Topography Effects on Microwave Emission Transmission of Snowpack and Snow Depth Retrieval

Advancements in snow depth retrieval from passive microwave remote sensing data have mainly focused on influence of varying snow characteristics and forests, while neglecting the complicated mountainous terrain. Therefore, examining the influence of mountainous terrain on microwave radiation transmission of snowpack is beneficial for improvement of snow depth retrieval algorithms in mountainous areas. In this study, we established a microwave emission transfer model of snowpack in Mountainous areas within the framework of microwave emission transfer model of layered snowpack (MEMLS), thereafter, called MEMLS-T. MEMLS-T considers the influence of complicated terrain on the microwave radiation transmission of snowpack from three perspectives: 1) the varied hill slopes alter the local incidence angle; 2) the diverse hill slopes and aspects induce the polarization rotation; 3) the reduced sky visibility in mountainous regions results in an escalation of downward background radiation reaching the snow surface, as a consequence of the illumination from neighboring slopes. We simulate brightness temperatures at varying sky visibilities, slopes and aspects using MEMLS-T, and find that, in most conditions, the mountainous terrain weakens the brightness temperature difference between K and Ka bands (TbD), yet it also augment the TbD. The proportion of augmentation is approximately 8.9% for vertical polarization and 4.6% for horizontal polarization, and decreases as snow depth (or scattering intensity) increase or sky visibility decline. The uncertainties in snow depth retrieval in mountainous areas increase as snow depth increases. The brightness temperatures are simulated based on various spatial resolutions (30 m, 90 m, 270 m, 810 m) of the Digital Elevation Model (DEM) of Qilian mountain and integrated into a grid of 6.25 km×6.25 km. The results reveal that the coarser DEM results in less variation in brightness temperatures and TbD. Although the scale of DEM has a significant impact on the simulation of brightness temperature and TbD, its influence on average TbD at satellite scale is greatly reduced, resulting in approximately 3% variation in retrieved snow depth when using snow depth retrieval algorithm developed in flat terrain.