Mesoscale forcing of marine atmospheric boundary layer clouds along the U.S. West Coast

Hourly forecasts produced by a high-resolution mesoscale model for several months in 1999 provide an opportunity to study both offshore and near-shore cloud layers along the U.S. West Coast. Analysis of the mesoscale forcing on marine atmospheric boundary layer clouds is considerably less complex than over-land, convective clouds generated by sharp discontinuities in the land surface. Stratus over oceans also displays mesoscale structure driven by inhomogeneities in surface forcing but with significantly less diurnal variation. Furthermore, without the complexity of underlying topography, marine cloud layers offer a simpler setting in which to examine the relative impacts of multiple forcing mechanisms. Mesoscale variability in a coastal topographic barrier does have an influence on adjacent cloud layers to different degrees throughout the seasons. This coastal forcing along with large-scale and surface forcing of U.S. West Coast stratus layers are examined for 4 months during each season of 1999. Monthly average patterns of cloud occurrence, liquid water content, cloud top and base heights are correlated to patterns of sea surface temperature, low-level winds and surface fluxes to determine potential sources of mesoscale variability. Coastal cloudiness and its relationship to monthly trends in sea-surface temperature indicate a tendency for stratus to form downwind rather than over the primary upwelling centers. And strong wind forcing in springtime reveals a correlation between convergent flow downwind of the low-level wind maximum and enhanced marine layer clouds. Further, these periods of strong winds produce substantially fewer clouds than do weaker wind regimes. The coastal topography elevates cloudy layers during the synoptically active month of January as they approach the Oregon coastline, while warm season clouds are generally channeled along the coastal barrier by a strong subsidence inversion. Hence, in summer months variations in the coastal orography, coupled with those imparted by strong coastal upwelling, produce ample mesoscale signatures in U.S. West Coast cloudiness.*