A 6-YEAR CLIMATOLOGY OF CLOUD OCCURRENCE FREQUENCY FROM STRATOSPHERICAEROSOL AND GAS EXPERIMENT-II OBSERVATIONS (1985-1990)

A 6-year climatology of subvisual and opaque cloud occurrence frequenc ies is established using observations from the Stratospheric Aerosol a nd Gas Experiment (SAGE) II between 1985 and 1990. The subvisual cloud s are observed mostly at high altitudes near the tropopause. The opaqu e clouds terminate the profiling, reducing the measurement frequency o f the SAGE II instrument in the troposphere. With its l-km vertical re solution, 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 freq uency distributions with the tropospheric mean circulation (Hadley and Ferrel cells); (3) the tropical cloud occurrence that follows the equ atorial circulation, including the Walker circulation over the Pacific Ocean; and (4) the overall higher cloud occurrence in the northern he misphere than in the southern hemisphere. The radiative impact of subv isual clouds is estimated to be a 1-W m(-2) reduction in outgoing long wave radiation. The maximum overall effect is a net positive cloud for cing 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 tro pics 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 deli cate connection between the stratospheric-tropospheric exchange and de hydration processes and the high-altitude cloud activities.