Retrieval of the Water Equivalent of Snow Cover in Finland by Satellite Microwave Radiometry

The use of satellite microwave radiometry to retrieve the water equivalent of seasonal snow cover in Finland was investigated. Data from the Scanning Multichannel Microwave Radiometer (SMMR) on board Nimbus-7 were employed to examine the feasibility of several water equivalent algorithms. The satellite data set covers the winters of 1978-1979 through 1981-1982. The ground truth consists of snow water equivalent maps of Finland, compiled by the National Board of Waters. The microwave response to the water equivalent of dry snow cover was observed to depend substantially on frequency. Only algorithms including the 37-GHz channel gave adequate agreement with the manually measured water equivalent values. The algorithm involving the brightness temperature difference between 18 and 37 GHz, vertical polarization, provided the highest overall linear correlation coefficient. Two distinct categories were observed in the microwave behavior of snow-covered terrain for dry snow conditions: 1) early and mid-winter (usually about three months in Finland), and 2) late winter (a few weeks, starts after several melt-freeze cycles). Short-term variations in the microwave response to snow water equivalent can be related to variations in the surface structure and, to some degree, the temperature of the snow cover. Especially for small snow depths, the microwave response is also affected by the temperature of the underlying ground. Annual variations were observed to correlate, in addition to snow parameters, with the water content and state (frozen or thawed) of the underlying ground. The microwave response to snow water equivalent was found to depend substantially on the land-cover category.