Mixed-layer models and large-eddy simulations of stratocumulus-topped marine boundary layers: Their nonlinear dynamics and sensitivity to cloud droplet concentration

Due to their high albedo and weak greenhouse effects, low stratiform clouds have a strong tendency to cool the earth and are an important factor in predicting global climate. However, they are difficult to accurately simulate in global climate models, leading to large disparities between the responses of leading global climate models to increased greenhouse gases and aerosols. Marine subtropical stratocumulus-topped boundary layers that persist over cool ocean waters such as off the coast of California or Chile are particularly challenging due to a strong and sharp capping inversion that is difficult for the grids of numerical models to resolve, as well as the clouds themselves being thin, radiatively active, and sometimes drizzling.;We use a simple model problem to look at effects of aerosols on the properties of stratocumulus cloud-topped boundary layer, which can help us better understand the effect of human-generated aerosols on climate. We compare simulations with two models, a simple Mixed-Layer Model (MLM) and a much more complex and computationally intensive Large-Eddy Simulations (LES). Our model problem is idealized from an observed case, Research Flight 1 of the Second Dynamics and Chemistry of Marine Stratocumulus Experiment (DYCOMS-II) 300 km southwest of San Diego. Simulations with different values of cloud droplet condensations are run to steady state and compared to each other.;The MLM is a 3-variable autonomous system of ordinary differential equations which we study using phase plane analysis. This leads to the idea of a one-dimensional slow manifold and multiple long-term evolutions depending on the initial boundary layer depth, which prove to be very helpful in understanding the LES as well as the MLM. For high droplet concentrations, the LES displays two slow manifolds leading into two stable steady states, a well-mixed stratocumulus layer and a decoupled boundary layer with thin, broken cloud. This is the first time that an LES of boundary layer clouds has been shown to have multiple equilibria, which has important consequences for the sensitivity of cloud-topped boundary layers to their initial and boundary conditions as well as their sensitivity to climate perturbations.