Identifying Synoptic Controls on Boundary Layer Thermodynamic and Cloud Properties in a Regional Forecast Model

Abstract. Most of our understanding of boundary-layer cloudiness is based on idealized, subtropical, barotropic marine environments, yet boundary-layer clouds exist across a range of conditions. In this study, we investigate marine boundary-layer clouds associated with a midlatitude synoptic cyclone. We use the Naval Research Laboratory’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) and an automated cold-front-relative analysis framework to explore low cloud properties across a transect from the warm sector, through the cold front, and northwestward into the cold sector. The model credibly captures boundary-layer structure in line with conceptual models. However, the simulated clouds are too thick, with too much liquid water and too little cloud-base drizzle, compared to observations. The transects reveal a shallow, conditionally unstable boundary layer in the warm sector, accompanied by shallow clouds with low liquid water content. The frontal region exhibits forced convection associated with weak stability and upward vertical motion. Northwest of the cold front, the boundary layer is well-mixed with increasing depth and stability. Further northwest in environments of high stability and subsidence, the model produces clouds and associated upward grid-scale vertical motion. We interpret these features as the model’s attempt to represent cumulus or mesoscale organization of closed cellular convection typically observed in the wake of midlatitude cyclones. The deep, well-mixed boundary layers and shallow cumulus are maintained by strong surface fluxes, as in cold air outbreaks. Our analysis framework serves as a unique approach to model verification, and our results offer unique insights into cloud and boundary layer evolution throughout a cyclone.