## 3 Change Detection Approaches  
The change from snow to snow-free conditions is usually not distinguished from the change from frozen to unfrozen soil conditions when active microwave data are applied in the high latitudes. The term freeze/thaw detection is frequently used in both cases. This may cause confusion on the actual meaning in regions with seasonal snow cover. The snow melt process can take several days to weeks, depending on the snow depth and the continuity of the meteorological conditions leading to the melt. Advanced change detection approaches thus need to also consider multiple thaw periods. This differs for the detection of mid-winter melt events or melt days on ice caps. In such cases each single day needs to be detected. Although the available record of NSCAT is rather short (eleven months) it provided a first dataset covering an entire northern hemisphere winter and spring period at this wavelength (Table 2). Its suitability for detection of freeze/thaw was tested by [11]. They introduced a change detection algorithm which considers differences between five day averages (separately for morning and evening measurements) and location specific differences from the overall mean value. They discussed also an approach previously developed for C-band scatterometry [31]. This method is based on the typical summer (July) and winter (February) backscatter level in order to determine the thaw timing. When a minimum of 50% of the winter-summer difference is exceeded for at least two consecutive measurements ground thaw is detected. In case of NSCAT summer backscatter was determined based on June measurements since it stopped operating during that month. It has been stressed that the backscatter signal is increasing after melt at C-band and decreasing at Ku-band. The C-band method can therefore not be directly applied. This first NSCAT study at three sites in Canada [11] was extended for a number of follow-up studies with Ku-band scatterometry. [PERSON] _et al._[15] adapted the C-band freeze thaw classifier of [31, 32] to NSCAT data over Alaska. The five day average method from [11] was extended for NSCAT by extraction of three specific dates of thaw: the start, the end and the primary thaw date which is the day with the largest negative backscatter difference and applied over part of Canada [16]. The five day moving average approach has been subsequently transferred to QuikSCAT (launched 1999) for final thaw date extraction and also applied in a similar way to autumn refreeze over Alaska for the years 2000 and 2001 [33]. The investigation has been then extended for application on circumpolar scale [34]. A further method considers winter (February) mean backscatter applying fixed thresholds for daily mean values in order to determine the onset of snowmelt over the Canadian Arctic for the years 2000-2004 [23]. A fixed value for melt duration (23 days) has been added to determine the end of snowmelt. As QuikSCAT provides sufficient morning and evening measurements, a new adaptive approach based on diurnal thaw and refreeze of snow cover could be developed [12, 18, TUW method]. The algorithm was initially setup over central Siberia covering the years 2000-2004. Thresholds are defined for each single grid cell dependent on the estimated standard deviation of long-term noise (Figure 2) and the actual number of measurements available during each 12 hour period. Significant diurnal backscatter changes occur throughout the snowmelt period several times but not necessarily on subsequent days. This occurrence of multiple events has been solved with a clustering method. In case of multiple melt periods (several clusters of at minimum two days with diurnal thaw and refreeze), the last one is identified as the major melt period [35, 36]. An analysis limited to evening measurements using the five day average approach [11] plus the summer mean backscatter (August) has been carried out on pan-arctic scale for 2000-2005 by [24]. The evening values have been taken from the BYU \"egg\" product [20].