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1995 .

(4 publications)

O. Boucher and T. L. Anderson. General circulation model assessment of the sensitivity of direct climate forcing by anthropogenic sulfate aerosols to aerosol size and chemistry. Journal of Geophysical Research, 100:26117, December 1995. [ bib | DOI | ADS link ]

Climate response to atmospheric changes brought about by human activity may depend strongly on the geographical and temporal pattern of radiative forcing [Taylor and Penner, 1994]. In the case of aerosols stemming from anthropogenic sulfur emissions, geographical and temporal variations are certainly caused by variations in local mass concentration [Charlson et el., 1991; Kiehl and Briegleb, 1993], but could also arise from variations in the optical properties of sulfate aerosols. Since optical properties (including their relative humidity (RH) variation) depend fundamentally on aerosol size and chemical form and since size and chemical form are features of the aerosol which are not likely to be modeled on the global scale in the near future, geographical and temporal variations in optical properties could represent a stumbling block to accurate climate change forecasts. While extensive measurements of aerosol optical properties are needed to fully assess this problem, a preliminary assessment can be gained by considering the sensitivity of climate forcing to realistic variations in sulfate aerosol size and chemical form. Within a plausible set of assumptions (sulfate aerosol resides in the accumulation mode size range and only interacts with water vapor and ammonia vapor), we show that this sensitivity is fairly small (20%). This low sensitivity derives from a number of compensating factors linking the three optical parameters identified by Charlson et al. [1991]. By implication, these optical parameters, low RH scattering efficiency, the ratio of hemispheric backscatter to total scatter, and the RH dependence of scattering efficiency, should not be treated independently in either theoretical or experimental investigations of direct climate forcing. A suggested logical focus for such investigations is the backscatter efficiency at high RH. If borne out by future research, low sensitivity to sulfate aerosol size and chemistry would mean that direct sulfate climate forcing can be incorporated in global climate models with only a knowledge of sulfate mass concentration. We emphasize, therefore, the need to study the extent to which our assumptions break down, in particular, the fraction of anthropogenic sulfate that forms on coarse mode particles (i.e., those with diameters 1 μm) and the extent and effects of sulfate interactions with other accumulation mode components. Finally, we find that a significant fraction of direct aerosol forcing occurs in cloud-covered regions, according to a simple bulk parameterization.

O. Boucher, H. Le Treut, and M. B. Baker. Precipitation and radiation modeling in a general circulation model: Introduction of cloud microphysical processes. Journal of Geophysical Research, 100:16395, August 1995. [ bib | DOI | ADS link ]

Cloud microphysical processes are introduced in the precipitation parameterization of a general circulation model (GCM). Three microphysical processes are included in this representation of warm cloud precipitation: autoconversion of droplets, collection of droplets by falling raindrops, and evaporation of raindrops falling in clear sky. The mean droplet radius, r, is calculated from the cloud water mixing ratio, which is computed in the model, and the cloud droplet number concentration, N, which is prescribed. The autoconversion rate is set to zero for r r0, a prescribed threshold mean droplet radius. We investigate the model sensitivity to r0 and to N, the cloud droplet concentration, which is linked to the concentration of cloud condensation nuclei and is likely to vary. We find that an increase in N leads to an increase in the amount of cloud water stored in the atmosphere. In further experiments the mean droplet radius used in the parameterization of cloud optical properties is calculated in the same way as in the precipitation parameterization in order to bring more consistency between the different schemes. We again investigate the model sensitivity to r0 and to N and we find that an increase in N significantly enhances cloud albedo.

O. Boucher and U. Lohmann. The sulfate-CCN-cloud albedo effect. Tellus Series B Chemical and Physical Meteorology B, 47:281, July 1995. [ bib | DOI | ADS link ]

Aerosol particles, such as sulfate aerosols, can act as cloud condensation nuclei (CCN). The CCN spectrum and the water vapour supply in a cloud determine the cloud droplet number concentration (CDNC) and hence the shortwave optical properties of low-level liquid clouds. The capability of anthropogenic aerosols to increase cloud reflectivity and thereby cool the Earth's surface is referred to as the indirect effect of anthropogenic aerosols. To obtain an estimate of this effect on climate, we empirically relate the CDNC, and thus the cloud optical properties, of two general circulation models (GCM) to the sulfate aerosol mass concentration derived from a chemical transport model. Based on a series of model experiments, the normalized globally averaged indirect forcing is about - 1W/m2 and ranges from 0.5 to - 1.5W/m2 in both GCMs for different experiments. However, it is argued that the total uncertainty of the forcing is certainly larger than this range. The overall agreement between the two climate models is good, although the geographical distributions of the forcing are somewhat different. The highest forcings occur in and off the coasts of the polluted regions of the Northern Hemisphere. The regional distribution of the forcing and the land/sea contrast are very sensitive to the choice of the CDNC-sulfate mass relationship. The general patterns of the forcing, and the appropriateness of the different CDNC-sulfate mass relationships, are assessed. We also examine the simulated droplet effective radii and compare them with satellite retrievals.

O. Boucher. GCM Estimate of the Indirect Aerosol Forcing Using Satellite-Retrieved Cloud Droplet Effective Radii. Journal of Climate, 8:1403-1409, May 1995. [ bib | DOI | ADS link ]

In a recent paper, Han et al. analyzed satellite data radiances to retrieve cloud droplet effective radii and reported significant interhemispheric differences for both maritime and continental clouds. The mean cloud droplet radius in the Northern Hemisphere is smaller than in the Southern Hemisphere by about 0.7 [mgr]m. This hemispheric contrast suggests the presence of an aerosol effect on cloud droplet size and is consistent with higher cloud condensation nuclei number concentration in the Northern Hemisphere due to anthropogenic production of aerosol precursors. In the present study, we constrain a climate model with the satellite retrievals of Han et al. and discuss the climate forcing that can be inferred from the observed distribution of cloud droplet radius. Based on two sets of experiments, this sensitivity study suggests that the indirect radiative forcing by anthropogenic aerosols could he about 0.6 or 1 W m2 averaged in the 0deg-50degN latitude band. The uncertainty of these estimates is difficult to a assess but is at least 50%.

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