Quality Assessment of Cloud Detection and Cloud Top Height Products from FY-3D/MERSI-II in Polar Regions

The Medium Resolution Spectral Imager II (MERSI-II) is a core payload onboard the Fengyun-3D (FY-3D) satellite. It is designed for monitoring atmospheric and environmental conditions and is crucial for retrieving key parameters such as cloud and aerosol properties. This study aims to evaluate the cloud detection and cloud top height (CTH) retrieval accuracy of FY-3D/MERSI-II in polar regions (Arctic and Antarctic) against the CALIOP (Cloud–Aerosol Li-dar with Orthogonal Polarization) cloud products. The results demonstrate that MERSI-II-derived cloud coverages in the Arctic are 7.2% lower than CALIOP observations, while they are overestimated (underestimated) in Antarctic in summer (other seasons). Seasonal variations in cloud detection accuracy are notable, with Arctic accuracy reaching 90.4% in summer but declining to 51.3% in winter. The detection accuracy is consistently lower in Antarctic than in Arctic, though MERSI-II shows significantly better detection capability over water surfaces than snow/ice-covered areas. For CTH retrievals, correlation coefficients between MERSI-II and CALIOP reach 0.70 (Arctic) and 0.71 (An-tarctic), with systematic underestimations of −0.46 km (Arctic) and −1.52 km (Antarctic) on average. Larger deviations occur in multi-layer clouds and ice cloud conditions. Notably, MERSI-II tends to overestimate (underestimate) CTH for low-level (high-level) clouds and demonstrates superior retrieval accuracy for water clouds than ice clouds. These findings provide robust data support for satellite algorithm optimization and polar environment monitoring.