Effects of urban pollution on downwind storms: An energetic perspective

In two previous studies. the effects of the Houston Metropolitan area on the characteristics and intensity of convection and precipitation were investigated for events triggered by the sea-breeze circulation . Carrió et al (2010) isolated the effects of the land-use change and examined the indirect effects of urban pollution considering sources of varied intensity linked to sub-grid urban area fractions. The Regional Atmospheric Modeling System developed at Colorado State University (RAMS@CSU) was validated against radar observations for the case used as a benchmark for these sensitivity experiments. With regard to the aerosol effects, as other authors have also found, enhancing cloud condensation nuclei (CCN) can produce an invigoration of downwind convective cells due to additional latent heat release. However, results showed an interesting non-monotonic behavior of convection intensity and precipitation when more intense CCN surface sources were considered. A second study (Carrió et al, 2011) consisted of a large number of multi-grid simulations (more than one hundred) and varied convective instability in addition to the intensity of urban pollution sources. The non-monotonic 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. Even though, Carrió et al, (2011) strongly supports the relationship between the behavior of the simulated precipitation and the aforementioned microphysical mechanism, the evidence could be considered somewhat "circumstantial". For that reason, the problem was revisited with a new modeling study that approaches it from a more energetic perspective. These new numerical experiments used RAMS@CSU coupled to the Town Energy Budget (TEB) urban model and a microphysical module that considers the explicit activation of CCN (and giant CCN), a bimodal representation of cloud droplets, and a bin-emulation approach for droplet collection, ice-particle riming, and sedimentation. Model outputs every 30s were used to analyze the (indirect) effects of urban pollution on the efficiency of microphysical processes leading to the generation of precipitation particles (involving ice-phase and warm rain), latent heat release rates, buoyancy, vertical momentum, as well as several integral quantities linked to the convective cells simulated downwind of the urban complex. Results clearly support the explanations inferred in the previous study, show a non-monotony behavior of several macroscopic characteristics of the downwind convective cells, and indicate that the expected increase of the particulate pollution is more likely to selectively enhance precipitation of convective events characterized by higher instability and extreme precipitation events.