Taha then a Staff Scientist with the Lawrence Berkeley National Laboratory led modeling for multi-state evaluation of the energy, meteorological, and air-quality impacts of heat island mitigation. One goal of this research is to help facilitate the deployment of heat-island control measures. The urban heat island modeling and research at Altostratus is undertaken at the meso- and meso-urban scales. The modeling is then focused on those areas where mitigation would be most beneficial.
The extensive multi-year modeling of those areas shows that the implementation of heat island control can help reduce energy use, reduce emissions, and reduce air pollution, e. These areas can be cooled by up to C. The multi-episodic and seasonal modeling of major metropolitan areas in California, for example, shows that the cooling effects from heat-island mitigation are relatively consistent regardless of the variations in local summer weather conditions.
In terms of air quality, the impacts on reducing ozone concentrations are significant and can help several regions attain or maintain the air quality standards. For example, the largest daily reductions in 1-hour average ozone can range from 2 to 8 ppb or more in various areas in California. Reductions in the 8-hour peak can reach up to ppb. The modeling also suggests that heat-island control is not always desirable; there are times at certain locations when heat island control can adversely impact flow and mixing and can thus increase ozone concentrations. While such events are relatively shorter, they nevertheless highlight the need to tailor the mitigation strategies for each region according to its local climate, emissions, and urban characteristics, so as to minimize any potential negative impacts.
Over the years, Altostratus has developed data, information, and findings about urban heat islands and their mitigation potentials. Of note is that the mitigation measures can be equally effective regardless of whether urban heat islands exist or not. In other words, even if an urban area is cooler than its surroundings, the mitigation measures will still have an impact.
In the U. Solar photovoltaic PV technologies have many benefits. The direct benefits are those of providing electricity and the indirect ones are those of avoiding power generation, e. The latter translates into reduced emissions of greenhouse gas and other pollutants, such as ozone precursors, thus reducing the atmospheric loading of particulates and ozone, reducing associated radiative-forcing effects, and improving air quality. One question that has been raised recently is whether large-scale deployment of solar PV arrays in urban environments can inadvertently impact the atmosphere, e.
Thus the goal of this effort at Altostratus Inc. The meso-urban atmospheric effects of PV deployment were evaluated via simple energy-balance calculations as well as full and detailed atmospheric modeling for the Los Angeles Basin as an example. In other words, there are no negative impacts on air temperature or the wind field.
The increase is small, reaching up to 0. With higher conversion efficiencies, expected in the future, the ambient cooling effect will be larger. It is important to note that these results are specific to the Los Angeles Basin and the assumptions made in the study. While the results can provide an idea for anticipated impacts, they are non-transferable to other regions.
For each region, specific analysis and modeling must be carried out to evaluate the local impacts of PV deployment. Among a host of potential ozone air-quality control measures, the SMAQMD and the Air Districts in the region are considering and evaluating the feasibility of using urban reforestation replacement of high-emitting species with a lower emitting mix to help the region attain and maintain the 8-hour ozone standard. Urban forests can impact air quality via several means, e.
The goal of this modeling effort includes developing and updating a state-of-science modeling system, developing higher-resolution data surface, canopy cover, meteorology, biogenic emissions, carrying capacity and simulations to better quantify the meteorological, emissions, and air-quality impacts of an urban forest program in the Sacramento Federal Non-Attainment Area SFNA. The modeling shows that as a result of canopy-cover increase over time, the urban area experiences a cooling effect. For example, by the year , the maximum cooling in the afternoon reaches up to 0.
By the year , the largest daily afternoon cooling reaches up to 1 to 1. With the addition of 2. Scores of federal workers and even the President experienced transportation and work delays as a result.
During the summer of , the country experienced record-breaking urban flooding in places like Atlanta, Georgia, Nashville, Tennessee, and Oklahoma City Shepherd et al. In , the south central U. During this regional-scale heat wave event, urban heat islands in cities like Dallas and Oklahoma City likely caused prolonged heat exposure at night Figure 3. Urban heat islands in cities like Dallas and Oklahoma City likely caused prolonged heat exposure at night. There is also emerging interest in understanding the role of urbanization on the climate system, which includes weather IPCC, Cities change properties of the land surface and subsurface, and are also known to influence atmospheric circulation patterns at various spatial scales Hidalgo et al.
The high building densities and sparseness of vegetation in cities makes urban surfaces. In addition, the three-dimensional nature of urban environments affects a number of parameters such as evaporation rates, absorption and reflection of solar radiation, and storage of heat, as well as wind and turbulence fields Figure 3.
Gaseous and particulate matter emissions Figure 3. Classifications for urban land cover zones have been developed Stewart and Oke, a, b that are useful for documenting metadata of urban monitoring sites and for characterizing surface features in urban modeling tools. Reprinted with permission.
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Overall, urbanization results in a suite of complex land surface atmosphere interactions that modify thermodynamic, radiative, dynamic, and hydrometeorological processes within the urban area and its surrounding regional footprint, particularly downwind of the city. Extreme weather and climate events in cities typically occur during unfavorable regional-scale conditions exacerbated by global warming trends and local, urban effects Hunt et al.
As an example, regional-scale heat waves, which are. Research shows that it is spatially correlated with regional land-use and land-use change. During the early phases of urban development, multiple land covers—bare land, vegetated areas, agricultural plots, and built-up areas—emerge in close proximity with each other. As urbanization increases, resulting in reduced vegetated surfaces, the spatial pattern of the urban heat island becomes less scattered and more intense Imhoff et al. Stone et al. It should, however, be noted that the types of data used to define UHI characteristics play an important role, and the discussion of UHI often lacks important information about data sources and site characteristics Stewart, In situ observations of air temperatures from measurement sites within the urban canopy layer provide information about the urban-canopy-UHI.
Remotely sensed observations provide information about boundary-layer heat islands above the urban canopy layer in the atmosphere and surface heat islands. In general, weather patterns in and near cities depend both on the degree of urbanization—characterized by the above mentioned changes in land surface characteristics, subsurface properties, and chemical composition of the atmosphere—and on larger scale meteorological conditions Mestayer and Anquetin, For situations with moderate-to-high wind speeds, an urban plume with warmer, polluted air is advected downwind of the city Figure 3.
Under such conditions, the lowest portion of the urban boundary layer UBL , the surface layer, can be divided into two main. TABLE 3. The column headings represent three ways that urban environments can affect weather and climate. The row headings represent different weather processes affected by each of the three. This shear layer also controls the turbulent exchange and ventilation between the UCL and the flow above average roof level, which is typically highly instantaneous and controlled by coherent structures Christen et al.
Overall, the RSL plays a critical role, and its properties need to be properly resolved in numerical models for accurate urban weather and air-quality forecasts. The values are in units of kW h m 2 day Figure courtesy of R. Sass, Rice University. The RSL typically extends from the surface to a height equivalent to times the average building height Raupach et al.
Within the UCL, atmospheric patterns are spatially inhomogeneous, strongly influenced by local effects, and very hard to predict Klein et al. At the same time, the UCL is the region where most of the urban anthropogenic emissions of atmospheric pollutants occur and where people spend most of their time, and is thus of great relevance. In the upper part of the RSL, above average roof level, a strong shear layer—a layer with high wind velocity gradients—develops and dominates the RSL wind and turbulence patterns.
SVF in box c stands for sky view factor. For large-scale weather patterns with weak dynamic forcings i. Canopy-layer UHI signatures are typically strongest under such conditions, with the highest values recorded at night. Historical and current literature has also persistently shown that UHI destabilization, urban surface roughness, and pollution can independently or synergistically initiate, modify, or enhance precipitation cloud systems. However, there is discourse about the sign of precipitation change i. Studies continue to verify that urbanization may also modify lightning Rose et al.
Further, urban land use accelerates hydrologic response through surface runoff variability and stresses on conveyance systems Shepherd et al.go site
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Human activities in urban areas e. Although urban areas have significantly higher carbon dioxide concentrations than in rural areas, greenhouse gas emissions per capita may be lower for urban dwellers than those for rural dwellers Dodman, Jacobsen has recently discussed the implications of urban carbon domes on public health and the climate system.
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Urban modification of winds, temperature, and turbulence including mixing height affect the concentration, dispersion, and transport of atmospheric pollutants which, in addition to higher emission rates, contributes to poor air quality in cities Grimmond et al. Numerous studies have focused on predicting and reducing urban air pollution, yet deficiencies in both observational and modeling capacity are still evident NRC, Ozone and particulate matter up to 2. Beyond conventional air quality concerns, accidental and intentional releases of chemical, biological, or radiological pollutants in cities remain as potential threats and pose particular challenges.
The NRC b concluded that no model system exists that fulfills all critical requirements for emergency response. Aerosols affect climate, directly and indirectly, through radiative forcing Kaufman et al. Most aerosols, including sulfates found in urban environments, promote a cooling effect in the radiative budget; however, carbon-based aerosols absorb solar radiation and may warm the atmosphere and surface. Such warming can affect the atmospheric stability profile and thereby alter cloud and precipitation development.
Climate-aerosol interactions are quite complex and beyond the scope of this discussion, but it is clear from emerging literature that the negative and positive effects associated with the urban production of aerosols must be placed in the context of scale: local or urban , regional, and global. For example, aerosols augment UHI-effects mainly in the surface or boundary layer on temperature through direct interactions with solar radiation Jin et al.
Souch and Grimmond and Grimmond et al. Expanding on these assessments, the current state of urban observational and modeling techniques in research and operations, emerging technologies, and remaining needs and challenges for urban meteorology were discussed at the workshop.
Meteorological and Air Quality Models for Urban Areas
The following section summarizes these discussions and relevant literature. Meteorological observations and modeling are tightly coupled and require continual emergence of new understanding, measurements, and technology Dabberdt abstract in Appendix A. The measurements needed for particular meteorological processes are typically a function of the spatiotemporal scale of the process, the latency requirements of the application, and technological capacity.
These models require detailed four-dimensional representation of the atmosphere provided by surface in situ measurements, surface-based remote sensing systems, upper air soundings, and satellite data often through data assimilation techniques.