Abstract
The imagery from the AVHRR on board NOAA polar orbiting satellites allows a description of cloud cover, oceanic, and continental surfaces that is used by Météo-France for nowcasting activities and as input for numerical weather prediction models (NWP). A real-time processing scheme has been designed at the Centre de Météorologie Spatiale (CMS) in Lannion to extract cloud cover and surface parameters from NOAA-11 AVHRR imagery received at CMS. The key step of this scheme is cloud detection. It is based upon threshold tests applied to different combinations of channels. Its main originality is its complete automation by the computation of the 11 μm infrared threshold from a monthly sea surface temperature (SST) climatology over the oceans and from air temperature (near the surface) forecast by NWP over land. A special test has been implemented to detect cloud edges and subpixel clouds over continental surfaces during daytime. It is applied daily in deferred time only to compute normalized difference vegetation index (NDVI). This scheme has been used operationally since February 1990, and its quality has been checked. It has enabled the routine production of various products. A nighttime cloud classification is sent to all French forecasters; NDVI values are computed daily and used to map the vegetation cover; and SST and thermal fronts are derived operationally from nighttime imagery.
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