On the validity of using ground-based observations to validate geostationary-satellite-derived direct and diffuse surface solar irradiance: Quantifying the spatial mismatch and temporal averaging issues

Abstract

This paper discusses the differences, due to different measuring mechanisms, of satellite-derived and ground measured surface solar irradiance (SSI) direct and diffuse components, and the impacts of the differences when validating satellite-derived data using ground measurements, using SSI products derived from the Advanced Himawari Imager. The issue of satellite-ground mismatch, though previously recognized, has not been given sufficient attention in the relevant scientific literature due to: (i) satellite SSI have mostly focused on relatively longer-term data, such as hourly, daily and longer; and/or (ii) most studies assessed only global (or total) irradiance. Also, studies addressing this satellite-ground mismatch issue often require a dense ground network to characterize the spatial representativeness of ground measurements, which likely limited performing such studies. With shorter-term satellite SSI data, such as 10-min or less, and the direct and diffuse components becoming increasingly available, further investigation of this satellite-ground mismatch issue is warranted. Our analysis, using direct and diffuse component SSI observations from satellite and sparse ground sites showed that a large part of apparent validation errors may have been due to the satellite-ground mismatch, often exceeding 50% in the case of 10-min direct normal irradiance. Greater appreciation and understanding of this issue should lead to more objective assessment of the real quality of satellite SSI products. Our study also showed that several factors, such as spatial averaging, temporal averaging, solar zenith angle and view zenith angle, all have often substantial impacts on the validation outcomes, over 100 W/m2 in some cases, highlighting the necessity for the ground- and satellite-based solar radiation communities to co-develop protocols for satellite SSI data validation.