Numerical Case Study of the Aerosol-Cloud-Interactions in Warm Boundary Layer Clouds over the Eastern North Atlantic with an Interactive Chemistry Module

Abstract. The presence of warm boundary layer stratiform clouds over the Eastern North Atlantic (ENA) region is commonly influenced by the Azores High, especially during the summer season. To investigate comprehensive aerosol-cloud interactions, this study employs the Weather Research and Forecast model coupled with a chemistry component (WRF-Chem), incorporating aerosol chemical components that are relevant with formation of cloud condensation nuclei (CCN) and accounting for aerosol spatiotemporal variation. This study focuses on aerosol indirect effects, particularly long-range transport aerosols, in the ENA region under three different weather regimes: ridge with surface high-pressure system, post-trough with surface high-pressure system, and weak trough. The WRF-Chem simulations conducted at a near the Large-Eddy Simulation scale offer valuable insights into the model’s performance, especially regarding its high spatial resolution in accurately capturing the liquid water path (LWP) and cloud fraction across various weather regimes. Our result shows that introducing five times more aerosols to either non-precipitating or precipitating clouds significantly increases ambient CCN numbers, resulting in varying degrees of higher LWP. The substantial aerosol-cloud interaction especially occurs in the precipitating clouds and demonstrates the LWP susceptibility to changes in CCN under different regimes. Conversely, non-rain clouds at the edges of a cloud system are prone to evaporation, exhibiting an aerosol drying effect. The aerosols released during this process transition back to the accumulation mode, facilitating future activation. This dynamic behavior is not adequately represented in prescribed-aerosol simulations.