CLOUD FIELD IDENTIFICATION FOR EARTH RADIATION BUDGET STUDIES .2. CLOUD FIELD CLASSIFICATION FOR THE SCARAB RADIOMETER

Gaining a better understanding of the influence of clouds on the earth 's energy budget requires a cloud classification that takes into accou nt cloud height, thickness, and cloud cover. The radiometer ScaRaB (sc anner for radiation balance), which was launched in January 1994, has two narrowband channels (0.5-0.7 and 10.5-12.5 mu m) in addition to th e two broadband channels (0.2-4 and 0.2-50 mu m) necessary for earth r adiation budget (ERB) measurements in order to improve cloud detection . Most automatic cloud classifications were developed with measurement s of very good spatial resolution (200 m to 5 km). Earth radiation bud get experiments (ERBE), on the other hand, work at a spatial resolutio n of about 50 km (at nadir), and therefore a cloud field classificatio n adapted to this scale must be investigated. For this study, ScaRaB m easurements are simulated by collocated Advanced Very High Resolution Radiometer (AVHRR) ERBE data. The best-suited variables for a global c loud classification are chosen using as a reference cloud types determ ined by an operationally working threshold algorithm applied to AVHRR measurements at a reduced spatial resolution of 4 km over the North At lantic. Cloud field types are then classified by an algorithm based on the dynamic clustering method. More recently, the authors have carrie d out a global cloud field identification using cloud parameters extra cted by the 3I (improved initialization inversion) algorithm, from Hig h-Resolution Infrared Sounder (HIRS)-Microwave Sounding Unit (MSU) dat a. This enables the authors first to determine mean values of the vari ables best suited for cloud field classification and then to use a max imum-likelihood method for the classification. The authors find that a classification of cloud fields is still possible at a spatial resolut ion of ERB measurements. Roughly, one can distinguish three cloud heig hts and two effective cloud amounts (combination of cloud emissivity a nd cloud cover). However, only by combining flux measurements (ERBE) w ith cloud field classifications from sounding instruments (HIRS/MSU) c an differences in radiative behavior of specific cloud fields be evalu ated accurately.