Orographically and thermally forced coastal marine boundary layer structure and dynamics

Coastal orography and thermal forcing can impose distinct imprints upon the structure and dynamic behavior of the marine atmospheric boundary layer (MABL). Several recent field experiments adjacent to mountainous coastlines have noted abrupt changes in MABL depth, wind speed, and inversion strength. These variations tend to be particularly prominent leeward of coastal points and capes where regions of enhanced stress, and curl of stress, frequently occur. There is a strong correlation between these regions and areas of pronounced oceanic upwelling, thereby signifying the potential importance of coupled air-sea interaction. Diurnally varying cross-coast baroclinity, supercritical flow effects, and mesoscale mountain wave activity all can contribute to such localized coastal MABL inhomogeneity.

In this study we use the atmospheric portion of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) to investigate MABL variations along the U.S. west coast. COAMPS features a complete operational mesoscale NWP system, having a suite of advanced physical parameterizations and utilizing an intermittent data assimilation procedure, while the oceanic data assimilation component is undergoing final development.

Observational studies along the California coast have established that during summer the MABL often exhibits supercritical flow (Froude number > 1) behavior in climatological northerly, coast-parallel flow regimes (e.g., Winant et al., 1988; Rogers et al., 1998). Also, observations and modeling have demostrated that diurnally varying coastal baroclinity drives not only the sea/land breeze, but the coastal low-level jet that frequently is observed in this region as well (e.g., Burk and Thompson, 1996; Holt, 1996). In this paper we investigate, using data from the Coastal Waves 1996 (Rogers et al., 1998) field experiment and modeled results from COAMPS, thermally forced circulations and supercritical MABL flow around Cape Blanco, OR and Cape Mendocino, CA. The results demonstrate that there is strong, diurnally varying interaction in the supercritical flow between these Capes that cannot be analyzed by treating the flow about each Cape independently (a common practice with shallow water models).

Recently we have generated COAMPS reanalysis fields along the U.S. west coast covering the full year of 1999, and continuing into 2000, with an inner grid having 9 km spacing. Monthly mean plots of COAMPS surface fields, including sensible and latent fluxes, stress and its curl, will be presented illustrating robust coastal structural features and their seasonal changes. Also, statistics will be compiled and presented comparing COAMPS forecast values with the ~15 moored coastal buoys in the model domain.

Key Word(s): (1) Marine & Oceanic BL’s; (2) Surface Layers