A cloud-detection scheme for use with satellite sounding radiances in the context of data assimilation for numerical weather prediction

A scheme for detecting cloud-affected radiances is described. The method is used to determine the probability of cloud-free conditions given the obser vations and the prior knowledge we have about the atmosphere from a numeric al weather prediction (NWP) model. This is achieved using a likelihood meth od. It combines the strengths of some alternative methods (e.g. comparison of infra-red and microwave channels sounding the lower troposphere and comp arison of infra-red window channels with sea surface temperature) in a powe rful and flexible method. It is powerful because it uses different types of information simultaneously. It is flexible because it makes no assumption about which instrument is being processed, or what type of prior informatio n (NWP, climatology etc.) is used. Therefore, it can readily be extended to new situations and data types (e.g. Advanced TIROS Operational Vertical So under (ATOVS)). It is suitable for use on general cloud-detection problems, using combined microwave and infra-red data. It has been tested using TIRO S Operational Vertical Sounder (TOVS) radiances. The new method has been co mpared with an alternative cloud-detection method tailored specifically for TOVS and has been developed to a level of robustness adequate for operatio nal use. The new method gave very similar results to the alternative method , especially over the ocean. The differences that did occur have been inves tigated by comparing with cloud information derived from the Advanced Very High Resolution Radiometer (AVHRR). Both the alternative method and the new scheme were found to have deficiencies when dealing with very low cloud. S ome cloud missed by the existing scheme is identified by the new scheme. Ov er land, cloud detection is more difficult. The two schemes disagree more o ften, but validation using AVHRR is also more difficult because of increase d surface heterogeneity and more variable emissivity and surface temperatur e errors. The new method is therefore shown to perform at least as well as an alternative method in operational use, whilst gaining the flexibility re quired for future systems. The implications for ATOVS are discussed.