Influence of Environmental Conditions on Forecasting of an Advection-Radiation Fog: A Case Study from the Casablanca Region, Morocco

Abstract

In this study, a dense advection-radiation fog that occurred over the Grand Casablanca region, Morocco, during the night of 23–24 December 2013, is investigated. The adverse visibility induced by the fog patch led to a series of collisions and loss of life on a highway of the region. This fog event is simulated by the Meso-NH research model. Conventional observations from two synoptic stations, satellite imagery, and the European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis are used to analyze the physical processes during the whole life cycle of the event. Some hypotheses on the influence of environmental conditions (topography, land-sea heterogeneity, urbanization) on the numerical fog prediction are presented. Observational analysis together with numerical results show that the horizontal moisture transport, linked to a northerly wind during the afternoon, drove the onset of this fog event. The formation stage was governed by weak turbulence and nocturnal radiative cooling at both stations (costal station, GMMC, and inland station, GMMN). Horizontal moisture flux convergence was observed at the top of the fog layer, over the coastal station, during the mature phase, in addition to the radiative-turbulent interactions. The latter was the main mechanism leading to fog thickening at the inland station. The dissipation phase was governed by a zone of horizontal moisture flux divergence linked to a dry wind from the south. Sensitivity experiments show that numerical coastal fog prediction is influenced by local topography, urbanization and aerosol types, but less by land cover.