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 (re) 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 τand smaller than 18 μm for re. 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. Archshaped curves in the TBB and BTD domains resembled those for ice clouds, depending on τand re. 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 τand re, showing some agreement with re for the solar reflection but needing more improvement for τin the current case study.