Cloud Twomey effect

The first indirect climatic effect of aerosol (the Twomey effect) is based on the assumption that with a constant equivalent liquid water content (LWC) of clouds an increase in atmospheric aerosol number density (and, hence, concentration of CCN)... 

The condensed phases also are important to the physical processes of the atmosphere however, their role in climate poses an almost entirely open set of scientific questions. The highest sensitivity of physical processes to atmospheric composition lies within the process of cloud nucleation. In turn, the albedo (or reflectivity for solar light) of clouds is sensitive to the number population and properties of CCN (Twomey, 1977). At this time, it appears impossible to predict how much the temperature of the Earth might be expected to increase (or decrease in some places) due to known changes in the concentrations of gases because aerosol and cloud effects cannot yet be predicted. In addition, since secular trends in the appropriate aerosol properties are not monitored very extensively there is no way to know... 

Platnick and Twomey (1994) have applied Eq. (KK) to marine clouds off the coast of California and southern Africa, to fogs in central California, and to ship tracks. Figure 14.42 shows a typical range of susceptibilities as a function of cloud droplet size. The measured susceptibilities in these studies covered three orders of magnitude, from 5 X 10-5 cm3 for fogs to 0.8 X 10-3 cm3 for marine clouds off south Africa and 2 X 10 2 cm3 for thin stratus clouds off the California coast. Similarly, Taylor and McHaffie (1994) report cloud susceptibilities in the range from 10-4 to >8 X 10-3 at various locations around the world. The highest susceptibilities were those with the smallest aerosol particle concentrations below the cloud base. As the particle concentration increased beyond 500 cm3, the susceptibility was relatively constant at 5 X 10"4 cm3. This means that the addition of new particles to a relatively clean air mass is far more effective than for a polluted one in terms of the effect on clouds. In short,... 

Another piece of evidence for anthropogenic emissions leading to increased CCN and hence effects on cloud properties such as albedo and extent is found in ship tracks. These are lines of clouds that trace ship movements, either in initially cloud-free regions (Conover, 1966 Platnick and Twomey et al., 1994) or superimposed on preexisting clouds (Coakley et al., 1987). Emissions associated with the ship exhausts serve as CCN. This allows clouds to form where the background CCN concentration is too small for cloud formation. Alternatively, the CCN can modify existing cloud properties in the exhaust plume by changing the number and size distribution of the cloud droplets as well as the liquid water content (e.g., Ferek et al., 1998). 

Warner, J., and S. Twomey, The Production of Cloud Nuclei by Cane Fires and the Effect on Cloud Droplet Concentration, J. Atmos. Sci., 24, 704-706 (1967). 

In determining their ability to nucleate clouds, the chemical composition of aerosol particles is much less important than their size, a result that will clarify aerosol effects on climate (Rosenfeld 2006). This is what is expected from theory because the radius-to-volume ratio determines the molecular transfer from the gas phase, whereas the hygroscopicity (surface characteristics of CCN) determines the uptake coefficient (Chapter 4.3.7.4). The other very important parameter for cloud formation is the CCN number, determining the cloud droplet number (cf Fig. 2.20). As shown originally by Twomey (1991), and recently reviewed by Lohmann and Feich-ter (2005), the sensitivity of climate to CCN number density is nonlinear, with the effect being much stronger at low particle numbers. 

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