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Local heterogeneity of CO 2 sources and sinks is a key factor for the variability of carbon dioxide flux (F C ) in urban areas. Information on the urban structure around a site, especially the related emission characteristics, is thus of great importance to the understanding of observed F C . Strong spatially confined sources like major roads inhibit a direct correlation of F C to area-averaged features of the urban structure and may lead to a heavily biased signal.Four years of F C measured at Basel Aeschenplatz, Switzerland, are analysed with respect to the controlling factors and the cause for variability on different time scales. The source area is segregated into equal sectors to address heterogeneous emission patterns. Residential areas to the east are bordered by business areas and major roads to the west, which leads to a fundamental dependence of F C on wind direction. Besides, its diurnal course is explainable with traffic emissions while its annual course follows heating-related combustion emissions. Vegetation fraction is rather considered to be an indicator for urban land use types (residential/business) and the attributable emission characteristics than to be a measure for biological sink effects. Inter-annual variability occurs as a result of anomalies in wind direction patterns or air temperature. Average yearly F C is 16.4 μmol m -2 s -1 with slight variations (±0.55 μmol m -2 s -1 ) over the 4 years. It likely originates from an average of 70% traffic and 30% heating-related emissions with significant sectoral differences.As a continuous measure for the emissions of each sector, the expected CO 2 flux (eF C ) per sector is introduced, leading to an enhanced comparability. Relating sectoral eF C instead of F C to urban surface fractions of buildings and vegetation results in a better agreement (also with data from other studies).

Meteorologists are most interested in understanding how energy in the form of radiation and heat influences the urban climate and how this energy is transported, transformed and stored (e.g. in urban building structures). They also are interested in the effects of precipitation on cities, how storm water runoff is changed and how much water is emitted into the atmosphere through evapotranspiration. In addition, they want to know how much cities worldwide contribute to climate change through their emissions to the global carbon cycle. For meteorologists to address the challenges of sustainable cities and urban planning, information on the distribution and flows of energy, water and carbon in typical urban systems have to be known.From a meteorological perspective, the urban metabolism of a city is strongly dependent on the prevailing regional and local climate and its built-up structure. Together these define the microclimate within the street canyons, on the roads, in the buildings, and at any other place in an urban area. In this context, the urban energy, water and carbon balances are presented in this Chapter. URBAN ATMOSPHERE Layers and ScalesA key issue of importance for urban investigations is the definition of the appropriate scale of a study area. A classification of urban canopy layer (UCL) elements according to scale considerations is given in Table 4.1. Vertically, the urban atmosphere can be divided into layers as illustrated in Figure 4.1. The lower atmosphere that is influenced by the urban structure is called the urban boundary layer (UBL). From the ground up to roughly the average height of roughness elements like buildings or trees ( ℎ ) is the UCL. It is produced by micro-scale processes in their immediate surroundings. The UCL is part of the roughness sublayer (RSL) which is dependent on the height and density of roughness elements and extends to the height * = ⋅ ℎ where ranges between 2 and 5 (Raupach et al. 1991). Above this is the inertial sublayer (ISL) where under ideal conditions vertical fluxes of energy or matter can be expected to be constant with height. The upper part of the UBL, which is to a large extent determined by meso-scale advective processes, is referred to as the outer urban boundary layer (Rotach et al. 2005).

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