Radiative Effects of Clouds

Hartmann, D. L., Radiative Effects of Clouds on Earth s Climate , in Aerosol-Cloud-Climate Interactions (P. V. Hobbs, Ed.), pp. 151-173, Academic Press, San Diego, 1993. 

Walcek, C. J., W. R. Stockwell, and J. S. Chang, Theoretical Estimates of the Dynamic, Radiative, and Chemical Effects of Clouds on Tropospheric Trace Gases, Atmos. Res., 25, 53-69 (1990). 

Similarly, Chou et al. (1998) used measurements of surface radiative fluxes and satellite radiance data in the Pacific warm pool region to conclude that the effect of clouds was similar to that expected, i.e., that the excess absorption, if it exists, is small. 

Liao, H., and J. H. Seinfeld, Effect of Clouds on Direct Aerosol Radiative Forcing of Climate, J. Geophys. Res., 103, 3781-3788... 

Estimates of radiative forcing due to tropospheric ozone are listed in Table 2. CTM calculations of changes in tropospheric ozone and the radiative forcing are based on a variety of assumptions. Most models, but not all, include temperature adjustment in the stratosphere and effects of clouds. Neglect of either of these effects has been estimated to lead to an overestimate of radiative forcing by approximately 10-25% (Berntsen et al., 1997). 

Brenguier et al. (2003) discussed the results of eight series of aircraft measurements of the microphysical characteristics of marine stratocumulus clouds in a broad range of observation conditions (different physico-chemical properties of aerosol, number density values in the interval 50 cm 3-25 cm-3, etc.)- The unique complex of synchronous observations of the microphysical and radiative characteristics of cloud cover obtained can be used to assess the indirect impact of aerosol on clouds and climate based on analysis of the ratio between the cloud optical thickness and effective radius of cloud droplets. Correlation between these values is usually negative, but in a heavily polluted atmosphere it can be positive. From the observational data obtained during ACE-2, the polluted systems of clouds turned out to be somewhat drier and therefore thinner, resulting in the positive correlation between the indirect impact of aerosol on climate and the effective radius of droplets. 

The effect of aerosol on cloudiness consists in the following (i) a certain part of aerosols acts as condensation nuclei, causing an increase in the amount of suspended small droplets, and (ii) aerosol particles, caught by clouds, change the radiative properties of clouds and, consequently, the vertical temperature profile. In its turn, this affects the conditions of the formation of cloudiness and of its subsequent transformation. 

Two clond types, stratocnmnlns and cirras, cover the greatest portion of the earth s snrface and are therefore the most important clond types from the standpoint of the effect of clouds on climate. The focused observations reveal that stratocumulus have a net cooling effect on the planet. However, the net effect of cirrus on climate is still nnclear. The details of the cirrus cloud microphysics, including the mean, maximum, and effective (radiatively important) particle size, crystal shape, temperature, and height in the atmosphere, affect their radiative properties. This is an active area of research. 

If this excess absorption by clouds is ultimately shown to be a real phenomenon, then an increased cloud formation and extent due to anthropogenic emissions may alter the radiative balance of the atmosphere not only through increased reflectance but also through increased absorption of solar radiation. Such an effect could impact atmospheric temperatures, their vertical distribution, and circulation, as well as surface wind speeds and the surface latent heat flux (Kiehl et al., 1995). Hence establishing if this is truly excess absorption, and if so, its origins, is a critical issue that remains to be resolved. 

Andronache, C., L. J. Donner, V. Ramaswamy, C. J. Seman, and R. S. Hemler, The Effects of Atmospheric Sulfur on the Radiative Properties of Convective Clouds A Limited Area Modeling Study, Geophys. Res. Lett., 25, 4423-1426 (1998). 

Ghan, S. J., and R. C. Easter, Comments on A Limited-Area-Model Case Study of the Effects of Sub-Grid Scale Variations in Relative Humidity and Cloud upon the Direct Radiative Forcing of Sulfate Aerosol, Geophys. Res. Lett., 25, 1039-1040 (1998). 

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