COST 715 WORKSHOP ON URBAN BOUNDARY LAYER PARAMETERIZATIONS BY
MATHIAS W. ROTACH, BERNHARD FISHER, AND MARTIN PIRINGER
State-of-the-art parameterizations of the urban atmospheric boundary layer applied to air pollution dispersion modeling were reviewed by leading experts. Current theories with modifications just survive critical appraisal.
T
he workshop on Urban Boundary Layer Parameterizations was organized as an activity within COST 715 (Meteorology Applied to Urban Air Pollution Problems) by Working Groups 1 and 2 of this COST action. COST (an acronym translated from the French for European Cooperation in the Field of Scientific and Technical Research) is a European framework for the coordination of nationally funded research within Europe. A “COST action” operates within a bottom-up structure that is administered by individual scientists through working groups and a so-called management committee. This particular workshop was held in Zurich, Switzerland, on 24– 25 May 2001 and consisted of a series of scientific presentations and an extensive discussion by all the par-
AFFILIATIONS: ROTACH—Swiss Federal Institute of Technology,
Institute for Atmospheric and Climate Science, Zurich, Switzerland; FISHER—Environment Agency, London, United Kingdom; PIERINGER— Central Institute for Meteorology and Geodynamcis, Vienna, Austria CORRESPONDING AUTHOR: Mathias Rotach, Institute for Atmospheric and Climate Science ETH, Swiss Federal Institute of Technology, Winterhurerstrasse 190, Zurich CH-8057, Switzerland E-mail:
[email protected] DOI: 10.1175/BAMS-83-10-1501 In final form 20 May 2002 ©2002 American Meteorological Society
AMERICAN METEOROLOGICAL SOCIETY
ticipants (about 50 people from 18 countries). Extended abstracts of the individual presentations will be published as a proceedings volume by the European Commission. They can be downloaded from the Web site of Working Group 1 (www.iac.ethz.ch/en/ research/cost715/cost715_2.html). In this contribution a short outline of each presentation is given and the discussion is summarized. OVERVIEW OF THE PRESENTATIONS. S. E. Belcher (University of Reading) presented a new model for the flow in the lowest part of the urban boundary layer, that is, the roughness sublayer. The approach does not resolve individual buildings but rather treats the spatially averaged flow field with all its consequences. Scaling considerations were presented as well as an approach for the turbulence closure (mixing length approach). The model was shown to favorably correspond to available datasets (mainly from regular array wind tunnel or “semi-full-scale” physical modeling). An overview of various modeling approaches for urban applications in numerical models was given by R. Bornstein (San Jose State University). In broad terms, three types of approaches can be distinguished. • the “traditional” approach (termed the flat sandbox type by the author), in which only parameter OCTOBER 2002
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values (e.g., for the roughness length, surface emissivity, etc.) in conventional schemes are modified for urban conditions; • inclusion of grid-averaged urban “topographic blocks” to mimic the barrier effect due to the presence of buildings; and • more dynamical approaches, where the turbulent exchange of momentum, heat trapping in street canyons, etc. is parameterized explicitly. Bornstein’s presentation gave an overview of these different approaches. Unfortunately, the simplest traditional approach is still the most widely used although it cannot appropriately handle the region near the surface (i.e., the roughness sublayer). Bornstein also provided an extensive list of references on the subject. Near-surface turbulence characteristics over urban surfaces as observed in full-scale and wind tunnel studies were summarized by P. Kastner-Klein (University of Oklahoma). In contrast to smoother surfaces, Reynolds stress is observed to be strongly height dependent and parameterizations for its profile were presented. Little is known from experimental evidence concerning profiles of other turbulent fluxes (sensible and latent heat). Knowledge of the Reynolds stress profile in conjunction with an approach for local scaling was furthermore shown to be appropriate (above the urban canopy) for describing other turbulence statistics (e.g., velocity variances) relevant to the dispersion process. A generally satisfying correspondence between full-scale and wind tunnel observations was reported. Combining the advantages of the two approaches would probably yield the best results. C. Soriano (Universitat de Catalunna) presented first results of an ongoing activity of Working Group 1 (WG1) withing COST 715. For practical purposes it is often desirable to estimate the wind speed at a certain urban site from observations at nearby rural stations (e.g., the city’s airport). Examples from a number of available urban–rural pairs show that the “urban effect” has to be isolated by carefully selecting specific situations (e.g., according to wind direction) and extracting the influence of topography and other mesoscale features. Also, and most important, it is necessary to define an appropriate urban reference height [similar to the World Meteorological Organization’s (WMO) guideline for rural stations]. A comparison of urban and rural reference wind speeds can only become meaningful if all the urban observations stem form such a reference height. For this purpose WG1 has devised a “recipe” mainly based on the findings presented by Kastner-Klein, in order to estimate mean wind speed at the urban reference 1502 |
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level from observations at any height within the urban roughness sublayer. Future analysis will assess the success of this approach. Parameterizations for urban roughness parameters were presented by P. G. Mestayer (Ecole Centrale de Nantes). These parameters are required if the traditional approach (recall Bornstein’s presentation) is chosen for urban-scale numerical modeling. Various approaches were presented that are either based on detailed or sparse information concerning the morphology of the urban surface. The somewhat disappointing conclusion from an extensive intercomparison study (Grimmond and Oke 1999) of these approaches with the available high quality experimental data (in situ as well as from wind tunnel experiments) is that they all yield substantially different results. About half of the experimental evaluations lie outside the physically plausible range, and there is little evidence to indicate which approach is best. One conclusion of the study was that sensitivity studies should be undertaken to show to what extent additional factors/effects should be included in order to obtain “better” parameterizations for urban roughness parameters. Z. Janour (Institute of Thermomechanics, AS-CR, Prague) presented a model for the flow in the boundary layer over a rough surface that is based on Rossby number similarity. Only the simplest case of a horizontally homogeneous, stratified flow had been considered to this point. Janour’s approach yields universal wind speed profiles for a vertical range between about 10 times the roughness length up to the boundary layer top. A preliminary comparison to radio sounding data from a release in Barcelona exhibits some qualitative resemblance to the predicted profile, but neither the model’s state nor the dataset is yet optimal for comparison. A new dataset of urban wind speed and turbulence profiles up to 250 m from the surface was reported by M. Schatzmann (University of Hamburg). The observations were (and still are being) taken in the city of Hamburg on a tall radio transmitter tower. For certain wind direction sectors the urban fetch appears to be close to ideal and shows no significant topographic disturbances.The presentation focused on data quality and analysis and the assessment of urban turbulence characteristics, with an eye on comparability between full-scale and wind tunnel experiments. A presentation by A. Karppinnen (Finnish Meteorological Institute and a joint presentation by E. Berge and L. Slørdal (Norwegian Meteorological Institute) dealt with problems of current existing boundary layer parameterizations under extreme conditions. In particular, over snow-covered urban surfaces at high
latitudes numerical models have severe problems reproducing the strength and height of the near-surface or elevated inversion. Interestingly enough the two papers reported either strong underestimation or strong overestimation of the inversion strength. Possible reasons for this type of failure and strategies to circumvent it were discussed. “Why not?” was, in short, the reaction of S.-E. Gryning (Risø National Laboratory) to the question of whether rural parameterizations for the boundary layer height could work in urban areas. For urban areas it was suggested that one discriminates between class A cities (large urban fetch) and class B cities (nonhomogeneous urban regions such as in coastal areas, urban/rural transitions). As far as surface fluxes are concerned (as an input in both types of models), they should be considered above the blending height (which can be taken as a surrogate for the height of the roughness sublayer at least under certain conditions and which is discussed in the summary below). Examples of the successful application of these ideas under various conditions were presented. A new diagnostic parameterization of the boundary layer height over rough (and smooth) surfaces was presented by S. Joffre (Finnish Meteorological Institute). It is based on one nondimensional parameter that is composed from the Obukhov length and a second length scale derived from the friction velocity and the Brunt–Väisälä frequency. Based on the presented data the approach can be applied for both stable and (not too) unstable conditions. The author’s conclusion was also an affirmative answer to the “why not” question of the previous author, as long as representative urban surfaces fluxes, and especially the friction velocity, are correctly estimated over cities. SUMMARY AND CONCLUSIONS FROM THE GENERAL DISCUSSION. Urban areas present a difficult problem to numerical modelers and experimentalists due to the complicated three-dimensional structure of the urban boundary layer (UBL). In particular both the vertical and horizontal structure of the UBL do not satisfy the conditions under which current knowledge of (uniform and homogeneous) boundary layer processes apply, thus making fast progress very difficult. The vertical structure of the UBL was discussed first. Given a large enough urban surface (fetch) so that an equilibrium flow can be established, it was agreed that the concept of a blending height1 corresponds to the existence of an inertial sublayer base. In other words, the blending height corresponds to the interface between the top of the roughness AMERICAN METEOROLOGICAL SOCIETY
sublayer and the bottom of the inertial sublayer. There was some disagreement concerning the terminology for layers above the surface-layer region (above the roughness layer and the inertial sublayer). The urban mixed layer refers to the polluted layer generated during unstable conditions (and hence its application in mixed-layer scaling), but it is not appropriate during stable conditions (which may not be typical but indeed can exist, in particular in high-latitude cities in winter). On the other hand, urban mixing layer emphasizes the process rather than the state but risks confusion with the prototype turbulence paradigm of a mixing layer. It is therefore suggested the layer above the surface layer be called the urban boundary layer without any reference to its state or stability. The horizontal structure of the UBL is complicated by the rural–urban transition, various regions within a city (downtown, residential, industrial, etc. ), and possible topographic features or large bodies of water. Relatively little evidence is available at this time concerning internal boundary layers that are thought to grow at the transitions between such city regions. In this sense every urban area is different and results are difficult to generalize. It was therefore agreed that the community should first concentrate on ideal, sufficiently large, flat and horizontally homogenous urban areas. These will allow for an assessment of the vertical flow and turbulence structures over the urban surface (in such ideal cases) and may later serve as an “urban Kansas” reference case. Numerical models with exchange parameterizations that are able to reproduce these features may then be used to investigate more complicated cities where local circulation and topography play an important role. For ideal cities with such an “infinite” urban fetch the inertial sublayer and the turbulent fluxes therein may play crucial roles as the basis for urban boundary layer parameterizations. For the larger-scale properties like the boundary layer height and the profiles of flow statistics in the bulk of the UBL, the inertial sublayer fluxes may serve as surface (effective) fluxes. On the other hand they are key parameters for describing the (spatially averaged) profiles of turbulence statistics within the urban roughness sublayer including the canopy. Hence, inertial sublayer turbulent fluxes may also provide the link between the urbanscale properties on the one hand and local-microscale properties on the other hand.
1
The term blending height is used by the plant canopy community to denote the height above which turbulent mixing has smoothed out the differences due to different surface patches. OCTOBER 2002
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TABLE 1. Key parameters to be studied for models of different scales in the urban environment: zo = roughness length (subscript T for temperature); d = zero plane displacement; (x) denotes a spatial variability; hUBL = height of the UBL; LC, La are characteristic length scales as defined in the contribution of Belcher (section 2); z* is the height or the roughness sublayer, and lowest height within the inertial sublayer; u*IS refers to the friction velocity in the inertial sublayer (and correspondingly for the other fluxes; cf. x*IS ); and h = average building height. Models Mesoscale
Submesoscale
Street-canyon scale
zo, zoT
zo(x), d(x)
hUBL
LC, La, z*
Detailed geometry
Surface fluxes (effective)
u*IS, HIS, general: x*IS
uG(h), second velocity scale for horizontal transport
Anthropogenic heat flux (nonsurface) at some representative height
Dispersive flux
Heat exchange at vertical and horizontal building surfaces
Profiles of turbulent fluxes
Profiles of turbulent fluxes
Characteristic velocity variance in street canyon
Higher-order moments?
Higher-order moments (skewness, . . . )
Higher-order moments?
Synoptic forcing, average albedo
Mesoscale stability, albedo (x)
It was therefore concluded that research is needed concerning the vertical extension and the lower boundary of the inertial sublayer, as well as concerning the relation between effective turbulent fluxes to surface and forcing conditions. High towers or turbulence sondes (e.g., attached to the tethered balloon) would be required. As an alternative, extended suburban areas, as is typical in many U.S. cities,2 may reduce the requirement for very high towers. There was a general consensus that both dynamic and thermodynamic effects and their variation with respect to the surrounding rural areas have to be studied and to be taken into account at the same time. In a final part of the discussion, a list of key parameters on which to base a description of the UBL was compiled (Table 1). For this it was felt that a distinction between the different scales (traditional mesoscale, submesoscale, and “street-canyon” scale) is necessary with the submesoscale acting as a link between the other two. As can be seen from Table 1 some of
the variables appear in more than one column. This again emphasizes the link between the various scales involved. Generally, and not apparent from Table 1, different models (or surface exchange parameterizations) must be sought for low wind conditions. In summary the key issue to be investigated is the degree of detail required to describe the UBL. The hope expressed in the majority of the presentations is that it is possible to extend the ideas of the rural boundary layer to the UBL. One can imagine a situation in which it is possible to define slowly varying effective surface fluxes for application to mesoscale models, while mesoscale models themselves may provide the upper-boundary conditions for submesoscale and street-canyon scale models that include detailed and complex flow descriptions. Justification of the linking of the different scale models at the so-called blending height would be a valuable advance.
2
Grimmond, C. S. B., and T. R. Oke, 1999: Aerodynamic properties of urban areas derived from analysis of surface form. J. Appl. Meteor., 38, 1262–1292.
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Note that such studies—more concerning the energy balance and less the dynamic aspects of urban surfaces—do already exist. OCTOBER 2002
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