Abstract
Abstract. Cloud microphysical process rates control the amount of condensed water in clouds and impact the susceptibility 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 condensate budget in most regions. A simple steady-state model of warm rain formation is used to illustrate that the diagnostic 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 conducted to analyze how perturbations to the precipitation microphysics for stratiform clouds impact process rates, precipitation susceptibility and aerosol–cloud interactions (ACI). With similar liquid water path, corrections for the diagnostic 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 process 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.
Highlights
Aerosols have many direct, semi-direct and indirect effects on clouds
If accretion dominates over autoconversion for shallow (Stevens and Seifert, 2008) and stratocumulus (Wood, 2005) clouds, this would tend to dampen the aerosol–cloud Interactions (ACI): reducing the role of autoconversion, which depends on clouddrop number, reduces the effect of aerosols on cloud radiative properties (Wood et al, 2009)
The analysis focuses on a single GCM but the parameterization of microphysical processes is similar to many global models
Summary
Semi-direct and indirect effects on clouds. The indirect effects, or aerosol–cloud Interactions (ACI), result from more cloud condensation nuclei (CCN) creating a population of more and smaller particles for a given amount of cloud water. If accretion dominates over autoconversion for shallow (Stevens and Seifert, 2008) and stratocumulus (Wood, 2005) clouds, this would tend to dampen the ACI: reducing the role of autoconversion, which depends on clouddrop number, reduces the effect of aerosols on cloud radiative properties (Wood et al, 2009) Consistent with this idea, the change in rain rate with respect to aerosols or drop number (called the “susceptibility” of precipitation to aerosols following Feingold and Siebert, 2009) seems to decrease at higher LWPs where accretion dominates (Jiang et al, 2010; Terai et al, 2012).
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