Diagnostic Boundary Layer Height for Cloud-topped Boundary Layers in a Mesoscale Model

The ability of COAMPS™ to accurately forecast the height and structure of coastal stratus is analyzed and compared to observations from the Dynamics and Evolution of Coastal Stratus (DECS) field study. 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, consistent with hydraulic flow theory and the development of a Low Level Coastal Jet. With some modifications, the model predicted the general evolution of these events but was slow to dissipate the cloud and frequently produced surface fog vice stratus.

A consistent tendency was found in the model predictions of low inversion heights by 200 to 300 meters, inversion strengths that were too weak, and integrated liquid water values that were too high. These observed biases are consistent with underestimating the cloud top entrainment and resulting buoyancy flux in the modeled boundary layer. It is believed that this is due to the representation of entrainment at discrete and relatively coarse vertical levels suitable for operational prediction. An explicit entrainment parameterization was developed to represent the cloud top sub-grid scale processes and tested in COAMPSTM, which required accurate determination of the inversion height. It was found that the current method of determining boundary layer height in COAMPS™ based on the bulk Richardson number frequently misdiagnosed the boundary layer height when a weakly stable surface-based inversion was present. An alternative method based on the liquid potential temperature gradient was implemented with good results.