## 2 Data Characteristics  
The first Ku-band scatterometer data used for snow studies were acquired by the NSCAT instrument onboard the Advanced Earth Observation Satellite (ADEOS). It was launched in August 1996 and operated until June 1997. The spatial resolution was 25 km and a 90% global coverage was achieved within two days. As it was a polar orbiting platform, twice daily acquisitions were available at high latitudes [11]. The later SeaWinds instruments (on QuikSCAT and ADEOS2) also covered 90% of the Earth's surface daily and provided up to ten measurements towards 75\({}^{\circ}\)N [12]. SeaWinds on QuikSCAT had also approximately 25 km spatial resolution and has been in operation for more than ten years from June 1999 [13] to November 2009. It was launched into a polar, sun-synchronous orbit with 98.6\({}^{\circ}\) inclination and with a local equator crossing time at the ascending node of 6:00 am \(\pm\) 30 min and a swath width of 1,800 km [14]. Backscatter measurements were collected simultaneously at constant incidence angles of 46\({}^{\circ}\) for the inner beam, and 54\({}^{\circ}\) for the outer beam, with horizontal and vertical polarizations respectively, using a scanning dish antenna operating at 13.4 GHz (Ku-band). The antenna has an elliptical footprint size of roughly 24 km \(\times\) 31 km at inner beam. Scatterometers are non-imaging radars. An important step in the data preprocessing is therefore the transformation to a discrete grid. Scatterometer data have often been gridded into rectangular cells of e.g., 25 km \(\times\) 25 km for NSCAT [15, 16, 17] and 10 km \(\times\) 10 km for SeaWinds [18, TUW (Vienna University of Technology) product]. This regridding for SeaWinds is carried out either based on Level 2A or Level 1B products of the Jet Propulsion Laboratory (JPL). The Level 1B Product is in temporal order and therefore defined by time boundaries. The Level 2A Product is in spatial order and defined by spatial boundaries [14]. Level 2A data contain backscatter measurements representing the elliptical antenna footprint area. Each \(\sigma^{0}\) is based on the sum of the echo energies measured among the center high resolution slices in a single scatterometer pulse. The Level 2A Product file also contains flags for land and ice area and it is used, e.g., for the TUW product. An alternative resolution enhanced QuikSCAT product has been developed based on the multiple overlapping measurements available during short time intervals [19]. It is derived from the JPL Level 1B product. In this case \(\sigma^{0}\) data are made available as \"eggs\" or \"slices\" depending on the actual processing method. Egg-based QuikSCAT images have a nominal pixel spacing of 4.45 km and an estimated effective resolution of 8-10 km [20, BYU (Brigham Young University) product]. Daily BYU \"eggs\" data have been used by [21, 22, 23, 24], for land applications outside glaciated areas. On a global level these data are assembled from four days of data and for polar regions for separated day times due to increased revisit intervals (applied in e.g., [24]). Especially in studies which have been carried out with the BYU product the data are referred to as QSCAT data. Microwave backscatter in general differs significantly due to changing dielectric properties between frozen and unfrozen ground (e.g., [25, 26, 27]). In case of Ku-band, the backscatter is low before snow arrival. It gradually increases with snow accumulation, then rapidly decreases when the snow starts melting. Eventually it increases again when all snow has melted [17](see also Figure 1). A melting snow surface acts similar to a water surface and thus causes specular reflection resulting in the low backscatter values. The level of summer backscatter is lower than winter backscatter. Microwaves emitted at 13.4 GHz penetrate deep into the snow pack in case of dry snow since the grain size is smaller than the wavelength. With increasing snow depth, however, the backscatter is increasing for dry snow [1, 28].