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Microphysical process rates and global aerosol-cloud interactions

by A. Gettelman, H. Morrison, C. R. Terai, R. Wood
Atmospheric Chemistry and Physics ()
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Cloud microphysical process rates control the amount of condensed water in clouds and impact the suscep-tibility of precipitation to cloud-drop number and aerosols. The relative importance of different microphysical processes in a climate model is analyzed, and the autoconversion and accretion processes are found to be critical to the conden-sate budget in most regions. A simple steady-state model of warm rain formation is used to illustrate that the diagnos-tic rain formulations typical of climate models may result in excessive contributions from autoconversion, compared to observations and large eddy simulation models with explicit bin-resolved microphysics and rain formation processes. The behavior does not appear to be caused by the bulk process rate formulations themselves, because the steady-state model with the same bulk accretion and autoconversion has reduced contributions from autoconversion. Sensitivity tests are con-ducted to analyze how perturbations to the precipitation mi-crophysics for stratiform clouds impact process rates, precip-itation susceptibility and aerosol–cloud interactions (ACI). With similar liquid water path, corrections for the diagnos-tic rain assumptions in the GCM based on the steady-state model to boost accretion indicate that the radiative effects of ACI may decrease by 20 % in the GCM. Links between pro-cess rates, susceptibility and ACI are not always clear in the GCM. Better representation of the precipitation process, for example by prognosticating precipitation mass and number, may help better constrain these effects in global models with bulk microphysics schemes.

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