Comparison of the urban heat island signatures of two Texas cities: Dallas and Houston

Both Dallas and Houston, Texas have comprehensive networks of surface meteorology and chemistry sensors. The similarities of the networks and lack of terrain in Dallas and Houston allow for the comparison of their urban heat islands (UHI). The Dallas UHI, unperturbed by thermal flows driven by the land/sea temperature difference, is a well-defined phenomenon over the summers of 2000-2006. Including all weather conditions, the average nighttime T(urban) – T(rural) temperature difference was between 1.5º and 2.0º C and the average daytime difference was ~ 1.0º C. Analysis of Houston temperature data, however, revealed a different picture due to the bay and gulf breezes. While the Houston UHI was a distinct phenomenon, even when including all weather conditions, the bay or gulf breeze modified the Houston UHI by cooling the city. Average nighttime T(urban) – T(rural) temperature differences in Houston were between 1.75º and 2.75º C. However, during the day, the rural areas to the north and west of the city were often warmer than the downtown area during afternoon hours as a result of the sea breeze. Averaging the Houston T(urban) – T(rural) temperature differences over the summers of 2000-2006 indicated a very small urban-rural temperature difference between 1400 to 1600 LST. In some individual years, such as 2000, 2003, 2005 and 2006, the urban areas were actually cooler than the rural areas, on average, in the mid-afternoon. These years had more bay breeze/gulf breeze activity to cool the urban area.

In addition to characterizing the Dallas and Houston UHIs under different weather conditions, we will look at the UHI effects on the surface winds of both cities. For Houston, we will also look at the UHI impact on boundary layer height, using backscatter measurements from TOPAZ, an ozone and aerosol profiling lidar that was deployed on a NOAA Twin Otter in the summer of 2006. We will look at surface ozone patterns to assess if changes in surface winds and boundary layer heights due to the UHI impact the transport and horizontal variability of surface ozone.