The search for robust estimates of the first aerosol Indirect effect for the southeast Pacific stratocumulus region

The first aerosol indirect effect tells us that clouds with smaller cloud droplets are brighter, all else equal, because of the increase in cloud optical depth. Only the first aerosol indirect effect is represented in the 2007 IPCC, in which it contributes the largest uncertainty of all the radiative forcings considered. Modeled estimates of the first aerosol indirect effect also differ widely from observed values. This has been attributed at least partly to inadequate constraints and issues of scale. We have used data from in situ probes, and aircraft- and space-based remote sensors to assess the first aerosol indirect effect for the southeast Pacific stratocumulus region. We find agreement between diverse observational approaches when constraints are properly considered, producing estimates of the first aerosol indirect effect that exceed those typically produced by large-scale models. A more regional analysis of global climate model results is suggested to help diagnose the microphysical processes influencing the model’s first aerosol indirect effect estimate.

Paquita Zuidema

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