Upper Level Cloud Climatology from an Orbiting Satellite

Abstract

The Selective Chopper Radiometer on Nimbus 5 includes two narrow-band channels in the 2.7 μm absorption bands of carbon dioxide and water vapor. Because of strong absorption by these two constituents, no radiation reflected by the surface or low and mid level clouds is detected by the instrument, but for clouds higher than 6 km, a differential absorption technique can be used to determine cloud heights and reflectances (or amounts). A set of empirical equations is derived and used to analyze two and one-half years of Nimbus 5 data from December 1972 launch. Distributions of upper level clouds are derived on a 10° latitude-longitude grid and average monthly and seasonal maps of cloud cover are given. Reliable distributions are obtained in both the tropics and the temperate latitudes of the summer hemisphere, but in other situations the quality decreases as the relevant path lengths increase due to both higher solar zenith angles and lower “upper level” clouds. The maps are highlighted by the excessive cover in the region of the southeast Asian monsoon and the persistently clear area in the eastern Pacific associated with the descending arm of the Walker Circulation of the Southern Oscillation. As an example o year-to-year variation the monthly distributions for each January are discussed in relation to the 1972–73 El Niño phenomenon.