Comparison of Cloud Optical Thickness of GK-2A and HIMAWARI Satellites by Using UV-A Irradiance in Daejeon

Cloud optical thickness, also known as cloud optical depth, is an indicator that quantitatively shows the attenuation effect of solar radiance by clouds in the atmosphere and is calculated through statistical models or the radiative transfer model of each satellite using its observation data and ancillary data. Therefore, even for observations performed at the same location and time, the retrieved COT differ from satellite to satellite. Thus, this study aims to verify the retrieved COT of the GK-2A and HIMAWARI satellites, which are Korean and Japanese geostationary satellites, respectively, covering the Korean Peninsula. To verify the COT data, the method used by Qin et al. (2019) was used, where the COT was indirectly verified by calculating downward radiation using satellite-retrieved data such as COT, cloud phase, and cloud top pressure as parameters and comparing this with ground-observed radiation. In this study, a radiative transfer model, libRadtran 2.0.4, was used to calculate UV-A radiation and compare it with observed data in Daejeon as the true value for the cloud phase retrieved from each satellite. When comparing the COT from both satellites directly, the values from HIMAWARI tended to be larger than the data from GK-2A. A comparison of the UV-A radiation shows that the observed values are seemingly larger than the satellite results, indicating that both satellites may overestimate the cloud optical depth. Additionally, when both satellites were estimated to have the same cloud phase, HIMAWARI showed better parameters for RMSE and MAE, whereas GK-2A was better when GK-2A and HIMAWARI had different cloud phase estimates. By comparing the freezing level from the vertical profile and the cloud top height from each satellite, the actual cloud phase was estimated, which showed that GK-2A had better performance in estimating cloud phases.