## 5 Summary  
SeaWinds (1999-2009) seasonal snow melt studies largely build upon previous experience with NSCAT (1996/1997). Additionally approaches developed for the C-band scatterometers (ERS1/2, 1992 onwards) have been considered, although they operate at a longer wavelength with different snow scattering properties from Ku-band scatterometry. The sensitivity of backscatter to changes in liquid water content of the snow surface is exploited in both wavelengths. NSCAT analyses for seasonal snow applications started over Alaska and Canada. First application of QuikSCAT data was published for Siberia [18] followed by studies over Canada and Alaska and eventually on circum-polar scale [24, 34]. Comprehensive processing of SeaWinds L2A and L1B data have been carried out in cooperation with NASA JPL (e.g., [11, 16]), by TUW (e.g., [12, 18]) and BYU (e.g., [23, 24]). Whereas passive microwave sensors are used to determine geophysical parameters such as SWE, the Ku-band scatterometer is to date only used for change detection analyses intended for identification of freeze-thaw characteristics for specific days of the year. This is achieved by definition of thresholdsfor the backscatter deviation from long-term, seasonal, multi-day or diurnal averages. Static as well as adaptive measures are applied. An adaptive threshold is for example defined as multiples of the standard deviation of location specific noise [12] or seasonal variation [11]. Most studies have focused on high latitudes, since continuous snow cover and higher number of observations benefit time series analyses in that region. Sufficient snow depth and limited evaporation are also required for a distinctive snow melt signal. Derived parameters which relate to spring snowmelt are the start, duration and end of snowmelt (or snow-off day). Days with maximum deviations are defined as primary thaw days [16]. Single melt days during winter do not necessarily show a backscatter decrease during events (such as rain-on-snow) but result in later backscatter increase when the snow surface is refreezing. This allows for the detection of thaw and refreeze days [40]. The detection of autumn refreeze has been attempted only in one case [33] as this change is less distinctive in Ku-band. Beside its use for seasonal snowmelt detection, SeaWinds has been also investigated at high latitudes for glaciological studies over Greenland [7,8], lake ice phenology [50] and sea ice monitoring [9,10,51]. Although the QuikScat mission stopped on November 23, 2009 due to antenna problems it provides a valuable dataset. A full decade of data is now available which gives insight into the spatial variability of snowmelt on circumpolar scale (Figure 4). The record is too short in order to access significant trends and the interannual variability is large. The type of satellite data employed (scatterometer) provides rather coarse spatial resolution maps, but better than daily data. Ku-band and also X-band have been identified as most suitable for snow applications and are considered for possible future SAR satellite missions which are dedicated to snow monitoring [52]. A Ku-band scatterometer for the purpose of ocean wind detection is also aboard Oceansat II (launched by the Indian Space Research Organisation on September 23, 2009; [[http://database.eohandbook.com/](http://database.eohandbook.com/)]([http://database.eohandbook.com/](http://database.eohandbook.com/)), status March 2010). Surface melt and refreeze patterns can, however, be also observed with C-band as demonstrated by [32]. C-band scatterometry such as the recently launched Metop ASCAT offer near real time mapping capability [53] at similar resolution like SeaWinds. A similar Ku-band sensor which will additionally acquire in C-band (Dual Frequency Scatterometer: DFS) is in preparation by the National Oceanic and Atmospheric Administration (NOAA) in cooperation with the Jet Propulsion Laboratory (JPL) and the Japan Aerospace Exploration Agency (JAXA) for launch in 2016 as part of the Global Climate Observation Mission-W2 [54].