The onset of snow melt estimated from SMMR and SSM/I data in Eurasia from 1979-2006

Snow is an important physical parameter in hydrology and in weather forecast and climatological models. Hydropower plants can adjust waterflow accordingly if the timing of the snow melt is known. Typically this knowledge should be available in a timeframe of one week or even one day. Snow covered terrain affects the radiation budget and thus to the climate directly. The onset of snow melt is directly related to the beginning of growing season and the amount in the carbon dioxide in the atmosphere. Carbon dioxide is a important greenhouse gas. The time series of the onset of snow melt can also give insight to the effects of global warming. Finnish Meteorological Institute has initiated a research project called Snow-CLIM which aims to determine such time series and compare the results to existing climate models.

Spaceborne radiometer data is ideally suited for global monitoring of the onset of snow melt. Radiometer data covers the globe in daily basis and weather effects or the lack of sunlight are no concern. The rather coarse resolution does not pose a problem since climate models today have even coarser resolution. The channel differences T19v,h-T37v,h have been widely used in determining the onset of snow melt. Abdalati et al. as well as other authors have applied the snow melt onset detection in arctic tundra or snow over glacier conditions. Takala et al. have been developing algorithms for taiga belt conditions first with hydrological model data as a reference. Mognard et al. attempted a global approach but with no analysis on accuracy. Takala et al. presented in IGARSS 07 first results of the onset of snow melt in Eurasia using temperature data as a reference. Lately the interest of the scientific community has concentrated on the diurnal melt cycles, for example Ashcraft et al. In this paper the authors use a time series analysis on the channel difference T37v-T19v to determine the onset of snow melt for the years 1979-2006 together with reference data.

The radiometer data used in this paper is SMMR-data (Knowles et al.) for the winters 1979-1987 and SSM/I-data (Armstrong et al.) for the years 1988-2006. As a reference data INTAS (International Association for the promotion of co-operation with scientists from the New Independent States of the former Soviet Union) -SCCONE (Snow Cover Changes Over Northern Eurasia) data is used covering years 1979-2001. SCCONE data contains snow depth values (cm), visual estimates of the snow melt and descriptive information about the each measurement station. From the 8 days average of the channel difference T37v-T19v a threshold is estimated. If the values of the channel difference go over this value the melt is estimated to take place. A post filtering is applied to tweak out non permanent wet snow cases. Error analysis of 9 years of SMMR based estimates and 14 years of SSM/I based estimates are presented. Also maps of snow melt in Eurasia and temporal development of the onset of snow melt through different years are discussed.

The results obtained are accurate and no such validated time series exist before this date. The dataset is very valuable for the climatological research and the study of the global warming. The historical time series can be a valuable tool for the population study of lemmings and other flora or fauna in arctic areas.