INFLUENCE OF URBAN AREAS ON DIURNAL RAINFALL PATTERNS

During urbanization natural land covers are removed and replaced by artificial structures and surfaces (e.g., buildings, roads, parking lots, and sidewalks), natural trees and vegetation are replaced with ornamental landscapes, and the soil profile and structure are modified. These alterations impact the land-atmosphere exchanges of water and energy, which is manifested in modified temperature and moisture profiles, augmented precipitation patterns, and wind perturbations and enhanced turbulence. The urban impact on rainfall is due to one or a combination of four factors: (1) enhanced thermal mixing due to Urban Heat Island (UHI), (2) increased turbulence and mechanical mixing due to increased roughness created by tall buildings, (3) modified low-level atmospheric moisture, and (4) increased concentrations of cloud condensation nuclei (CCN) from automobiles and industry. Identification of which factors contribute to urban rainfall modification for a particular city requires in-depth data analysis and focused numerical modeling experiments. A recent conjunctive analysis of space-based and ground-based rainfall data in Houston, Texas has shown elevated rainfall amounts and rates within and directly downwind of the city. Part of this work involved the analysis of 13 years of rainfall data from a dense rain gauge network in Houston. Data were summed into four time periods: midnight to 6 am, 6 am to noon, noon to 6 pm, and 6 pm to midnight. The data were also stratified by season (winter, spring, summer, and fall) to better define the warm season (June, July, and August) rainfall impacts. On average, 198 mm and 143 mm of rain falls from noon to midnight during the warm season in the urban area and climatologically downwind area, respectively, compared to 94 mm of rain in the climatologically upwind area. This represents an increase of rainfall during the noon to midnight time period of approximately 110% in the urban area and 52% in the downwind region, compared to the upwind region suggesting an urban influence on the spatio-temporal rainfall pattern. Further results show that there are nearly two times as many occurrences of rainfall from noon to midnight in the urban area compared to the upwind area. The presentation will focus on these results and the implications of the findings for flood control in Houston.