Urban growth and aerosol effects on convection over Houston. Part II: Dependence of aerosol effects on instability

Our studies of the urban growth and aerosol effects on convection over Houston were performed in two phases. The first phase (Part I) used a sea-breeze-induced convective case (August 24 2000) as a benchmark for a series of sensitivity experiments varying the city size and the intensity of aerosol sources linked to sub-grid urban area fractions. The use of National Land Cover Dataset (NLCD) for three different years allowed an objective way to design the land-use sensitivity experiments as well as to consider the spatial distribution and differential intensities of the urban aerosol sources. This paper extends Part I to explore how the relative intensification of the cells downwind of the city (due to urban aerosol sources) can change for environments with more or less instability. This second series of numerical experiments consists of a fairly large number of multi-grid simulations (almost 100) that varied not only the intensity of the urban sources but also the value of CAPE. Our results were in agreement with previous studies; higher cloud condensation nuclei (CCN) concentrations reduced the sizes of cloud droplets and increased their probability of reaching supercooled levels and thus reducing coalescence. Therefore, downwind convective cells were intensified by additional release of latent heat of freezing. However, our results show a non-monotonic behavior on surface precipitation. This interesting response was linked to the riming efficiency reduction of ice particles when aerosol concentrations are greatly enhanced. Therefore, a greater fraction of the ice-phase condensed water mass is transported out of the storm as 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 initially increased and then decreasing when aerosol concentrations are further enhanced. The CCN concentration needed to reach maximum precipitation efficiency is higher for more unstable environments. (C) 2011 Elsevier B.V. All rights reserved.