Aerosol-Induced Closure of Marine Cloud Cells: Enhanced Effects in the Presence of Precipitation.

The Weather Research Forecasting (WRF) V4.2 model is configured within a Lagrangian framework to quantify the impact of aerosols on evolving cloud fields. Simulations employing realistic meteorological boundary conditions are based on 10 case study days offering diverse meteorology during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA). Cloud and aerosol retrievals in observations from aircraft measurements, ground-based Atmosphere Radiation Measurement (ARM) data at Graciosa Island in the Azores, and A-Train and geostationary satellites are in good agreement with the simulations. Higher aerosol concentration leads to suppressed drizzle and increased cloud water content. These changes lead to larger radiative cooling rates at cloud top, enhanced vertical velocity variance, and increased vertical and horizontal wind speed near the base of the lower-tropospheric inversion. As a result, marine cloud cell area expands, narrowing the gap between shallow clouds and increasing cloud optical thickness, liquid water content, and the top-of-atmosphere outgoing shortwave flux. While similar aerosol effects are observed in lightly to non-raining clouds, they tend to be smaller by comparison. These results show a strong link between cloud cell area expansion and the radiative adjustments caused by liquid water path and cloud fraction changes. These adjustments scale by 74 % and 51 %, respectively, relative to the Twomey effect. Given the limitations of traditional global climate model resolutions, addressing mesoscale cloud-state transitions at kilometer-scale resolutions or higher should be of utmost importance in accurately quantifying aerosol radiative forcing. [ABSTRACT FROM AUTHOR]