Comparative Study on the Different Downscaling Methods for GPM Products in Complex Terrain Areas

Fine spatial information of precipitation plays a significant role in regional eco-hydrological studies but remain challenging to derive from satellite observations, especially in complex terrain areas. Sichuan Province, located in the southwest of China, has a highly variable terrain, and the spatial distribution of precipitation exhibits extreme heterogeneity and strong autocorrelation. Multi-scale Geographically Weighted Regression (MGWR) and Random Forest (RF) were employed for downscaling the Global Precipitation Measurement Mission (GPM) products based on high spatial resolution terrain, vegetation, and meteorological data in Sichuan province, and their specific effects on gauged precipitation accuracy and spatial precipitation distributions have been analyzed based on the influences of environmental variables. Results show that the influence of each environmental factor on the distribution of precipitation at different scales was well represented in the MGWR model. The downscaled data showed good spatial sharpening effects; additionally, the biases in the overestimated region were well corrected after downscaling. However, when based on spatial autocorrelation and considering adjacent influences, the MGWR performed poorly in correcting outlier sites adjacent to the high–high clusters. Compared with MGWR, relying on independently constructed decision trees and powerful regression capabilities, superior correction for outlier sites has been achieved in RF. Nevertheless, the influence of environmental variables reflected in RF differs from actual conditions, and detailed characteristics of precipitation spatial distribution have been lost in the downscaled results. MGWR and RF demonstrate varying applicability when downscaling GPM products in complex terrain areas, as they both improve the ability to finely depict spatial information but differ in terms of texture property expression and precipitation bias correction.