A global stratospheric aerosol database employed for climate simulations is described. For the period 1883–1990, aerosol optical depths are estimated from optical extinction data, whose quality increases with time over that period. For the period 1850–1882, aerosol optical depths are more crudely estimated from volcanological evidence for the volume of ejecta from major known volcanoes. The data set is available over Internet.

Current uncertainties in the effects of aerosols and clouds on the Earth radiation budget limit our understanding of the climate system and the potential for global climate change. The CALIPSO satellite will use an active lidar together with passive instruments to provide vertical profiles of aerosols and clouds and their properties which will help address these uncertainties. CALIPSO will fly in formation with the EOS Aqua and CloudSat satellites and the other satellites of the Aqua constellation. The acquisition of simultaneous and coincident observations will allow numerous synergies to be realized by combining CALIPSO observations with complementary observations from other platforms. In particular, cloud observations from the CALIPSO lidar and the CloudSat radar will be complementary, together encompassing the variety of clouds found in the atmosphere, from thin cirrus to deep convective clouds. CALIPSO is being developed within the framework of a collaboration between NASA and CNES and is scheduled for launch in 2004.

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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 1‐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 Niño‐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.

Abstract. Ten data sets coveting the period 1954-2000 are analyzed to show a 1%/yr increase in stratospheric water vapor. The trend has persister for at least 45 years, hence is unl•ely the result of a single event, but rather indicative of long-term climate change. A long-term change in the transport ofwater vapor into the stratosphere is the most probable cause.