A 6-year climatology of cloud occurrence frequency from stratospheric aerosol and gas experiment II observations (1985-1990)

A 6-year climatology of subvisual and opaque cloud occurrence frequencies is established using observations from the Stratospheric Aerosol and Gas Experiment (SAGE) II between 1985 and 1990. The subvisual clouds are observed mostly at high altitudes near the tropopause. The opaque clouds terminate the profiling, reducing the measurement frequency of the SAGE II instrument in the troposphere. With its l-km vertical resolution, the climatology shows many interesting features, including (1) the seasonal expansion and migration behavior of the subvisual and opaque cloud systems; (2) the association of the zonal mean cloud frequency distributions with the tropospheric mean circulation (Hadley and Ferrel cells); (3) the tropical cloud occurrence that follows the equatorial circulation, including the Walker circulation over the Pacific Ocean; and (4) the overall higher cloud occurrence in the northern hemisphere than in the southern hemisphere. The radiative impact of subvisual clouds is estimated to be a 1-W m(-2) reduction in outgoing longwave radiation. The maximum overall effect is a net positive cloud forcing of 0.5-1 W m(-2) in the tropics. During the 1987 El Nino-Southern Oscillation (ENSO), cloud frequency was generally enhanced in the tropics and midlatitudes and reduced in the subtropics and high latitudes. The present study shows a distinct negative correlation between the high-altitude-cloud occurrence and the lower stratospheric water vapor mixing ratio in the tropics, providing intrinsic evidence on the delicate connection between the stratospheric-tropospheric exchange and dehydration processes and the high-altitude cloud activities.