Limits to Existing Observationbased Studies

Despite many observational campaigns, no large body of observational evidence exists to provide a positive correlation between cloud albedo and aerosol perturbations on a large scale. Although it has been established that an increase in aerosol concentration results usually in an increase of CDNC, this does not necessarily lead to an increase in cloud albedo, since the LWP of the cloud also changes. Although the LWP may be responding to aerosol-induced changes, it may be determined to a first order by differences in large-scale meteorological forcings.

Ship Track Analyses

Studies of plumes that interact with clouds, as typified by ship tracks, provide a natural laboratory to sample a partial derivative with respect to aerosol concentration or cloud droplet concentration, unlike, for example, a regional-scale plume of polluted air, in which the meteorology co-varies with the aerosol properties. Heat and moisture are also injected into the atmosphere within ship exhaust, but these perturbations can be regarded as negligible after the plume spreads over a few kilometers in width. However, ship tracks occur only in one type of cloud (marine stratocumulus), and they appear to be more likely in shallow boundary layers (Coakley et al. 2000). Despite these limitations, analyses of ship tracks might be exploited to yield further insights into indirect effects of aerosols, since they reveal information about the important processes (e.g., entrainment) that control the cloud response to aerosol perturbations (see Cotton, this volume).

Statistical Relationships

Correlations between cloud properties (cloud droplet radius, CDNC) and column aerosol concentration (aerosol index or aerosol optical depth, or a proxy such as hemispheric or land-sea contrasts) from satellite data may be used to infer some clues about aerosol-cloud interactions (Nakajima and Schulz, this volume). Since it is, however, impossible with a passive satellite instrument to quantify the relevant cloud and aerosol parameters simultaneously at a particular location, these analyses rely on the assumption that aerosol properties in clear scenes are similar to those in near-by cloudy situations. Long-term measurements from ground-based sites are necessary to establish similar correlations (Feingold and Siebert, this volume).

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