Enhanced Shortwave Cloud Radiative Forcing Due To Anthropogenic Aerosols

Abstract

Berkshire RG12 2SY C K It has been suggested,, originally by Twomey (SCEP, 1970), that anthropogenic aerosols in the troposphere can influence the microphysical properties of clouds and in turn their reflectivity (albedo), thereby exerting a radiative influence on climate. This chapter presents the theoretical basis for of this so-called indirect forcing and reviews pertinent observational evidence and climate model calculations of its magnitude and geographical distribution. We restrict consideration to liquid-water clouds, in part because these clouds are the principal clouds thought to be influenced by anthropogenic aerosols, and in part because the processes responsible for nucleation of ice clouds are not sufficiently well understood to permit much to be said about any anthropogenic influence. The argument for anthropogenic influence on cloud albedo rests on the premise that aerosol particle number concentrations are substantially increased by industrial emissions. As a consequence, the number concentration of cloud droplets Ned, which is determined by the number concentration of aerosol particles in the precloud air, is also increased. This in turn leads to an enhanced multiple scattering of light within clouds isnd to an increase in the optical depth and albedo of the cloud. In contrast the physical thickness, liquid water content, and liquid water path of the cloud, which are governed to close approximation by large-scale thermodynamics, are considered to be unchanged oir at least not greatly influenced by the increase in cloud droplet concentration. There is evidence that this assumption may not be entirely correct. In particular there is indication that the decrease in drop size may inhibit precipitation development, increasing cloud liquid water content and cloud lifetime, both of which effects would contribute 1 further to reflection of solar radiation by clouds; this phenomenon is also considered here.The present chapter reviews the theory of the indirect forcing phenomenon and estimates of its magnitude, field measurements addressing aspects of this phenomenon, and approaches to describing this phenomenon in climate models. This review is necessarily fairly selective. For more extended accounts see Hobbs (1993), Andreae (1995), andHeintzenberg (1995). THEORETICAL BASIS OF INDIRECT FORCINGAerosol influences on cloud albedo and shortwave forcing. The theoretical basis for a dependence of cloud albedo on cloud droplet number density, and in turn on the number density of the aerosol particles on which cloud droplets form, is outlined by Twomey (1974Twomey ( , 1977a. To relate cloud albedo to optical depth and thus to microphysical properties we employ an analytical expression obtained from the two-stream approximation for the reflectance (albedo) RCT of a nonabsorbing, horizontally homogeneous cloud (e.g., Bohren, 1987): Here g is the asymmetry parameter for single scattering (mean cosine of the scattering angle), approximately 0.85 for cloud droplets of radius much greater than the wavelength of visib...

Sea-going hardware for the cloud albedo method of reversing global warming

“…Clearly, seasonal migration of the spray vessels is desirable and the southern oceans are particularly suitable for treatment in the southern summer. The very best all-year Assumptions obtained from Charlson et al (1987), Schwartz & Slingo (1996) and Smith et al (1991). sites are off the coasts of California, Peru and Namibia. Regions in which marine currents are flowing towards the Arctic are of special interest partly because cooling this water might contribute to preserving Arctic ice cover, which is itself a powerful reflector of solar energy, and partly because a reduction in the release rate of methane from the melting of Siberian permafrost might be achieved.…”

“…Cloud albedo A depends on cloud depth Z in metres, liquid water content L, usually expressed in ml m K3 , and the concentration of cloud drops n, usually expressed as the number per ml of air. For engineering purposes the equations in Twomey's classic paper (1977), usefully discussed by Schwartz & Slingo (1996), can be condensed by the use of a variable K(Z, L, n). Following figure 2 of Schwartz & Slingo, this expression has been plotted in figure 2 of this paper against the concentration of cloud condensation nuclei for a typical liquid water content of 0.3 ml m K3 and for a wide range of cloud depths.…”

Effects of aerosol organics on cloud condensation nucleus (CCN) concentration and first indirect aerosol effect

“…The uncertainty in N CCN translates into uncertainties in cloud droplet number concentration and albedo, and ultimately to the uncertainty in indirect aerosol effects. The uncertainty in average first indirect aerosol effect, F c , can be estimated by (Schwartz and Slingo, 1996):…”

“…For global mean calculations, the cloud fraction is taken with the assumption (Charlson et al, 1987) that nonoverlapped marine stratus and stratocumulus clouds having a fractional area of 30%, and T r is 76% (Schwartz and Slingo, 1996). For the range of cloud albedo 0.28≤R c ≤0.72, which is characteristic of the prevalent and climatically important marine stratus clouds, R c can be estimated using the following equation with error less than 10% (Schwartz and Slingo, 1996;Wang, 2007):…”

Description and validation of an AOT product over land at the 0.6 μm channel of the SEVIRI sensor onboard MSG

“…The indirect effect (Albrecht, 1989;Twomey, 1991;Lohmann and Feichter, 2005) is the increase of the condensation nuclei generation. The origin of the cloud formation being the presence in the atmosphere of cloud condensation nuclei (CCN) around which water vapour condensates, an increase of CCN can therefore affect cloud microphysics and consequently cloud physics (formation, evolution, dissipation) (Denman et al, 2007;Haywood and Boucher, 2000;Schwartz and Slingo, 1995;Hegg et al, 1993). These first and second indirect effects are currently a source of major uncertainties in cloud and climate models (Ghan et al, 2001;Hansen et al, 1997;Quaas et al, 2008;Schulz et al, 2006;Lohmann et al, 2010).…”