# Mapping Radar Glacier Zones and Dry Snow Line in the Antarctic Peninsula Using Sentinel-1 Images
## 3 Methodology
### RGZ Classification Using Sentinel-1  
RGZs show latitudinal zonation on the glaciers. In general, bare ice, wet snow, frozen percolation, and dry snow radar zones distribute successively with the elevations [8]. Melting signals were found within the areas lower than 1700 m by the SSM/I in this study. [PERSON] et al. [34] reported that the elevation is higher than 800 in the dry snow radar zone, and lower than 500 m for bare ice in the AP. RGZs can be also characterized by their backscattering characteristics. Dominated by specular  
Figure 2: Ascending special sensor microwave/imager (SSM/I) 19.35 GHz \(T_{b}\) in the horizontal polarization of two pixels from July 2014 to June 2015. The red line represents a melting pixel with a threshold of 190 K (red dot line) on the Larsen C Ice Shelf. The green line represents a frozen pixel with a threshold of 232 K (green dot line) on Palmer Land. reflection, the bare ice radar zone shows minimal change throughout the freeze/thaw cycles. The wet snow radar zone is dark because of the strong absorption and limited penetration of the C-band microwave; scattering occurs only at the wet-snow surface [28; 30]. Freezing and thawing repeatedly, the frozen percolation radar zone shows high winter backscatter intensities due to the ice layers and coarse grain in the snowpack. The wavelength of the C-band microwave is much greater than the snow grain size in dry snow packs, so the backscatter returns in the dry snow radar zone is weak, because of the low dielectric losses and deep penetration [2; 48; 49; 50]. The MEMLS38&a developed by [PERSON] et al. (2015) [51] was used in this study to simulate the backscatter of RGZs due to the limited observations in the AP. As an extension of the microwave emission model of layered snowpacks (MEMLS), MEMLS38&a applies six-flux theory to describe multiple volume scattering and absorption. The model outputs \(T_{b}\) and \(\sigma^{0}\) in the snowpacks by inputting the snow density, thickness, salinity, temperatures of interfaces, exponential correlation length, and liquid water content of each layer. Other general model parameters are also needed, including the snow-ground reflectivity, cross-polarization ratio, and mean slope of surface undulation. Simulations were in good agreement with in situ observations [51; 52; 53]. A set of controlled experiments was conducted using MEMLS38&a. We assume that the snowpacks are homogeneous in the vertical direction, and both the ground and snow temperatures are 273.15 K for slight snowmelt. The incidence angle was set to 30\({}^{\circ}\), and the corresponding sky brightness temperature for the C-band is 8 K [54]. Snow-ice reflectivity at 5.4 GHz was set to 0.15 and 0.6 in horizontal and vertical polarizations, respectively, according to a study on sea ice [55]. The mean slope of snow surface undulation and the cross-polarization ratio were set to 0.1 and 0.15, respectively [51]. The exponential correlation length in MEMLS38&a is used to describe the snow structure, which is a function of snow density and grain size [56; 57], and is illustrated as follows:  
\[P_{ex}=0.75\left(\frac{P_{snow}}{P_{ice}}-\frac{P_{snow}^{2}}{P_{ice}^{2}} \right)d \tag{3}\]  
where \(P_{ex}\) is the exponential correlation length, \(\rho_{snow}\) and \(\rho_{ice}\) are snow and ice density, respectively, and \(d\) represents the snow grain size. Ice density was set as 917 kg/m\({}^{3}\) in this study. Snow profiles designed in the simulations referenced a study that links the SAR data with snow pit measurements in the AP [19]. We simulated the changes of \(\sigma^{0}\) in the transitions from dry to wet snow regimes (with snow liquid water from 0 to 1% in volume). With snow density ranging from 350 kg/m\({}^{3}\) to 540 kg/m\({}^{3}\) and snow grain size ranging from 0.5 mm to 3 mm, different snow profiles were designed in the simulations. As shown in Figure 3a, \(\sigma^{0}\) changes slightly and maintains less than \(-6.5\) dB when snow depth was set as 1 cm (regarded as bare ice).