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.

The Stratospheric Aerosol and Gas Experiment (SAGE) II satellite experiment measures the extinction due to aerosols and thin cloud, at wavelengths of 0.525 and 1.02/am, down to an altitude of 6 km. The wavelength dependence of the extinction due to aerosols differs from that of the extinction due to cloud and is used as the basis of a model for separating these two components. The model is presented and its validation using airborne lidar data, obtained coincident with SAGE II observations, is described. This comparison shows that smaller SAGE II cloud extinction values correspond to the presence of subvisible cirrus cloud in the lidar record. Examples of aerosol and cloud data products obtained using this model to interpret SAGE II upper tropospheric and lower stratospheric data are also shown. INTRODUCTIONThe Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation experiment was designed for the study of stratospheric aerosols and gases [Mauldin et al., 1985]. In the absence of high-altitude cloud, measurements are possible down into the troposphere, with a vertical resolution of 1 km [Kent et al., 1988, 1991]. The principal SAGE II aerosol wavelength is 1.02 /am, and if the atmosphere is clear of clouds, measurements are possible down to the Earth's surface. SAGE II has additional aerosol channels at shorter wavelengths. Use of these wavelengths is limited by increased atmospheric attenuation, but in the absence of clouds, data are obtainable at a wavelength of 0.525 /am, down to an altitude of 6 km. At tropospheric altitudes the extinction may be due to aerosol alone or to a combination of aerosol and thin cloud lying along the optical path from the Sun to the satellite. The effective length of this path in the atmosphere is of the order of 200 km, and the optical properties of the atmosphere along it may be quite inhomogeneous. Several publications have described the statistical properties of the 1-/am extinction values [Kent et al., 1988, 1991], and on the assumption that the higher extinction values are due to the presence of cloud, climatologies of upper tropospheric cloud have been developed [Woodbury and McCormick, 1983, 1986; Chiou et al., 1990]. These climatologies have used an arbitrary extinction level to separate aerosol from cloud, and although statistical comparisons with other cloud databases have been made, direct validation has not been possible. Recently, Kent and McCormick [1991] have discussed the functional relationship between the measured extinction at 1.02/am and at 0.525 /am and interpreted this in terms of a simple extinction model. In this model there is assumed to be little or no wavelength dependence of the extinction due to cloud, whereas that due to aerosol shows a strong wave-1Science and Technology Corporation, Paper number 93JD00340. 0148-0227/93/93 JD-00340505.00 length variation. The purpose of this paper is to present the development of the model for systematic application to the SAGE II data set. A series of airborne measurements made in April 1991 near Califo...

Abstract. A comparison of global stratospheric aerosol levels measured in 1979 by the Stratospheric Aerosol and Gas Experiment (SAGE) and in 1989-1991 by SAGE II is presented. These periods exhibit the lowest stratospheric aerosol levels in the era of modern measurements and are often referred to as background periods. We find that, depending on latitude, the 1-/,m aerosol optical depth in 1989-1991 was 10 to 30% higher than that observed in 1979. We demonstrate that the latter period (prior to the June 1991 eruption of Mount Pinatubo) was characterized by an ongoing global recovery from the eruptions of E1 Chich6n in 1982 and Nevado del Ruiz in 1985, with a further complication introduced by the February 1990 Kelut eruption. Therefore the differences between 1979 and 1989-1991 cannot be completely attributed to nonvolcanic sources.

Measurements of aerosol extinction at wavelengths of 0.525 and 1.02 μm, made by the Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation satellite experiment, have been used to study the global‐scale characteristics of the upper tropospheric aerosol. Extinction measurements, in which only aerosols occurred along the optical path, have been separated from those that included high‐altitude cloud by examining the wavelength variation of the extinction. Data for the time period October 1984 to May 1991 show that the two main influences on the upper tropospheric aerosol were seasonal lifting of material from below and downward transfer of volcanic aerosol from the stratosphere. Maximum lifting of surface material occurs in local spring in both hemispheres and is observed at all latitudes between 20°N and 80°N and 20°S and 60°S; the data also show a strong hemispheric asymmetry with more aerosol in the northern hemisphere. Downward transfer of volcanic aerosol is particularly observed poleward of 40° latitude, where a substantial enhancement of material occurs down to altitudes 2–3 km below the tropopause. By comparing tropospheric aerosol concentrations at different times during the period of observation, it has been possible to differentiate the effects of volcanic aerosols from those of the background, or baseline, aerosols. A simple model, based on the ratio of the extinctions at the two measurement wavelengths, has been used to calculate the aerosol mass density and effective radius. It was found that in 1984–1985, approximately 15% of the volcanic aerosol still present from the eruption of El Chichón in 1982 resided in the upper troposphere. Particle sizes for the volcanic aerosol in the lower stratosphere and upper troposphere were of the order of 0.5 μm, while those for the baseline aerosol were about 0.15 μm. Slightly larger aerosol sizes, of the order of 0.25 μm, were observed at altitudes 6–8 km during the springtime enhancements. The low‐latitude aerosol enhancements in both hemispheres appear to have the characteristics of material derived from arid surface regions, while the higher‐latitude aerosol in the northern hemisphere appears more likely to be derived from anthropogenic sources.

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