# Mapping Radar Glacier Zones and Dry Snow Line in the Antarctic Peninsula Using Sentinel-1 Images
## 5 Discussion  
Simulations of MEMLS3&a suggest that C-band \(\sigma^{0}\) with the incidence angle of 30\({}^{\circ}\) in wet snowpacks decreased for at least 4 dB compared with dry snow regime in the AP. The threshold is close to the value of 3 dB, which has been successfully used in Radarsat and ERS [(32; 61)]. Horizontally polarized \(\sigma^{0}\) increases and decreases with snow depth in coarse-grained and fine-grained snowpacks, indicating a clear boundary between frozen percolation and dry snow radar zones. The transition from bright to dark can be easily recognized at a threshold of \(-6.5\) dB, according to the simulations (Figure 4). This value agrees well with \(-6\) dB used in [PERSON] et al. (2000)'s work in the AP [(62)]. Backscatter decreases quickly from the frozen percolation to dry snow zone because of the differences in snow structure. As can be seen in Figure 10, \(\sigma^{0}\) drops rapidly from \(-5\) dB to \(-10\) dB in the transition, suggesting that the threshold is reasonable within a range. A lower threshold of \(-8\) dB was used by [PERSON] et al. [(8; 34)]. The result was supported by the higher winter \(\sigma^{0}\) with the increased summer melting days derived by SSM/I data (see in Figures 7b and 10). However, this trend is not significant when the number of summer melting days is higher than 50, which may result from the increase of specular reflections from ice-saturated fir layers in winter [(63)]. The model error of MEMLS3&a, and some assumptions we made, may lead to the uncertainties in the simulations. For example, the model assumes that the layer interfaces are smooth. Some empirical parameters in the model also need further verification [(51; 53)]. In the AP, snow profiles are much more complex than what we set in the simulations. RGZs were classified on SAR images based on their differences in the liquid snow content, snow grain size, and the relative elevations. The temporal and spatial distributions of RGZs in the middle of the AP were mapped based on nine repeat-pass Sentinel-1 images. Compared with snowmelt detected by radiometer, the SAR-derived melting signals show a great improvement of spatial resolution, and can show detailed patterns of snowmelt. Snowmelt expanded from the grounding zone of the Larsen C Ice Shelf to the coastal area, but from the coastal area to the inland for the George VI Ice Shelf. The phenomenon agreed well with the melting days calculated from SSM/I  
Figure 12: Boxplot of the DSL elevations at a 0.5\({}^{\circ}\) interval of latitude on the Palmer land. data (Figure 6b). Fohn winds may explain the relatively long melting seasons in the western Larsen C Ice Shelf (Figures 6b and 8). Low relative humidity of the air mass makes snowmelt or sublimation easier due to the adiabatically-dried fohn winds on the east side of the AP, leading to its ponding and thinning [32; 64]. The wet snow radar zone detected by Sentinel-1 was smaller than the melt area derived from SSM/I data for several reasons. More extensive snowmelt was recognized by the SSM/I because of the dual observations. The much coarser spatial resolution of the SSM/I data also matters. A melt area of 625 km\({}^{2}\) may be counted when only a small patch in the SSM/I pixel experiences snowmelt. The melting thin snow in periods of higher air temperatures would expose bare ice [65], leading to the underestimate of melt area on SAR images. Minor misclassifications of RGZs exist in our study because of the special imaging mode of SAR. Figure 13a shows the misclassifications of the crevasses and mountains in Agassiz Cape on 14 November 2015. Crevasses with weak backscatter returns on ice shelves may be mistaken for bare ice (Figure 13b). Similarly, for the mountainous areas, the very high local incidence angles of the back slopes in the range direction lead to much lower \(\sigma^{0}\) (Figure 13c). These places were mistaken for bare ice or dry snow with the elevations below or above 500 m. It is also difficult to map the DSL in glaciers flowing through narrow valleys [34].