low observed with a satellite microwave sounding unit

Abstract

A polar low which occurred over the Labrador Sea on 17 and 18 January 1989 was examined with the Microwave Sounding Unit (MSU) on the satellites NOAA-10 and 11. Surface reports indicated a pressure deficit of at least 4 mb with the polar low, and surface wind speeds of at least 20 m/s were retrieved with the Special Sensor Microwave/Imager. MSU brightness temperatures at 53.74 GHz were warmer over the polar low than in all of the adjacent MSU field-of-views. The 53.74 GHz brightness temperatures over the polar low were between 1 and 2 K warmer than the adjacent values. Since some polar lows are warm core systems, results of this type are expected. The possibility that the warming observed in the 53.74 GHz data was caused by factors other than warming of the air column was investigated. MSU 50.30 GHz brightness temperatures did not show a maximum over the polar low, as would be expected if the 53.74 GHz warming was caused by surface and precipitation effects. A sensitivity analysis indicates that the measured warming is greater than that expected from cloud and surface effects. This type of measurement should be a useful tool for monitoring polar lows, particularly when the higher resolution Advanced Microwave Sounding Unit becomes available.