Direct and indirect effects of urban pollution and land-use change impacts: A comparison

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 and radiative transfer 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 the direct radiative effects of urban pollution.

Our studies focused on events triggered by the sea-breeze circulation and were performed in four 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 (reference Carrio and Cotton, 2009) focused on the land-use change effects. This first series of simulation 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. The second phase(Carrio and Cotton 2009) examined the indirect effects of urban pollution considering sources of varied intensity linked to sub-grid urban area fractions. The dependence of the intensification of the cells downwind of the city due to urban pollution indirects effects on the convective instability was analyzed in a third phase(Carrio and Cotton 2010). Finally, the fourth series of simulations focused on direct (radiative) impact of urban pollution on precipitation and convection, and on the comparison of the various aforementioned effects.

In summary, our results indicate that changes in land-surface properties associated with growth of the urban complex and its interaction with the sea-breeze circulation is the dominant effect. The invigoration of downwind convective cells due to additional latent heat release linked to the enhanced enhancement of supercooled liquid production in polluted aerosol plumes ranks second to land-use changes in importance. Finally, the direct radiative heating effects of pollution generates surface cooling and a non-negligible reduction of convective instability and precipitation. However, the later ranks third in importance , especially over the area downwind of the city.