Observations of Coastal Urban Influences on Convective Precipitation Over Houston, Texas
Abstract
Urban-induced impacts on convective thunderstorms in coastal environments are analyzed. Data collected during the Convective-cloud Urban Boundary Layer Experiment (CUBE) and Tracking Aerosol-Cloud interaction Experiment (TRACER), conducted in Houston, Texas, are used to study urban rainfall intensification considering coastal processes. The observation-driven analysis uses surface weather station and operational radar data of thunderstorms for an entire warm season (JJAS). From this period, two distinct thunderstorm cases were further detailed. Overall, the results show that for Houston, urban sea breeze front (SBF) nimpacts preceded urban thunderstorm modifications. Specific results for urban–rural diurnal differences reveal a daytime peak in urban moisture excess (1.8 g·kg−1) compared to rural areas, which is enhanced on SBF days (2.3 g·kg−1). This urban moisture excess contributed more than the urban heat island to midday surface equivalent potential temperature (θe), an indicator of favorable storm air masses. Results from a spatial aggregate across 24 SBF passages show θe is enhanced along the city edges. Heavy rainfall patterns across all storm times showed little urban signal, while rainfall patterns for storms in the post sea breeze environment revealed an increased urban influence. A case involving bifurcation of the onshore coastal flow was consistent with averaged results, with a split moisture field preceding heavy (>75 mm) over-city rainfall in one hour. The results are significant in documenting a rainfall modification pathway by coastal cities, which emerges from their enhancement of surface moisture (and correspondingly θe). These findings underline the importance of considering urban–rural moisture gradients in understanding coastal urban rainfall modifications.
