Explicit aerosol-cloud interaction in the Dutch Atmospheric Large-Eddy Simulation model DALES4.1-M7

Abstract

Abstract. Large-Eddy Simulations (LES) are an excellent tool to improve our understanding of the aerosol-cloud interaction (ACI). These models combine a spatial resolution high enough to resolve cloud structures with domain sizes large enough to simulate macroscale dynamics and feedback between clouds. However, most research on ACI using LES simulations is focused on changes in cloud characteristics. The feedback of ACI on the aerosol population remains relatively understudied. We introduce a prognostic aerosol scheme with multiple aerosol species in the Dutch Atmospheric Large-Eddy Simulation model (DALES), especially focused on simulating the feedback of ACI on the aerosol population. The numerical treatment of aerosol activation is a crucial element in the simulation of ACI. Two methods are implemented and discussed: an explicit activation scheme based on κ-Köhler theory and a more classic approach using updraft strength. Model simulations are validated against observations using the Rain in Shallow Cumulus over the Ocean (RICO) campaign, characterised by rapidly precipitating, warm-phase shallow cumulus clouds. We find that in this pristine ocean environment virtually all aerosols enter the cloud phase through activation while in-cloud scavenging is relatively inefficient. Despite the rapid formation of precipitation, most of the in-cloud aerosol mass is returned to the atmosphere by cloud evaporation. The strength of aerosol processing through subsequent cloud cycles is found to be particularly sensitive to the activation scheme and resulting cloud characteristics. However, the precipitation processes are considerably less sensitive. Scavenging by precipitation is the dominant source for in-rain aerosol mass. About half of the in-rain aerosol reaches the surface, while the rest is released by evaporation of falling precipitation. Whether ACI increases or decreases the average aerosol size depends on the balance between the evaporation of clouds and rain, and ultimate removal by precipitation. Analysis of typical aerosol size associated with the different microphysical processes shows that aerosols resuspended by cloud evaporation are only 5 to 10 % larger than the originally activated aerosols. In contrast, aerosols released by evaporating precipitation are an order of magnitude larger.