## 1 Introduction  
The interactions between seasonal soil frost and snow cover have a substantial impact on the soil moisture balance and energy exchange processes between the soil and atmosphere, as the hydraulic and thermal properties of unfrozen and frozen soils are quite different. The influence of snow cover on the ground thermal regime depends on the accumulation, duration, timing and thickness of the seasonal snow layer, as well as melting processes and geographical location [1]. The soil frost depth is inversely correlated with snow cover depth, as snow acts as an insulating medium between colder air temperature and the soil surface [2]. The snow cover retained by vegetation on the ground surface can enhance the insulation of the soil, which may reduce periodic variation in soil temperature, slowing the freezing and thawing processes [3] and even altering the density of the snow layers [4]. An excellent review is given by [PERSON] _et al._[5] to address the effects of seasonal freeze-thaw and snow cover processes on the ground thermal regime. Traditionally, investigation of the freeze-thaw cycles are mainly characterized by using invasive _in situ_ point-measurements (e.g., frost tubes, thermistors, dielectric permittivity sensors), which provide limited local scale information and disturb the natural soil system during installation. For instance, [PERSON] _et al._[6] used thermistors to monitor the temperature variation along a cylindrical sample placed in an environmental cell. Frost tubes (plastic fluorescein dye tubes) are usually filled with a solution of methylene blue that undergoes a color change (blue to clear) as a result of freezing. Frost tube readings are taken manually and therefore significantly disturb the subsurface structure during installation, which may influence local soil moisture and local heat exchange. Time domain reflectometry (TDR) has also been used to monitor the dynamics of seasonally-frozen soils and meltwater infiltration due to its ability to measure the soil dielectric permittivity [7, 8]. Nevertheless, the TDR method requires the installation of several vertically- and horizontally-positioned probes at fixed soil depths, thereby restricting its applicability in larger spatial investigations. In the last few decades, satellite remote sensing techniques have been used to measure the surface soil freeze-thaw condition at both regional and global scales [9, 10]. Frozen surface and soil conditions have been monitored by both active and passive microwave sensors [11, 12, 13, 14, 15, 16], as well as using optical sensors [17]. The large contrast between the dielectric properties of ice and water makes it suitable to monitor the freeze-thaw cycle of soil using microwave measurements. Current approaches employing satellite microwave remote sensing to, e.g., to classify landscape freeze-thaw states, involve temporal change detection schemes applied to time series of backscatter (e.g., from ESA's ERS-1 satellite [11]) or applied to brightness temperatures (e.g., from the special sensor microwave imager (SSM/I) [18] or from the advanced microwave scanning radiometer - earth observing system (AMSR-E) [19]). The retrieval of information on snow and soil information from remote active microwave data (backscatter) is less developed in comparison to the corresponding retrievals using passive microwave data (brightness temperatures) [20]. Additionally, most remote sensing products are either coarse in spatial (especially passive sensors) or in temporal resolution [21, 22]. In this respect, ground-penetrating radar (GPR) has been used as a non-invasive method to monitor the spatial and temporal dynamics of the shallow freeze and thaw processes at field scales [23, 24, 25].