Retrieval of Snow Characteristics from Spaceborne Scatterometer Data Martti Hallikainen, Pekka Halme, Panu Lahtinen, Jouni Pulliainen Laboratory of Space Technology Helsinki University of Technology P.O. Box 3000, 02015 HUT (Espoo), Finland Martti.Hallikainen@hut.fi Abstract— We study the feasibility of using space borne scatterometer (QuikScat onboard SeaWinds) data for retrieval of snow parameters in Finland: onset of snow melt, end of snow melt, and fraction of snow-covered area during the seasonal snow melting period. The results are based on satellite and ground truth data for 21 test sites in Finland covering the winters of 1999-2000, 2000-2001 and 2001-2002. Radar-derived time series of snowmelt are produced and compared with corresponding snow products based on optical MODIS spectrometer data. Keywords-Microwave radiometry; scatterometry; snow I. INTRODUCTION Space-borne microwave sensors can monitor characteristics of seasonal snow cover at high latitudes regardless of lighting conditions and time of day. Microwave Synthetic Aperture Radars (SAR) have been demonstrated to be feasible for determining the snow-covered area (SCA) during the snow melt period by discriminating snow-free ground from melting (wet) snow [1]. The basic method relies on comparing the backscattering coefficient to two reference values for the same geographical area, e.g. wet snow (early melting period) and snow-free ground (immediately after snow has disappeared) [1]. Inaccuracy is caused by variation in snow and soil wetness, and in temperature (melt/refreeze periods); these parameters directly affect the backscattering coefficient. The main advantage of SAR data is good spatial resolution and the main problem is poor temporal availability. II. TEST SITES AND DATA SETS A. Test Sites 70 % of the land area in Finland is forested. We use a total of 21 test sites, with 13 sites mainly consisting of boreal forest (Norwegian spruce, Scotch pine) and 8 sites with nearly comparable fractions of forested and agricultural land, Table I. When selecting the locations, the fraction of water (ice- covered sea/lakes/rivers) was minimized and a safety zone was left around water bodies to eliminate their backscatter contributions. Test sites 4 and 5, and 12 and 13, respectively, are close to each other. Hence, the same SWE values are used for the nearby test sites, Table I. The average SWE maximum values and standard deviations (in brackets) for the test sites are the following: 1999-2000: 173 mm (92 mm); 2000-2001: 94 mm (37 mm); and 2001-2002: 131 mm (51 mm). This demonstrates that inter-annual variations in the nation-wide snow water equivalent are considerable. B. Ground-Based Data Sets The Finnish National Survey provided us with land-use and forest data. Additional data for the 21 test sites include for each of the three winters 1999-2000, 2000-2001 and 2001-2002: • Maximum and minimum daily air temperatures (Finnish Meteorological Institute) • Snow water equivalent values based on snow course data and daily precipitation-based interpolations (Finnish Environment Institute). TABLE I. DATA FOR THE 21 TEST SITES IN FINLAND Location Land-Cover Distribution (%) Max. Snow Water Equivalent (mm) Test Site No. Lat. N Lon. E Forest Agric. Land Water 1999- 2000 2000- 2001 2001- 2002 1 66.68 26.57 94.2 0.6 5.2 282 137 149 2 67.14 29.11 90.0 2.2 7.8 333 141 201 3 65.40 28.69 87.9 1.4 10.7 283 149 194 4 65.30 27.98 90.8 1.8 7.4 240 114 191 5 65.30 27.98 90.8 1.8 7.4 240 114 191 6 65.10 27.73 91.3 2.3 6.4 336 123 216 7 64.88 28.65 92.1 1.6 6.3 266 138 174 8 64.45 29.30 92.3 1.3 6.4 261 144 178 9 62.71 24.64 89.6 3.4 7.0 156 115 153 10 62.61 25.07 88.4 5.0 6.6 133 104 145 11 60.84 24.96 74.8 18.1 7.1 80 67 85 12 60.82 23.96 78.5 10.1 11.4 105 51 82 13 60.82 23.96 78.5 10.1 11.4 105 51 82 14 61.49 22.06 64.4 30.1 5.5 60 67 80 15 61.27 26.36 55.6 30.7 13.7 100 71 97 16 60.95 25.56 63.8 33.0 3.2 92 68 81 17 61.01 23.14 53.9 40.8 5.3 93 43 93 18 63.17 22.43 55.8 41.4 2.8 60 66 77 19 60.88 22.55 50.1 45.2 4.7 92 34 62 20 64.58 25.42 87.8 8.7 3.6 155 81 90 21 64.89 25.17 80.7 14.8 4.5 162 92 130