THE EFFECTS OF SNOWPACK GRAIN-SIZE ON SATELLITE PASSIVE MICROWAVE OBSERVATIONS FROM THE UPPER COLORADO-RIVER BASIN

Understanding the passive microwave emissions of a snowpack, as observ ed by satellite sensors, requires knowledge of the snowpack properties : water equivalent, grain size, density, and stratigraphy. For the sno wpack in the Upper Colorado River Basin, measurements of snow depth an d water equivalent are routinely available from the U.S. Department of Agriculture, but extremely limited information is available for the o ther properties. To provide this information, a field program from 198 4 to 1995 obtained profiles of snowpack grain size, density, and tempe rature near the time of maximum snow accumulation, at sites distribute d across the basin. A synoptic basin-wide sampling program in 1985 sho wed that the snowpack exhibits consistent properties across large regi ons. Typically, the snowpack in the Wyoming region contains large amou nts of depth hear, with grain sizes up to 5 lmm, while the snowpack in Colorado and Utah is dominated by rounded snow grains less than 2 mm in diameter. In the Wyoming region, large depth hear crystals in shall ow snowpacks yield the lowest emissivities or coldest brightness tempe ratures observed across the entire basin. Yearly differences in the av erage grain sizes result primarily from variations in the relative amo unt of depth hear within the snowpack. The average grain size for the Colorado and Utah regions shows much less variation than do the grain sizes from the Wyoming region. Furthermore, the greatest amounts of de pth hear occur in the Wyoming region during 1987 and 1992, years with strong El Nino Southern Oscillation, but the Colorado and Utah regions do not show this behavior.