Fog and low clouds in a coastally trapped disturbance

Abstract

One of the most distinctive features of coastally trapped disturbances (CTDs) is the tongue of fog and low clouds propagating poleward along the west coast of continents against the typical coast‐parallel equatorward flow. In this study, characteristics of the fog and low clouds observed in conjunction with a CTD event on 15–16 June 2000 are investigated. In particular, we examine the relative importance of radiation, cloud microphysical processes, advection, entrainment, and surface fluxes to the fog and cloud layer associated with the CTD. A simulation of the event is produced using the Naval Research Laboratory's nonhydrostatic Coupled Ocean/Atmosphere Mesoscale Prediction System. Propagation of the wind shift and cloud tongue and the extent of fog and low clouds in the model results are compared with coastal buoy data and satellite imagery. A critical value of the Bowen ratio delineates the extent of fog with lower values to the south in the low cloud area. Because the Bowen ratio provides limited information concerning the relative roles of sensible and latent heat fluxes in the cloud layer, we examine the ratio β of integrated buoyancy generation due to liquid water flux to the integrated positive buoyancy flux. The timing difference between arrival of the CTDs' wind shift and the arrival of the fog is found to be central to the evolution of the cloud liquid water field. Several sensitivity studies are also carried out in order to further understand the roles of radiative cooling and surface latent and sensible heat fluxes.