Retrieval of optical thickness and effective radius of low-level water cloud using the split window data of Meteosat-8

We studied the optical properties of low-level water clouds using the multi-channel data of Meteosat-8. The split window data are used for retrieval of optical thickness and effective particle radius for optically thin water clouds. The idea of the method is that optically thin water clouds also show larger values of brightness temperature difference (BTD) between the split window (11ƒÝm and 12ƒÝm) as known for cirrus cloud. One of the advantages of this method is the equal retrieval accuracy for day and night. The optical thickness and effective particle radius can be retrieved using the method called solar reflection method which uses the visible, 3.7ƒÝm and 11ƒÝm. Here, we first classified water clouds using the 11ƒÝm and 8.7ƒÝm. Then we compared the optical thickness retrieved by the solar reflection method and BTD. We found the BTD showed good relationship with optical thickness for the water clouds whose optical thickness is less than 6. The BTD becomes larger for optically thinner water clouds. We constructed the BTD-TBB diagram for water clouds that are similar to the diagram for cirrus cloud. The diagram showed that BTD was function of optical thickness and effective particle radius when we know the cloud temperature. Therefore, we first identified optically thinner part and thicker part of water cloud using the 8.7ƒÝm and split window. Then we retrieved optical thickness and effective particle radius for optically thinner water clouds using the BTD-TBB diagram. The retrieved optical thickness and effective radius for optically thin water cloud showed reasonable agreement with the solar reflection method.