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Advances in Urban Climate Modeling TRENDS AND DIRECTIONS IN CLIMATE RESEARCH Advances in Urban Climate Modeling Julia Hidalgo,a,b Valery´ Masson,b Alexander Baklanov,c Gregoire´ Pigeon,b and Luis Gimenoa aUniversidad de Vigo, Departamento de F´ısica Aplicada, Facultad de Ciencias de Ourense, Ourense, Spain bCentre National de Recherches Met´ eorologiques/Groupe´ d’Etude de l’Atmosphere` Met´ eorologique,´ Met´ eo-France/CNRS,´ Toulouse, France cDanish Meteorological Institute, Research Department, Copenhagen, Denmark Cities interact with the atmosphere over a wide range of scales from the large-scale pro- cesses, which have a direct impact on global climate change, to smaller scales, ranging from the conurbation itself to individual buildings. The review presented in this paper analyzes some of the ways in which cities influence atmospheric thermodynamics and airborne pollutant transport. We present the main physical processes that character- ize the urban local meteorology (the urban microclimate) and air pollution. We focus on small-scale impacts, including the urban heat island and its causes. The impact on the lower atmosphere over conurbations, air pollution in cities, and the effect on me- teorological processes are discussed. An overview of the recent principal advances in urban climatology and air quality modeling in atmospheric numerical models is also presented. Key words: urban climate; urban meteorology; urban modeling; air pollution; urban heat island Introduction importance. As confirmed by the Fourth Assess- ment Report of the United Nations Intergov- Cities interact with the atmosphere over a ernmental Panel on Climate Change,2 anthro- wide range of scales. A schematic view of the pogenic influences, primarily from emissions interactions in the urban environment and the of greenhouse gases (GHGs) and aerosols,3–5 air quality and climate change processes that are the most significant causes of the current occur at various scales is displayed in Figure 1 global warming trend. Energy production and (after Ref. 1). However, a significant knowledge consumption are the principal sources of GHG gap exists in relation to the interactions be- emissions (CO2,CH4, see Fig. 2), with indus- tween the large-scale processes, which have a trial activities and transportation also making direct impact on global climate change (these a contribution. Agricultural activities and gen- will not be addressed here in detail), and the im- eral land use account for 22.5% of GHG emis- pact of these processes at other smaller scales sions. In order to reduce the extent of global ranging from the conurbation itself to individ- warming, there is an urgent need to reduce the ual buildings. anthropogenic emission of GHGs. The Kyoto It is in cities where the greater part of human protocol, which was signed and ratified by 174 activity occurs; hence, an understanding of the countries (as of November 2007) has set out role that cities play in global warming is of great the reduction objectives up to 2012, with ef- forts to continue after this date.6 In France, for example, a fourfold reduction of GHG Address for correspondence: Julia Hidalgo, Area´ de F´ısica de la Tierra is envisaged by 2050.7 Furthermore, ozone is (Grupo FA09), Universidade de Vigo (Campus de Ourense) As Lagoas – 32004, Ourense, Espana.˜ Voice: +34-988-38-72-70. [email protected] produced from urban pollutants (see the next Trends and Directions in Climate Research: Ann. N.Y. Acad. Sci. 1146: 354–374 (2008). doi: 10.1196/annals.1446.015 C 2008 New York Academy of Sciences. 354 Hidalgo et al.: Advances in Urban Climate Modeling 355 Figure 1. Schematic representation of interactions in the urban environment at various scales with regard to air quality and climate change processes (after GURME WMO1). section) and concentrations are increasing in ban areas affects the hydro–meteorological the troposphere, thereby also contributing to regime and pollutant deposition. global warming. • The release of anthropogenic heat fluxes The scope of this paper encompasses the im- by human activity affects the thermal pact of urbanization at the smallest scales. A regime. wide range of urban features can influence at- • The release of pollutants (including mospheric flow, its turbulence regime, and the aerosols) affects the transfer of radiation, microclimate, and can accordingly modify the cloud formation, and precipitation. transport, dispersion, and deposition of atmo- • The street geometry (‘street canyons’) af- spheric pollutants both within and downstream fects the flow regime and the exchanges of of urban areas (e.g., acid rains). Key examples heat between different surfaces (e.g., roads include the following: and walls). • The heterogeneity of building distribution, In this paper, the impacts of urban areas are and more generally of all rough elements described (section A) focusing on: (i) the Ur- of the earth’s surface, affects the turbulence ban Heat Island (UHI) and its causes, (ii) the regime of the flow. observation of the UHI and its main character- • The massive use of impervious materi- istics, (iii) the impact on the lower atmosphere als and the scarcity of vegetation in ur- (below 1 or 2 km), (iv) the city’s impact on the 356 Annals of the New York Academy of Sciences Figure 2. Annual greenhouse gas emissions by sector. Relative contributions of man- made greenhouse gases for the year 2000 as estimated by the emission database for the Global Atmospheric Research fast track 2000 project (http://www.globalwarmingart.com/ wiki/Image:Greenhouse_Gas_by_Sector.png). precipitation pattern, and (v) air pollution in surrounding area. The difference in tempera- cities. State-of-the-art techniques to simulate ture can reach up to 10◦C for large conurba- each of these topics in atmospheric numeri- tions under certain weather conditions.8 Even cal models are presented in section B. Some of though the effects of the UHI phenomenon are these models are, or soon will be, used to per- not usually catastrophic, as they are for hurri- form operational weather forecasts over urban canes, tornadoes, or some thunderstorms, they areas. can nevertheless intensify heat-related stress, es- pecially at night during heat waves, and can lead to tragic consequences for public health. This was the case in 2003 when a strong Description of Urban Microclimate heat wave affected Europe and caused more The Urban Heat Island and Its Causes than 70,000 casualties, with a higher percent- age of victims in urban areas, e.g., in France.9 The most striking characteristic of the urban The UHI phenomenon has been observed in microclimate is the UHI. This phenomenon is cities in a wide range of locations, from mid- generally experienced by an observer traveling latitude to high-latitude, as well as in tropical between a city center and its less urbanized sur- and equatorial areas.10 Other atmospheric pro- roundings. The UHI effect causes the tempera- cesses are also affected by urbanization, such as ture to be warmer in the city center than in the the dispersion of pollutants, fog formation, and Hidalgo et al.: Advances in Urban Climate Modeling 357 Figure 3. Typical air temperature characteristics in rural and dense urban areas. The spatial pattern of temperature is composed of the “cliff” at the rural–urban transition, the “plateau” in the suburban area, and the “peak” over the city center (adapted from Oke, 19878). dissipation. Since most economic activity and ral and the urbanized zones. Towards the city capital investment is concentrated in urban ar- center, there is a region of gradual tempera- eas, research in this field is of high strategic ture increase, with a maximum temperature value. being reached in areas of the most dense ur- The time-varying characteristics of a UHI banization, characterized by high buildings and generally follows a daily cycle, increasing dur- narrow streets (Fig. 4). Urban climatologists ing the late afternoon or early evening, and commonly use the ratio of building height reaching a maximum at night. Its intensity to street width to quantify the density of ur- starts to decrease after dawn and generally banization.11,12 The spatial distribution of the reaches a minimum during the morning hours UHI has commonly been studied using in- (see Fig. 3 for an example of a UHI in Toulouse, strumented cars traveling across a conurba- France, during winter). This temporal variabil- tion. This typical UHI pattern is clearly mod- ity of the UHI has mainly been studied using erated by the structure of each city and the pairs or groups of urban and rural or suburban regional flow characteristics. For example, in weather stations (commonly airport weather a city with different cores of high urban den- stations). From analysis of the temporal evo- sity and for low wind speeds, the UHI can be lution of an urban and a rural station (as in multicellular.13 Fig. 3), it may be inferred that the UHI re- The nocturnal UHI generally extends a few sults from a differential cooling of the air at the hundred metres above the ground. In the case two locations. The UHI is also affected by re- of a nocturnal UHI, a lower atmospheric layer gional weather conditions. In the presence of (commonly 150 to 300 m high) is character- strong winds or cloud cover, the UHI inten- ized by a linear reduction of the temperaturea sity is reduced, or the phenomenon may even difference between the urban core and the disappear altogether. Seasonal cycles also af- fect the frequency of UHI occurrence. During rainy, cloudy, or windy seasons, a UHI may be a In Figures 3 and 5 the temperature variable used is in fact the “po- less frequent or weaker, while the opposite is tential” temperature (θ), related to the true temperature (T) by: θ = T (p/100000)2/9,wherep is the pressure (in Pascal). The potential tempera- true for anticyclonic conditions.
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