Clouds and Climate

The Romps Group at the University of California, Berkeley

Romps and Kuang, Nature versus nurture in shallow convection, JAS, 2010
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We use tracers in a large-eddy simulation of shallow convection to show that stochastic entrainment, not cloud-base properties, determine the fate of convecting parcels. The tracers are used to diagnose the correlations between a parcel's state above the cloud base and both the parcel's state at the cloud base and its entrainment history. We find that the correlation with the cloud-base state goes to zero a few hundred meters above the cloud base. On the other hand, correlations between a parcel's state and its net entrainment are large. Evidence is found that the entrainment events may be described as a stochastic Poisson process.

We construct a parcel model with stochastic entrainment that is able to replicate flux profiles and, more importantly, the observed variability. Turning off cloud-base variability has little effect on the results, which suggests that stochastic mass-flux models may be initialized with a single set of properties. This model of shallow convection holds promise as the framework for a convective parameterization.

Figure 39. Animation of the trade-wind cumuli during BOMEX as simulated using DAM.

Animation of the trade-wind cumuli during BOMEX as simulated using DAM.

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Romps, A direct measure of entrainment, JAS, 2010  Romps and Kuang, Do undiluted convective plumes exist in the upper tropical troposphere?, JAS, 2010