SEASONAL VARIATION OF SNOW COVER OVER THE NORTHERN HEMISPHERE : PROGRESSION OF SNOWMELT

Seasonal variation of snow cover over the Northern Hemisphere is examined. The NOAA/NESDIS weekly snow cover data for the 14-year period from 1973 to 1986 are used. The data set is a digitized version of NOAA/NESDIS snow cover charts, which in turn are based on manual analysis of visible imageries taken by NOAA and other satellites. The data set tell whether each of 89×89 grid boxes on a polar stereographic map is covered with snow or not. We analyzed the progression of snowmelt in spring. By "the week of snowmelt" (for a certain grid box and a certain year), we mean the week just after the one when snow cover is last observed within the period between No.1 (early January) and week No.30 (late July). Fourteen-year mean and interannual standard deviation of the week number of snowmelt are calculated and mapped. The part of continents where snow cover usually exists until March can be classified into "plain" areas and "mountain" areas. In "plain" areas, in many-year average, snowmelt proceeds rather smoothly from south to north at a speed of 10 degrees latitude per month. In addition, a general trend in the progress of snowmelt from west to east is superposed in the European/Western Siberian as well as North American plain areas. Some irregularities in propagation are found. For example, across the Ural Mountains, snowmelt occurs about one week earlier to the east than to the west. In individual years, the progression is much more patchy. Snowmelt over a contiguous area of the order of magnitude of (1000km)^2 is often observed in one week. Some mountain areas, such as Pamir Highland/Himalayas, Altai Mountains, Stanovoi Highland/Yablonovyi Mountains, Alaskan and Canadian Rocky Mountains, etc. are characterized by late snowmelt season. In particular, snow cover lasts until June in the Stanovoi/Yablonovyi area, although the ground is only about 2000m above sea level. In other mountain areas, such as the Tibetan Plateau, Mongolian Plateau and Rocky Mountains in the U. S. A., interannual variability of the "week of snowmelt" is large