Large-eddy simulations of marine boundary-layer clouds and aerosol effects associated with cold air outbreak during the ACTIVATE campaign

Micro-/macro-physical properties of marine stratocumulus clouds at Western North Atlantic Ocean (WNAO) are investigated using in-situ measurements and large-eddy simulations (LES) for two cold air outbreak (CAO) cases: February 28 and March 1 2020. The LES is driven and constrained with measurements collected during the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) field campaign and ERA5 reanalysis data. Aerosol size distributions measured below cloud base (BCB) are fitted with log-normal size distributions and then fed into the LES. The bulk aerosol hygroscopicity for each size mode used in the LES is estimated from Aerodyne Mass Spectrometer (AMS) measurements. The LES is able to reproduce the measured vertical profiles of liquid water content (LWC), droplet effective radius reff and the cloud droplet number concentration Nc from fast cloud droplet probe (FCDP) in-situ measurements for both the February 28 and March 1 case. Aerosol size distributions exhibit strong spatial variation as indicated by AMS measurements at different BCB flight legs. Therefore, both spatial and temporal matrices should be considered to quantify the relationship between Na from AMS measurements and Nc from FCDP measurements. We also use the LES results to evaluate cloud properties from reanalysis products. Compared to LES, ERA5 reanalysis is able to capture the time evolution of liquid water path and total cloud coverage within the LES domain during both CAO cases while MERRA-2 underestimates them. Although both ERA5 and MERRA-2 capture well the evolution of boundary-layer structure of clouds, MERRA-2 yields closer vertical structure of LWC and cloud coverage to LES compared to those from ERA5.