A numerical study of interactions between surface forcing and sea breeze circulations and their effects on stagnation in the greater Houston area

High-resolution simulations from the Advanced Research Weather Research and Forecasting (ARW-WRF) model, coupled to an urban canopy model (UCM), are used to investigate impacts of soil moisture, sea surface temperature (SST), and city of Houston itself on the development of a stagnant wind event in the Houston-Galveston (HG) area on 30 August 2000. Surface and wind profiler observations are used to evaluate the performance of WRF-UCM. The model captures the observed nocturnal urban-heat-island intensity, diurnal rotation of surface winds, and the timing and vertical extent of sea breeze and its reversal in the boundary layer remarkably well. Using hourly SST slightly improves the WRF simulation of offshore wind and temperature. Model sensitivity tests demonstrate a delicate balance between the strength of sea breeze and prevailing offshore weak flow in determining the duration of the afternoon-evening stagnation in HG. When the morning offshore flow is weak (3-5 m s(-1)), variations (1 degrees-3 degrees C) in surface temperature caused by environmental conditions substantially modify the wind fields over HG. The existence of the city itself seems to favor stagnation. Extremely dry soils increase daytime surface temperature by about 2 degrees C, produced more vigorous boundary layer and faster moving sea breeze, favoring stagnation during late afternoon. The simulation with dry soils produces a 3 h shorter duration stagnation in the afternoon and 4 h longer duration in the evening, which may lead to more severe nighttime air pollution. Hourly variations of SST in shallow water in the Galveston Bay substantially affect the low-level wind speed in HG.