COMPARISON BETWEEN SAGE-II AND ISCCP HIGH-LEVEL CLOUDS .2. LOCATING CLOUD TOPS

A comparison is made of the vertical distribution of high-level cloud tops derived from the Stratospheric Aerosol and Gas Experiment II (SAG E II) occultation measurements and from the International Satellite Cl oud Climatology Project (ISCCP) for all Julys and Januarys in 1985 to 1990. The results suggest that ISCCP overestimates the pressure of hig h-level clouds by up to 50-150 mbar, particularly at low latitudes. Th is is caused by the frequent presence of clouds with diffuse tops (>50 % time when cloudy events are observed). The averaged vertical extent of the diffuse top is about 1.5 km. At midlatitudes where the SAGE II and ISCCP cloud top pressure agree best, clouds with distinct tops rea ch a maximum relative proportion of the total level cloud amount (abou t 30-40%), and diffuse-topped clouds are reduced to their minimum(30-4 0%). The ISCCP-defined cloud top pressure should be regarded not as th e material physical height of the clouds but as the level which emits the same infrared radiance as observed. SAGE II and ISCCP cloud top pr essures agree for clouds with distinct tops. There is also an indicati on that the cloud top pressures of optically thin clouds not overlying thicker clouds are poorly estimated by ISCCP at middle latitudes. The average vertical extent of these thin clouds is about 2.5 km.