Urban land-use and pollution impacts on mesoscale circulations and convection over Houston

This study investigates the effects of the Houston metropolitan area on the characteristics and intensity of convection and precipitation, using a convective event occurred on August 24 2000 . We implemented the Town Energy Budget (TEB) urban model into the an improved version of RAMS that considers the explicit activation of CCN (and giant CCN), a bimodal representation of cloud droplets, a bin-emulation approach for the microphysics , and direct radiative effects of aerosols. RAMS@CSU is configured to use four two-way interactive nested model grids with 42 vertical levels and horizontal grid spacings of 45.0, 15.0, 3.75, and 0.75 km centered over Houston. The Landsat Thematic Mapper T National Land Cover Data (NLCD) corresponding to the years 1992, 2001 and 2006 were used as benchmarks for the experimental design of the land-use sensitivity experiments. We considered aerosol sources with intensities proportional to the sub-grid urban coverage (using TexAQS/ GoMACCS field campaign data to estimate the maximum intensity).

We analyzed the impact on two distinct groups of convective cells that developed on August 24 2000. The first group of storms located southeast of the city attained its maximum at approximately 18:30 UTC. The second, occurred north of the city (downwind) a few hours later. For the first group, the effects of land-use on convection and precipitation were dramatic and mainly linked to a monotonic increase in the intensity of the sea breeze due to the urban land-use change. The total volume of precipitation increased monotonically 9, 11, and 30% (over the NOCITY run) for 1992, 2001, and 2006, respectively. However, liquid water paths and updrafts maxima did not change significantly as the cells covered a larger area. Conversely, the effects on the storm that developed north of the city were not dominated by land-use change in itself but by the enhanced concentrations of aerosols. In this case, lower precipitation rates were simulated consistent with several previous studies indicating that increases in CCN number concentrations tend to reduce warm rain efficiencies and increase cloud water contents. In addition, greater amounts of supercooled water are available in the polluted environment enhancing non-inductive charge separation mechanisms.