CLOUD-SCREENING FOR AFRICA USING A GEOGRAPHICALLY AND SEASONALLY VARIABLE INFRARED THRESHOLD

A spatially variable monthly, infrared cloud-threshold data base has been applied to screen cloud-contaminated observations from radiances measured by the NOAA-9 Advanced Very High Resolution Radiometer (AVHRR) over Africa. The infrared threshold for cloud identification was based on the method of Stowe et al. (1988) developed to retreive cloud amount and height from measurements made onboard the Nimbus-7 satellite, which uses surface air temperatures, with an empirical correction for water vapour attenuation, and adjustments for the spatial variance of surface temperature and water vapour. A comparison is made between this threshold and a constant threshold of 287 K for Africa for the purpose of cloud-screening in computing the normalized difference vegetation index (NDVI) from AVHRR data. Based on comparisons for a single day, a ten-day composite and a monthly composite for August 1985, the Nimbus-7 variable infrared (IR) threshold screens many cloud-contaminated measurements that the 287 K threshold does not remove. The variable IR threshold was found to be effective over persistently cloud-covered regions, such as the coastal region of the Gulf of Guinea, but may introduce some erroneous cloud identifications over mountains.