Drizzle drives model differences in the Southeastern Atlantic Stratocumulus Transitions with Aerosol-Rain-Radiation interactions (SEA STARR) large eddy simulation intercomparison project

Abstract. The transition from overcast stratocumulus clouds to broken cumulus cloud fields as air is advected over warmer waters is important for controlling cloud cover and thus the radiative budget in the subtropics. Stratocumulus clouds can transition into scattered cumulus via an entrainment-driven “deepening-warming” process or into an open mesoscale cellular convective organization via a precipitation-driven “drizzle-depletion” process. The Southeastern Atlantic Stratocumulus Transitions with Aerosol-Rain-Radiation interactions (SEA STARR) large eddy simulation (LES) intercomparison project described herein uses a composite trajectory of cloud transitions in the southeastern Atlantic to assess the extent to which different LES model setups simulate similar deepening-warming or drizzle-depletion transitions under identical meteorological and aerosol forcings. Results are evaluated in light of observations from the ORACLES, CLARIFY, and LASIC field campaigns.

The default setup includes abundant smoke (~1,000 mg^-1) from southern African biomass burning and is compared with simulations using the same large-scale meteorology but cleaner (100 or 30 mg^-1) free tropospheric aerosol concentrations. In the control, all five LES models simulate deepening-warming transitions that track satellite observations in terms of cloud fraction; exhibit boundary layer deepening somewhat greater than is consistent with observations; and maintain higher cloud droplet number concentrations than observed during nearby aircraft campaigns despite similar below-cloud aerosol number concentrations. In the cleaner cases, however, some models produce drizzle-depletion transitions while others maintain deepening-warming transitions. LES model differences are tied to large discrepancies in simulated rain formation for a given cloud state in terms of total condensate and droplet concentration.