Aerosol Pollution Impacts over an Urban Complex

The effects of the Houston Metropolitan Area on the characteristics and intensity of convection and precipitation were investigated. We used the Regional Atmospheric Modeling System developed at Colorado State University (RAMS@CSU) coupled to the Town Energy Budget (TEB) urban model. RAMS@CSU microphysical modules consider the explicit activation of CCN (and giant CCN), a bimodal representation of cloud droplets, a bin- emulation approach for droplet collection, ice-particle riming, and sedimentation, and direct radiative effects of aerosols. Our studies focused on events triggered by the sea-breeze circulation and were performed in two phases that used an identical modeling framework. RAMS@CSU was 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 first phase used a case study (August 24, 2000) as a benchmark and the 1992, 2001, 2006 high-resolution National Land Cover Data (NLCD) for an objective experimental design. In addition, CCN sources of varied intensity were linked to sub-grid urban area fractions. The relative intensification of the cells downwind of the city due to urban aerosol sources may change for environments with more or less instability. For that reason, the second phase examined how variations in convective instability can modify precipitation for sea-breeze-induced storms over a polluted urban complex. This fairly large number of multi-grid simulations (almost 100) varied not only the intensity of the urban sources but also the value of CAPE. In summary, our results were in agreement with previous studies, wherein enhancing CCN concentrations reduced the size of the liquid particles and increased the probability of liquid particles reaching supercooled levels as a consequence of reduced coalescence. Therefore, downwind convective cells were intensified by an additional release of latent heat as liquid particles became frozen. However, our results also show that the effect on surface precipitation of the additional LWC carried to subzero temperature levels with increasing CCN concentrations is not monotonic. The non-monotonic behavior was linked to the riming efficiency reduction of ice particles when aerosol concentrations are further enhanced. Therefore, a greater fraction of the ice-phase condensed water mass is transported out of the storm as anvil cloud pristine ice crystals instead of being transferred to precipitating water species. Moreover, the precipitation efficiency of cells downwind of the city exhibited a similar behavior, increasing when CCN concentrations are moderately enhanced and then decreasing when aerosol concentrations are further enhanced. The pollution level for which the precipitation efficiency reaches a maximum is higher for more unstable environments.