Monitoring Snow Cover on the Tibetan Plateau Using Passive Microwave Satellite Data

Output from satellite-borne passive microwave sensors has increasingly been recognized as an important means of monitoring snow cover. However, employing a single global algorithm t o extract a snow signal from multichannel microwave data does not accurately monitor snow across all landscapes. One such region where difficulties occur is the Tibetan Plateau, where global algorithms overestimate the extent of cover. Here, adjustments we have made to the Chang et al. [I] global algorithm are discussed. Given the linear fit between snow depth and brightness temperature in the algorithm, estimated snow depths for individual 1" x 1" cells are reduced in 10 mm increments. This continues until the number of regional cells estimated t o be snowfree agree with a detailed analysis of snow cover using shortwave satellite imagery. In 15 Plateau test cases, an average of 80 mm of snow is subtracted from original microwave estimates of cell snow depth over the more level portions of the Plateau and 60 mm over more mountainous areas t o reach agreement with shortwave values. Possible reasons for the original overestimation are discussed. The microwave offsets are applied t o over 500 snow charts derived from Scanning Multichannel Microwave Radiometer data using the Chang et al. algorithm between 1979-87. These data show that the average annual (Sep-Aug) percent coverage of snow on the Plateau is 3546, with the most snow observed in the 1982/83 season (39%) and the least in 1982/83 (26%). The mean is 10% larger than less accurate estimates obtained from lower resolution hemispheric snow charts that are derived from shortwave satellite data.