Cloud detection and classification for earth radiation budget experiments

Abstract

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), launched in 1993, has in addition to the two broad-band channels (0.2 - 4 micrometers and 0.2 - 50 micrometers ) necessary for earth radiation budget (ERB) measurements, two narrow-band channels (0.5 - 0.7 micrometers and 10.5 - 12.5 micrometers ) in order to improve cloud detection. Most automatic cloud classifications have been developed with measurements of very good spatial resolution (200 m to 5 km). Earth radiation budget experiments, on the other hand, work at a spatial resolution of about 40 km (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 4 km 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 40 km, 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.