Abstract
Abstract. Challenges in understanding the aerosol–cloud interactions and their impacts on global climate highlight the need for improved knowledge of the underlying physical processes and feedbacks as well as their interactions with cloud and boundary layer dynamics. To pursue this goal, increasingly sophisticated cloud-scale models are needed to complement the limited supply of observations of the interactions between aerosols and clouds. For this purpose, a new large-eddy simulation (LES) model, coupled with an interactive sectional description for aerosols and clouds, is introduced. The new model builds and extends upon the well-characterized UCLA Large-Eddy Simulation Code (UCLALES) and the Sectional Aerosol module for Large-Scale Applications (SALSA), hereafter denoted as UCLALES-SALSA. Novel strategies for the aerosol, cloud and precipitation bin discretisation are presented. These enable tracking the effects of cloud processing and wet scavenging on the aerosol size distribution as accurately as possible, while keeping the computational cost of the model as low as possible. The model is tested with two different simulation set-ups: a marine stratocumulus case in the DYCOMS-II campaign and another case focusing on the formation and evolution of a nocturnal radiation fog. It is shown that, in both cases, the size-resolved interactions between aerosols and clouds have a critical influence on the dynamics of the boundary layer. The results demonstrate the importance of accurately representing the wet scavenging of aerosol in the model. Specifically, in a case with marine stratocumulus, precipitation and the subsequent removal of cloud activating particles lead to thinning of the cloud deck and the formation of a decoupled boundary layer structure. In radiation fog, the growth and sedimentation of droplets strongly affect their radiative properties, which in turn drive new droplet formation. The size-resolved diagnostics provided by the model enable investigations of these issues with high detail. It is also shown that the results remain consistent with UCLALES (without SALSA) in cases where the dominating physical processes remain well represented by both models.
Highlights
Large-eddy simulations (LES) have been used to study the properties of clouds and the boundary layer for a few decades (e.g. Deardorff, 1974, 1980; Moeng, 1984; Stevens et al, 2005)
While in the early stages of the simulation the liquid water content (LWC) and the macroscopic cloud structure are quite similar between LEV3 and LEV4, after about 4 h the results start to diverge substantially marking a clear shift in the boundary layer dynamics
The model was tested and evaluated using two wellcharacterised cases, which have been simulated with large-eddy simulation (LES) models in previous work: one comprising marine stratocumulus clouds from the DYCOMS-II campaign and another based on measurements of a radiation fog event in Cardington, UK
Summary
Large-eddy simulations (LES) have been used to study the properties of clouds and the boundary layer for a few decades (e.g. Deardorff, 1974, 1980; Moeng, 1984; Stevens et al, 2005). Large-eddy simulations (LES) have been used to study the properties of clouds and the boundary layer for a few decades Deardorff, 1974, 1980; Moeng, 1984; Stevens et al, 2005) These models solve the low-pass filtered Navier– Stokes equations; i.e. the large energy-containing turbulent eddies are resolved, whereas the smallest length scales and energy dissipation are parameterised typically using closures based on the Smagorinsky model. This approach provides an attractive compromise between accuracy and computational cost, which is why LES models have become popular in studies of the properties of boundary layers and clouds.
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