Passive microwave remote and in situ measurements of artic and subarctic snow covers in Alaska

Abstract

Between 11 and 19 March 1988, airborne and satellite passive microwave measurements were acquired simultaneously with ground measurements of depth, density and stratigraphy of the snow in central and northern Alaska. Five aircraft flights were flown along a north-south transect between about 147 ° W and 152 ° W, and extending from about 63° N (south of Fairbanks, Alaska) to the Arctic Ocean coastline, with an Aircraft Multichannel Microwave Radiometer (AMMR) on-board operating at 92 GHz, 37 GHz, 21 GHz, and 18 GHz. Passive microwave data from the satellite-borne Special Sensor Microwave Imager (SSMI), operating at 85.5 GHz, 37 GHz, 21 GHz, 18 GHz, and 10 GHz, were obtained concurrently. A good correspondence in brightness temperature (T B ) trends between the aircraft and satellite data was found. However, an expected inverse correlation between depth hoar thickness and T B was not found to be strong. A persistent T B minimum in both the aircraft and the satellite data was detected along the northern foothills of the Brooks Range. In an area located at about 68° 50'N, 149° 20'W, the T B as recorded from the aircraft microwave sensor dropped by 55 K. Satellite microwave measurements showed a T B decrease of up to 45 K at approximately the same location. Snow pit measurements did not reveal notable differences in snow characteristics or depth in this location. An examination of passive microwave satellite data from 1978 to 1987 revealed that similar low latewinter T B values were found in approximately the same locations as those observed in March 1988. According to the satellite data, the zone of low T B develops as the snow deepens, and reaches the lowest values in March or April each year. The cause of this T B minimum is unknown but thought to be related to snow stratigraphy. The observed difficulty in relating the ground measurements to data collected using aircraft and satellite passive microwave sensors is attributed to the fact that the snow depth and character are highly variable in central and northern Alaska. This variability is exemplified in the field measurements as well as in the passive microwave measurements.