Mesoscale and Meso-Urban Meteorological and Photochemical Modeling of Heat Island Mitigation in California: Results and Regulatory Aspects

Mesoscale and meso-urban meteorological models were updated and applied, using new techniques and parameterizations, in fine-resolution photochemical modeling of the effects of heat-island control in California. A meso-urban meteorological model, uMM5, based on Taha (2008a) and Dupont et al. (2004), improved the simulation of fine-resolution meteorological fields in the urban canopy layer (UCL). Compared with roughness-length-based mesoscale models, drag-force-based meso-urban modeling can capture phenomena of interest that are not typically detectable at the coarser scales. These include, for example, cool islands, heat islands, flow convergence associated with heat-island circulation, flow divergence at the leading edge of urban areas, and vertical variations in turbulent kinetic energy in response to changes in densities of buildings and vegetation canopies. Enhanced modeling of these meteorological fields is crucial for improved emissions and photochemical modeling of urban heat island (UHI) mitigation. From a regulatory perspective, photochemical air-quality modeling is an important tool in the planning and demonstration processes toward implementing strategies that target improvements in or maintenance of air quality. For State Implementation Plan (SIP) modeling purposes, UHI mitigation has thus far been considered as an emerging control strategy that must be modeled and evaluated as a voluntary control measure, per USEPA requirements. Models developed for evaluating UHI reduction under funding from the California Energy Commission (Taha 2008b, 2008c) are now being used in modeling the ozone air-quality benefits of urban forest in the greater Sacramento, California, region. This effort is part of an ongoing study performed for and led by the Sacramento Metropolitan Air Quality Management District (SMAQMD). Mesoscale and meso-urban simulations to date suggest that implementation of heat-island control strategies, via increased urban albedo and forest cover, can be especially effective when large areas are modified, whereas limited-area control (e.g., a geographical area smaller than ~100 km 2 ) may have both positive and negative effects, albeit at different times and locations. Limited-area UHI control can produce local improvements in air quality but sometimes mixed impacts downwind. The latter situation arises because of changes in mixing, boundary-layer height, wind speed (flushing of pollutants), and their interaction with topography (i.e., the blocking effect of the eastern mountain ranges in coastal California). Such findings can shed additional light on the “threshold” effect (or plateau), previously identified in modeling California (Taha 2005, 2007), past which any further surface modifications result in smaller net ozone reductions. The results re-emphasize the need for region-specific planning and modeling to develop an optimal mix of surface modification strategies for UHI mitigation. The improved modeling system can capture fine-resolution features of meteorological fields of interest to this application. For example, the vertical profiles of temperature within the canopy layer reflect the effects of land-cover variations within an area. Vegetated areas in Sacramento can be up to 2o C cooler than are less-vegetated areas during the times of peak daily temperatures. The UCL simulations showed Bowen ratios β > 50, β ~ 3.5, and β ~ 1.3, respectively for high-rise, residential, and forested areas in Sacramento. The model results also showed that during nighttime hours, increased surface albedo and canopy cover have little or no effect, but that during the daytime, they have a significant impact on air temperature. The “cooler” air (i.e., temperature difference) can be transported downwind and thus can also have an impact on areas beyond the modified urban regions (e.g., where surface albedo and/or canopy-cover were increased). In the Sacramento area, air temperature can be reduced by up to 3o C in the high-albedo scenarios and up to ~2o C in the increased urban forest scenario. The fine-resolution photochemical simulations of the Sacramento region showed peak ozone concentrations