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There have been some pioneering studies of ice clouds (see Lohmann and Feichter, 2005) showing potential for an anthropogenic in- fluence on the number of ice nuclei.
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It has poten- tially a strong impact on the radiative balance, but is also very sensitive to the vertical distribution of aerosols and clouds (Johnson et al., 2004 Penner et al., 2003). Within the concept of the semi-direct aerosol effect we assume that absorbing aerosols cause the inhibition of cloud formation, evaporation of existing clouds, and blocking of surface heat- ing inhibiting convection and cloud formation. Lately, the semi-direct aerosol effect of the inhibition of cloud formation has attracted large at- tention (Ackerman et al., 2000 Cook and Highwood, 2004 Johnson et al., 2004 Kaufman et al., 2002 Koren et al., 2004 Menon et al., 2002 Ramanathan et al., 2001). Aerosols may thus lead to an increase in cloud optical thickness due to a combination of reduction in cloud droplet radius and in- creased water content. The reduction in cloud droplet size can inhibit precipitation (Al- brecht, 1989 Rosenfeld, 1999, 2000), the cloud lifetime in- creases and the clouds can evolve to an increased cloud top height (Andreae et al., 2004 Khain et al., 2005 Williams et al., 2002 Rosenfeld, 2006) with an increased LWP. This cloud albedo effect is seen in various measurements of clouds but early experimental measurements 50 years ago also showed that the size of newly-formed cloud droplets was dependent on the aerosol concentration (Gunn and Phillips, 1957). Fraser, 1997 Twomey, 1977) for a similar liquid water path (LWP), resulting in increased cloud optical thickness and in- creased reflectivity of solar insolation.