Cloud detection and classification for Earth Radiation Budget Experiments

In order to get a better understanding of the influence of clouds on the Earth's energy budget, one needs a cloud classification taking into account cloud height, thickness and cloud cover. The radiometer ScaRaB (Scanner for Radiation Balance), which will be launched in 1993, has in addition to the two broad-band channels (0.2-4μm and 0.2-50μm) necessary for Earth Radiation Budget (ERB) measurements, two narrow-band channels (0.5-0.7μm and 10.5-12.5μm) in order to improve cloud detection. Most automatic cloud classifications have been developed with measurements of very good spatial resolution (200m to 5km). Earth Radiation Budget Experiments, on the other hand, work at a spatial resolution of about 40km (at nadir), and therefore we investigated a cloud classification algorithm adapted on this scale. The algorithm is based on the dynamic clustering method and uses co-located AVHRR-ERBE data, simulating the ScaRaB measurements. This cloud field classification is compared on one hand to results obtained by a well tested threshold algorithm using AVHRR (Advanced Very High Resolution Radiometer) measurements at reduced spatial resolution of 4km and on the other hand to cloud parameters extracted from HIRS(High Resolution Infrared Sounder)/MSU(Microwave Sounding Unit) data. We find that classification of cloud fields is still possible at a resolution of 40km, and by combining AVHRR, ERBE and HIRS/MSU measurements one can undertake interesting studies on the influence of different cloud fields on the Earth radiation budget.