Assessment of FY-3F MERSI-III Cloud Optical Thickness and Cloud Effective Radius Over Polar Regions

Abstract

Polar clouds play a crucial role in Arctic amplification and the surface radiation budget in a warming climate. However, accurate retrievals of cloud optical thickness (COT) and cloud effective radius (CER) over polar regions remain challenging because snow and ice surfaces have high and variable reflectance, solar-viewing geometries are often extreme, and satellite radiometric accuracy strongly affects retrieval performance. This article extends a bi-spectral COT/CER retrieval algorithm for the MEdium Resolution Spectral Imager-III (MERSI-III) onboard Fengyun-3F (FY-3F), with an emphasis on polar applications. To reduce surface contamination from snow and ice, the algorithm uses the 1.64 and 2.13 μm channels instead of the conventional visible/shortwave-infrared channel combination. A sensor-specific reflectance recalibration scheme based on cloudy targets is introduced to correct radiometric biases in the channels used for retrieval, especially the 1.64 μm channel, which shows approximately 15% radiometric degradation over one year. The optimized MERSI-III retrievals show reasonable agreement with MODIS operational products in terms of both spatial distributions and magnitudes. Cross-seasonal comparisons from 2024 to 2025 yield correlation coefficients of 0.77 for COT and 0.87 for CER relative to MODIS products, comparable to the consistency obtained over mid- and low-latitude regions. These results demonstrate the capability of FY-3F MERSI-III for polar cloud optical and microphysical property retrievals and provide a basis for extending Fengyun operational cloud products toward high-latitude and polar coverage.