Large Eddy Simulation of Shallow Cumulus Convection during BOMEX: Sensitivity to Microphysics and Radiation

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Abstract
A number of large eddy simulations with the Regional Atmospheric Modeling System have been made to study the sensitivity of shallow marine cumulus convection to different microphysics and radiation schemes. In particular, the sensitivity of shallow marine cumulus convection to drizzle and radiation effects, and how drizzle and radiation modify turbulent fluxes, are investigated. It is shown that for the case of prescribed radiative heating, drizzle—albeit very slight—leads to reduced buoyancy fluxes and less turbulence. Consequently, drizzling boundary layers appear to entrain less than their nondrizzling counterpart. Heavy drizzle events are simulated in association with deeper clouds as high as 2 km, even though the majority of clouds are only a few hundred meters deep. A heavier and longer lasting drizzle episode associated with a deeper boundary layer is produced when a two-stream radiative parameterization replaces the prescribed radiative heating in the simulation. Simulated surface precipitation rates agree reasonably well with observations. The greatest alteration in boundary layer structure is obtained when radiative heating interacts explicitly with the broadened drop distribution associated with drizzle formation.

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