On the Parameterization of Convective Downdrafts to Represent Marine Stratocumulus Clouds

The role of non-local transport on the development and maintenance of marinestratocumulus (Sc) clouds in coarse resolution models is investigated,with a special emphasis on the downdraft contribution. A new parameterizationof cloud-top triggered downdrafts is proposed and validated againstlarge-eddy simulation (LES) for two Sc cases. The applied non-local massfluxscheme is part of the stochastic multi-plume eddy-diffusivity/mass-flux(EDMF) framework decomposing the turbulent transport into local and nonlocalcontributions. The complementary local turbulent transport is representedwith the Mellor-Yamada-Nakanishi-Niino (MYNN) scheme. ThisEDMF version has been implemented in the Weather Research and Forecasting(WRF) single-column model (SCM) and tested for three model versions:without mass flux, with updrafts only, and with both updrafts and downdrafts.In the LES, the downdraft and updraft contributions to the total heat and moisturetransport are comparable and significant. The WRF SCM results show agood agreement between the parameterized downdraft turbulent transport andLES. While including updrafts greatly improves the modeling of Sc cloudsover the simulation without mass flux, the addition of downdrafts is less significant,although it helps improve the moisture profile in the planetary boundarylayer.