Simulating Feedbacks Between Stratocumulus Cloud Dynamics, Microphysics and Aerosols Over Large Scales

The response of a stratocumulus cloud deck to aerosols involves a complex interplay between cloud microphysics, precipitation, cold pool dynamical interactions between neighboring cells, cloud top entrainment and the boundary layer structure over larger scales. Such feedbacks are thought to be involved in, for example, the formation of Pockets of Open Cells (POCs), which represent a large albedo change relative to the closed cell regime. However, they are not represented in GCM parameterizations and have also so far have not been simulated adequately in mesoscale models, which is a necessary step in order to develop parameterizations. Mesoscale models with domains of order 1000km that are driven by meteorological analysis, allow realistic forcing and large scale interactions, in contrast to idealized LES simulations. However, some important questions remain about how such models, which employ grid resolutions between those used in LES models (<~250 m) and regional models (>~10 km), represent stratocumulus. How important is the correct representation of aerosol number, cloud feedbacks upon aerosol, small-scale dynamical features such as narrow precipitating regions, sub-grid humidity, sub-grid updraft distribution, etc. to the accurate simulation of stratocumulus sheets?

As a first step towards more complex POC cases, results will be shown from high resolution (<1 km) mesoscale simulations of overcast stratocumulus using a new multi-moment microphysics scheme coupled to the UK Met Office Unified Model. The new scheme represents the processing of aerosol by clouds, allowing examination of the feedbacks between cloud dynamics, microphysics and aerosol. Results will be presented highlighting the performance of the model compared to satellite and ship-borne measurements, along with results demonstrating the sensitivity to the aerosol concentration and the treatment of aerosol scavenging by cloud. A cloud scheme to account for sub-grid humidity variability was also added and was found to be necessary in order to simulate realistic clouds. Additionally, we will show development results from the implementation of a representation of sub-grid vertical velocities based on resolved motions, which will allow consistent droplet activation across a range of horizontal model resolutions.

We will use this case to explore cloud-aerosol evolution of stratocumulus through the use of the joint phase space of liquid water path and droplet concentration along Lagrangian trajectories. This framework will be used to understand formation of POCs in response to a changing aerosol environment and the effects of interactions between multiple cells and POC boundaries over large spatial- and time- scales.