# Remote Sensing of Snow Cover Using Spaceborne SAR: A Review
## 1 Introduction  
Snow covered areas influence the global radiation balance, groundwater, runoff, glaciers, flora and fauna, and human activities such as tourism, civil engineering, and infrastructure. Within all cryospheric components, snow covers the widest area: During the wintertime, more than 40% of the northern hemisphere is covered by snow [1; 2; 3]. As a result of its multiple influences on the environment, snow cover is addressed in the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) and is identified as a critical climate variable within the Global Climate Observing System (GCOS) [4]. Several snow cover projects relying on remote sensing have been conducted in recent years, including the European Space Agency (ESA)'s Satellite Snow Product Intercomparison and Evaluation Exercise (SnowPEx) [5], the German Aerospace Center (DLR)'s Global SnowPack [6], the National Aeronautics and Space Administration (NASA)'s Snow Experiment (SnowEx) [7], and the ongoing ESA's Let It Snow project [8] as well as new Climate Change Initiative Extension (CCI+) Essential Climate Variables (ECV) mission [9]. Figure 1 gives an overview of some of the aspects related to snow cover, and also illustrates some of the more variable snow cover characteristics such as liquid water content, grain size, density, and snow water equivalent (SWE). Snow cover reflects incoming solar radiation, as fresh snow normally has an albedo between 0.8 and 0.9 while most land surfaces have an albedo ranging between 0.1 and 0.3 [10]; therefore, snow influences the regional and global energy balance [11; 12; 13]. A decrease in snow cover extent (SCE) and duration leads to a reduced albedo of the land surface, which increases the warming process and further accelerates the snowmelt process [14; 15; 16; 17; 18]. Additionally, snow cover affects the conditions and spatial distribution of other cryosphere components [19]: Snow is a prerequisite for temperate glaciers, controlling their equilibrium. The high albedo of snow preserves the retention of sea and lake ice and also influences the growth rate of ice thickness due to thermodynamic processes (freezing and melting) and snow types (dry or wet) [20; 21; 22]. Snow also interacts with permafrost due to its thermal insulation characteristic, which reduces the scale of variation of active layer thickness (ALT) caused by air temperature changing [23; 24; 25]. Consequently, snow cover is regarded as one of the most critical factors affecting the thermal regime of permafrost [26; 27]. In addition, snow cover directly affects ecology and the socioeconomic system. Studies suggested that water originating from snowmelt dominates the runoff regimes of downstream regions not only in terms of water amount but also in distribution, quality, and seasonality [28; 29; 30; 31; 32]. Therefore it also controls the available water resources for the inhabiting population. More than 50% of precipitation in mountainous regions is falling as snow in Norway, the French Alps, and also the northern and western United States [13; 33; 34; 35; 36; 37]. Moreover, snow cover and snowmelt in particular can also lead to natural disasters such as floods or avalanches (or in the absence of snow: Droughs). In order to identify possible flood events, detecting the onset of snowmelt in time is necessary [38; 39; 40; 41; 42]. Furthermore, snow cover is also an important aspect for winter tourism [43; 44]. Due to global warming, a significant decrease in spring SCE has been shown in both observations and models [45; 46; 47; 48; 49; 50; 51], and recorded in the Synthesis Report of IPCC AR5 [52]. Climate change influences the global snow cover spatial extent as well as the duration. [PERSON] et al. [23] concluded that temperature increase as well as large-scale atmospheric patterns is the most influencing factors changing global snow cover.