Large-Eddy Simulations of Namibian Stratocumulus Evolution during ORACLES-2016

Based on a companion study of satellite observations along multi-day boundary layer trajectories obtained from a continuous GEOS-5 assimilation-driven simulation of the ORACLES 2016 field campaign period, here we present quasi-Lagrangian large-eddy simulation (LES) results for selected conditions of stratocumulus-to-cumulus transition. Scenarios where a continental biomass burning plume is extensively overlying the marine boundary layer west of Namibia are contrasted with scenarios where overlying air is clean or a sizable gap separates the boundary layer from an overlying plume, based on in situ and airborne remote-sensing measurements. In situ measurements are used to provide realistic initial thermodynamic and aerosol profiles for simulations. Owing to relatively complex large-scale advection patterns overlying the boundary layer, in situ and remote-sensing measurements are also used to specify the overlying free tropospheric aerosol and moisture profiles in time along boundary layer trajectories. Sea surface temperature, horizontal wind and large-scale subsidence are taken from GEOS-5 output. Sea surface aerosol emissions are estimated from near-surface wind speed, and aerosol numbers are treated prognostically. Results are constrained with in situ measurements of aerosol number size distribution, liquid water content and drop size distribution, and satellite remote sensing measurements of cloud top height and cloud fraction. Baseline (observation-based) and sensitivity test simulations focus on evaluating the factors identified in a pre-campaign study as potentially important to regional indirect plus semi-direct shortwave and longwave radiative forcings from biomass burning plumes (Zhou et al. 2017): the duration of time before a plume makes contact with the MBL, the strength of drizzle prior to contact, and background and plume aerosol properties and moisture contents.