Abstract
A three-dimensional mesoscale computer model is used to assess the importance of urban effects, relative to non-urban effects, on mesoscale boundary-layer vertical air motion and on the height of the boundary layer downwind of St. Louis, Missouri. Simulations are made for south, southwest, west and northwest winds, with urban land uses replaced by rural land uses, both with and without topography. Simulations including urban effects indicated mesoscale upward air motion downwind of the city for all wind directions, strongest for southwest winds and weakest for northwest winds. With urban effects excluded, much weaker upward motion was found downwind for south, southwest and west winds, and downward vertical velocities occurred in the downwind areas for northwest winds.
The results of this study imply that mesoscale boundary-layer upward air motion occurs downwind of St. Louis, primarily as a result of urban effects. Local geographic influences may tend to enhance or suppress this upward air motion, depending on wind direction. Thus, the interaction of urban effects with those resulting from geographic features is important. Comparison of results obtained with and without topography indicates that topography is the primary source of non-urban effects. These simulated effects on boundary layer vertical velocities are reflected in perturbations in the model-predicted boundary-layer height. Comparison of model results with Metropolitan Meteorological Experiment (METROMEX) radar first echo frequencies suggests that the model results are consistent with the hypothesis that cloud and precipitation anomalies are related to perturbations in boundary-layer dynamics caused by the urban heat island and surface roughness.
