Coastal Stratocumulus Topped Boundary Layers and the Role of Cloud-top Entrainment

The ability of the U.S. Navy’s Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPSTM) to accurately forecast the height and structure of the Marine Boundary Layer (MBL) in the coastal zone is analyzed and compared to surface and aircraft observations from the Dynamics and Evolution of Coastal Stratus (DECS) field study conducted along the central coast of California from June, 16 to July 22, 1999. The stratus field was found to have significant mesoscale variability within 100 km of the coast due to interaction between the mean flow and the coastal terrain. This structure is consistent with general hydraulic flow theory and the development of a low level coastal jet; however the specific characteristics on any given day were very sensitive to flow direction, inversion height, and synoptic conditions. With some modifications, the model predicted the general evolution of these events with qualitative fidelity but was slow to dissipate the cloud and frequently produced surface fog vice stratus. A consistent tendency was found in the model predictions of inversion heights 200-300 meters too low, weak inversion strengths, high integrated liquid water content, and weak buoyancy flux near the cloud top. These observed biases are consistent with underestimating the cloud top entrainment velocity and entrainment fluxes in the modeled boundary layer. An explicit entrainment parameterization was developed to better represent the sub-grid scale processes at cloud top and tested in the single column and three-dimensional versions of COAMPS. The first step in this process is accurate determination of the inversion height. It was found that the current method of determining boundary layer height in COAMPSTM based on the bulk Richardson number frequently misdiagnosed the boundary layer height as occurring in the subcloud layer when a weakly stable surface-based inversion was present. Alternative methods based on the liquid water content and the liquid potential temperature gradient were tested and showed a more consistent diagnosis for the observed conditions in the marine boundary layer. The explicit entrainment parameterization was found to generally improve the boundary layer height and cloud liquid water content as compared to field observations, however the modeled boundary layer still exhibited a low bias and the entrainment velocity was higher than is generally expected from field studies for this regime.