There were no fundamental changes in state of research as it has been presented in the first application for the project. A focus of research in that field is still the investigation of the energy balance of urban areas with the intention to model the heat island effect of urban agglomerations (Kobayashi and Takemura, 1994; Moreno-Garcia, 1994; Roth and Oke, 1994; Hiyama et al., 1995; Kidder and Essenwanger, 1995; Oke, 1995; Spellmann, 1995; Tuller, 1995; Goldreich, 1995). It can be recognized that in general there is a lack of data which are suitable for developping and testing corresponding modelling ideas. The storage term in the energy balance of urban surfaces reaches during daytime the magnitude of the sensible heat flux and is dominating the energy balance during nighttime (Roth and Oke, 1994). The storage term can not be measured directly and has to be estimated as a residual of the energy balance equation.
Meaningful (planning-relevant) statements on urban areas can only be gained by a combination of the knowledge of the distributed surface properties deduced from remotely sensed data (land use type, roughness lenght, albedo etc.) and meteorological point measurements. Furtheron, the spatial aggregation of the different data types is a problem for which still solutions have to be found (Schmid and Bünzli, 1995).
In the field of the parametrization of the turbulent exchange the classical smooth surface relations are now as before compared to measurements conducted in the urban boundary layer to see whether they are valid (Hiyama et al., 1994; Oikawa and Meng, 1995; Roth and Oke, 1995; Grimmond and Cleugh, 1995). But only in a few cases the available data allow real testing and development of modelling approaches, e.g. on the vertical structure of the lower urban boundary layer, as has been done by Rotach (1995). The importance of a better knowledge of the turbulent exchange and its vertical structure is underlined by the results of Rotach (1996). There, the distribution of the concentration of the tracer gas, emitted as part of an experiment, could be calculated more accurate, if the special situation in the urban roughness sublayer (Rotach, 1993a,b) has been included in the model.
From the former proposal (SNF 20-40621.94) the following is still valid:
The biggest part of the human population lives in cities or urban agglomerations. Anthropogenic activities (living, working, traffic etc) modify more and more these urban areas in manifold ways. Cities consist of building structures of different vertical and horizontal extension with varying building material. The interaction between this complex urban morphology and the atmosphere close to the ground leads to the urban boundary layer. A city in its complete extension influences the physical and the chemical state of the lower 1000 m of the atmosphere. The urban meso climate has some specific features, because an urban agglomeration
One consequence of this "climate"-modification is the well known heat island effect: the central part of cities can be several degrees warmer than the surrounding suburban and rural area, especially during nighttime (Landsberg, 1981).
The discussion on the predicted global climate change has shifted again increasing interest to the topic "urban climate". Many measurements of air temperature were done at places, which during this century gradually changed from rural to urban sites, and therefore the question arises, whether the measured increased temperatures could not be explained by the heat island effect (Lindzen, 1990; Myrup et al., 1993).
The urban boundary layer can be divided into a roughness sublayer (RS) close to the roughness elements, followed by the inertial sublayer (IS) and topped by the (convective) mixed layer (ML). The RS and the IS together are also known as the surface layer. The RS extends up to a certain height above the roughness elements. The air flow in this layer is three dimensional due to the strong influences of the local building structures. Higher up in the IS the influences of a larger area are mixed in such a way, that the turbulent characteristics in the IS represent an area of average roughness. The Monin-Obukhov similarity relations can be applied to the flow in the IS, and therefore, the exchange of momentum, heat and mass can be calculated relatively easy. The IS is sometimes also referred to as the constant-flux layer.
It is a well known problem, that methods for estimating the turbulent exchange, which were developed for rather smooth, homogeneous surfaces, actually can not be transferred to the urban boundary layer. In spite of that, due to a lack of better knowledge, the semi-empirical methods of the Monin-Obukhov similarity theory are still used in flow and diffusion modelling (Gryning and Lyck, 1984; Beniston, 1987; Frenzen et. al, 1987; Gross, 1989).
However, a better knowledge of the turbulent structure of the urban boundary layer is necessary, to understand the exchange processes for to be able to describe them in models using an adequate parametrization. Especially the nondimensionalized variances of the wind vector components are of interest, because they are needed in many air pollution diffusion models.
A first and often used approach to the problem is to investigate and to describe the deviations of the turbulent charactaristics from their "ideal" values (Högström et al. 1982, Clarke et al. 1982, Rotach 1991, Wang 1992, Roth 1993a, Roth and Oke 1993b, Rotach 1993a, Rotach 1993b).
In a comprehensive study Rotach (1991) investigated the turbulence structure of the urban boundary layer in Zürich and applied, as proposed by Högström et al. (1982), a second approach, the local scaling: local values of turbulence (fluxes, variances) are used to calculate similarity relations, instead of using the values of the uppermost measurement level and assuming that these values are representative for the IS. The latter approach is difficult, as the upper limit of the RS is not known and has to be found out experimentally. Parametrizations for the height of the RS as have been found for smoother surfaces can not be simply transferred to the urban boundary layer. Rotach (1991) deduced from profile measurements, with a maximum height of 1.6 h (h = height of the roughness elements), that the upper limit of the RS is around 2.5 h. He finds, that the semi-empirical Monin-Obukhov relations are valid, if local scaling is applied. This interesting result has to be verified by measurements.
Considering the turbulence structure of the urban boundary layer there are, up to today, many open questions. Finding answers to them is important for improvements of the model calculations.
Open questions referring to exchange processes in the urban boundary layer:
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Clarke, C.F., J.K.S. Ching, J.M. Godowich (1982): A study of turbulence in an urban environment. EPA technical report, EPA 600-S3-82-062.
Frenzen, G., D. Heimann, M. Wamser (1987): Dokumentation des Regionalen Klimamodells (RKM) auf der Basis von FITNAH. DFVLR-Mitt. 87-07.
Goldreich, Y. (1995): Urban climate studies in Israel - a review. Atmospheric Environment, 29, 467-478.
Grimmond, C.S., Cleugh, H.A. (1994): A simple method to determine Obukhov lengths for suburban areas. Journal of Applied Meteorology, 33, 435-440.
Gross, G. (1989): Numerical simulation of the nocturnal flow systems in the Freiburg area for different topographies. Beiträge zur Physik der Atmosphäre, 62, 57-72.
Gryning, S.-E. & E. Lyck (1984): Atmospheric dispersion from elevated sources in an urban area: comparison between tracer experiments and model calculations. Journal of Climate and Applied Meteorology, 23, 651-660.
Hiyama, T., Sugita, M., Kayane, I. (1995): Variability of surface fluxes within a complex area observed during TABLE 92. Agricultural and Forest Meteorology, 73, 189-207.
Högström, U., H. Bergström, H. Alexandersson (1982): Turbulence characteristics in a near neutrally stratified urban atmosphere. Boundary-Layer Meteorology, 23, 449-472.
Kidder, S.Q., Essenwanger, O.W. (1995): The effect of clouds and wind on the difference in nocturnal cooling rates between urban an rural areas. Journal of Applied Meteorology, 34, 2440-2448.
Kobayashi, T., Takamura, T. (1994): Upward longwave radiation from a non-black urban canopy. Boundary-Layer Meteorology, 69, 201-213.
Kuttler, W. (1988): Spatial and temporal structures of the Urban Climate - a survey, in: Grefen, K. & Löbel, J.: Environmental Meteorology, 305-333
Landsberg, H.E. (1981): The urban climate. Academic Press, New York.
Lindzen, R.S. (1990): Some coolness concerning global warming. Bulletin of the American Meteorological Society, 71, 288-299.
Moreno-Garcia, M.C. (1994): Intensity and form of the urban heat island in Barcelona. International Journal of Climatology, 14, 705-710.
Myrup, L.O., C.E. McGinn, R.G. Flocchini (1993): An analysis of microclimatic variations in an suburban environment. Atmospheric Environment, 27B, 129-156.
Oikawa, S., Meng, Y. (1995): Turbulence characteristics and organized motion in a suburban roughness sublayer. Boundary-Layer Meteorology, 74, 289-312.
Oke, T.R. (1995): The heat island of the urban boundary layer: characteristics, causes and effects. In: Cermak, J.E., Davenport, A.G., Plate, E.J., Viegas, D.X. (1995): Wind Climate in Cities. NATO ASI Series Vol. 277, Kluwer, Dordrecht, 81-107.
Rotach, M.W. (1991): Turbulence within and above an urban canopy. ETH Diss. 9439, 240 pp. Published as ZGS, Heft 45, Verlag vdf Zürich 1991.
Rotach, M.W. (1993a): Turbulence close to a rough urban surface. Part I: Reynolds stress. Boundary-Layer Meteorology, 65, 1-28.
Rotach, M.W. (1993b): Turbulence close to a rough urban surface. Part II: Variances and gradients. Boundary-Layer Meteorology, 66, 75-92.
Rotach, M.W. (1995): Profiles of turbulence statistics in and above an urban street canyon. Atmospheric Environment, 29, 1473-1486.
Rotach, M.W. (1996): The turbulent structure in an urban roughness sublayer. Proceedings of the Conference on Flow and Dispersion Through Groups of Obstacles. In press.
Roth, M. & T.R. Oke (1993b): Turbulent transfer relationships oer an urban surface: II: Spectral characteristics.- Quarterly Journal of the Royal Meteorolgical Society, 119, 1071-1104.
Roth, M. (1993a): Turbulent transfer relationships over an urban surface: II: Integral statistics.- Quarterly Journal of the Royal Meteorological Society, 119, 1105-1120.
Roth, M., Oke, T.O. (1994): Comparison of modelled and "measured" heat storage in suburban terrain. Beiträge zur Physik der Atmosphäre, 67, 149-156.
Roth, M., Oke, T.O. (1995): Relative efficiencies of turbulent transfer of heat, mass, and momentum over a patch urban surface. Journal of the Atmospheric Sciences, 52, 1863-1874.
Sachweh, M., Koepke, P. (1995): Radiation fog and urban climate. Geophysical Research Letters, 22, 1073-1076.
Schmid, H.P., Bünzli, B. (1995): The influence of surface texture on the effective roughness length. Quarterly Journal of the Royal Meteorological Society, 121, 1-21.
Spellmann, G. (1995): The urban climatology of Barcelona - an ideal heat island model ? Journal of Meteorology, 20, 117-130.
Tuller, S.E. (1995): Onshore flow in an urban area: microclimatic effects. International Journal of Climatology, 15, 1387-1398.
Wang, J. (1992): Turbulence characteristics in an urban atmosphere of complex terrain. Atmospheric Environment, 26A, 2217-2724