Interactions between city and mesoscale sea-breeze circulations in the Greater Houston area: A study with the coupled WRF-urban model

Summertime meteorological conditions in the Houston-Galveston (HG) area are determined by interactions between mesoscale sea-breeze circulations, force by differential heating between daytime warm land, the relatively cooler offshore waters, and large-scale (background) geostrophic flows. The wind perturbation associated with the diurnal cycle of sea breeze can counteract the background wind to produce stagnant wind conditions in the afternoon, resulting in high ozone concentrations. This study investigates, using high-resolution mesoscale Weather Research and Forecasting (WRF)-urban model simulations, the role of city and background soil moisture in the formation and evolution of sea-breeze circulations for a high ozone-pollution event during the Texas Air Quality Study 2000 (TexAQS2000) field program over the HG area. The WRF-urban model was configured with 4 nesting domains with grid spacing of 27 km, 9km, 3km, and 1km. The urban areas for the GH region were specified with the USGS 2001 National Land Cover Dataset (NLCD) based on 30-meter Landsat data. Verified against surface network data, WRF-urban is able to reproduce observed diurnal wind rotation and stagnant wind over most of urban regions in early afternoon, but in general underestimates morning wind spend. It also captures the evolution of wind structures in the urban boundary layer reasonably well. A number of experiments were conducted for assessing the sensitivity of modeled evolution of land-sea-breeze circulations to different land characteristics, including: 1) replace the HG urban rland-use by cropland, 2) perturb soil moisture conditions at mesoscales, and 3) using hourly sea-surface temperature. The development of modeled land-sea breeze is substantially modified by city and background soil-moisture distribution. In the simulation without city, the sea-breeze front reaches the city about one hour earlier, and the boundary layer depth is shallower with higher wind speed than that in the control simulation. Compared to the simulation with higher soil-moisture contnent, the simulation with dry soil produces earlier onset and deeper inland penetration of sea breeze, and a delayed land-breeze return flow. Results illustrate that interactions between background soil moisture and urban surface layer significantly affect the onset and penetration of land- and sea-breeze, and hence modify the occurrence of wind stagnation in the HG area.