Retrieval of Optical Thickness and Effective Particle Radius of Thin Low-Level Water Clouds using the Split Window of Meteosat-8

Abstract

We developed a method to retrieve the optical thickness (τ) and effective particle radius (r_e) of water clouds using the split-window channels and the 8.7-µm channel of Meteosat-8. Valid ranges are approximately from 1 to 9 for τ and smaller than 18 µm for r_e. Water clouds were first identified using 8.7-µm and 11-µm data. The retrieval used the brightness temperature (TBB) and brightness temperature difference (BTD) between the split windows, as computed with the radiation code RSTAR5b for various properties of water clouds and vertical profiles of temperature and water vapor. Arch-shaped curves in the TBB and BTD domains resembled those for ice clouds, depending on τ and r_e. The retrieved cloud parameters were then compared to those retrieved by the solar reflection method, which uses the 0.6-, 3.9-, and 11-µm channels of Meteosat-8. Comparison of the two methods revealed that the split-window technique could capture spatial features for both τ and r_e, showing some agreement with r_e for the solar reflection but needing more improvement for τ in the current case study.