Observations of Coastal Urban Influences on Convective Precipitation over Houston, Texas

Urban-induced impacts on convective thunderstorms are analyzed herein. Data from the Convective-cloud Urban Boundary Layer Experiment (CUBE) and Tracking Aerosol-Cloud interaction Experiment (TRACER), conducted in Houston, Texas, are used to study two distinct episodes. The observation-driven analysis used data from surface meteorological stations, three radar sites, and radiosonde launches to study two urban rainfall cases considering coastal processes. Overall, the results show that for Houston, urban sea breeze front (SBF) impacts preceded urban thunderstorm (TS) modifications. Specific results reveal that boundary layer offshore and mid-tropospheric along-shore flows weakened SBF propagation and influenced rainfall location, respectively. These weakened SBFs were bowed over the city due to building barrier effects, which led to the formation of a moderate (~2 K) post-rainfall daytime UHI. Frontal passage over the urban core and its UHI determined the timing of urban rainfall modification. The isolated rainfall case saw enhancement of the typical urban downwind convergence pattern in the post-SBF regime, while the other case also saw progressive urban organization of widespread convective cells into a post-SBF bifurcated storm. The coastal bifurcation case shows novel observation of a small, mesocyclone pair within the building-stalled SBF flow. This effect developed a split rainfall rate maxima of >75 mm/hr over the city. The presented results are significant as they broaden the understanding of urban precipitation processes and SBF modification.