Cloud Aspects Critical for Aerosol Indirect Effect Quantifications

Cloud Albedo

The sensitivity of cloud albedo to perturbations in cloud condensation nuclei, and thus the strength of the Twomey effect (Twomey 1974), depends on the value of cloud albedo. To simulate aerosol indirect effects with fidelity, we must thus be able to simulate realistically the distribution of cloud albedo (Feingold and Siebert, this volume). Satellite data of this quantity exist, allowing for an observation-based evaluation of the distribution.

Boundary Layer Moisture Budget

Process modeling and theoretical studies suggest that for stratocumulus, the cloud thickness response to perturbations in CDNC may, in many cases, act to oppose the changes in albedo caused by the Twomey effect alone. The dynamic and microphysical processes responsible for this depend critically on surface relative humidity. In general, the total aerosol indirect effect may be strongly sensitive to the cloud macrophysical and boundary layer properties (see Brenguier and Wood; Feingold and Siebert; and Stevens and Brenguier, all this volume).

Cloud Subgrid-scale Variability

The aerosol and microphysical processes that determine cloud radiative properties occur on scales much smaller than those resolved by GCMs, and they are nonlinear. Thus, the application of GCM-resolved moisture and motion fields will produce unrealistic estimates of aerosol indirect effects. A general approach to address this problem is to construct PDFs for subgrid velocity and moisture distributions (Lohmann and Schwartz, this volume). Aerosol and microphysical processes are then evaluated using the PDF vertical velocity and moisture.

Microphysical Processes in Ice and Mixed-phase Clouds

Aerosols may have an important influence on the optical properties of ice clouds as well as on microphysical processes in mixed-phase clouds. To simulate such effects realistically, the various ice crystal nucleation pathways and mixed-phase microphysical processes must be accurately represented in the model (Lohmann and Schwartz, this volume).

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