Nocturnal Boundary Layer Evolution in Houston during the TexAQS II Campaign

The evolution of the nocturnal boundary layer in Houston, TX was examined using high-temporal resolution tethersonde profiles taken on four nights during the TexAQS II field campaign. The measurements were made at the University of Houston campus located approximately 4 km southeast of the downtown Houston central business district and consisted of vertical profiles up to 400 m AGL of potential temperature, water vapor mixing ratio, wind speed, wind direction, and ozone concentration. Profiles were taken at approximately 30 min intervals throughout the night including both the evening and morning transitional periods. Preliminary results from the night of 7-8 Sept. 2006 showed that the stability during the evening transition period remained near neutral in the lowest 200 m AGL. Cooling occurred uniformly within this layer but stronger surface cooling appeared to be delayed; it ultimately began between 2130 – 2315 CDT and continued through 0430 CDT. In the early morning an elevated bluff-like structure developed in the virtual potential temperature profiles indicating neutral stability within the 50 - 100 m AGL level. Just before sunrise a neutral layer with constant potential temperature developed between the surface and 50 m AGL indicating mixing processes occurring near the surface. Further analyses will be presented for one additional overnight period during the campaign.