Urban canopies

Fig. 16.7 (a) Schematic view of typical element of urban canopy - street canyon (b) Wind flow and pollution dispersion for the part of Copenhagen area... 

Nuterman R (2008) Modelling of turbulent flow and pollution transport in urban canopy. PhD Thesis, Tomsk State University, 156 p... 

Figure 1.10 Schematic wind distribution over an urban territory after Bottema, 1993 [71] and Fisher et al., 2005 [193] 1 - logarithmic portion high enough over the city 2 - decelerated and distorted velocity profile within the urban canopy 3 - transition layer (roughness sublayer).

It is natural that equations like (1.4) and (1.5) can be considered as correct for only forest or riparian structures but only as approximate for urban canopies. The fundamental difference between two canopy types lies in the fact that trees in a forest and grass in a water stream displace a negligibly small amount of air and so the volume of all obstructions Hobstrucrio s divided to the whole volume II is very small,... 

Many authors have successfully applied the models of a distributed force to a number of problems (see [318, 319] and the consequent chapters the general investigation of such models is presented in the Section 3.1). The structures thus affecting the flow may be equally called canopies or penetrable roughnesses. The hypothesis of the additivity of individual forces is applied to almost all canopies except, perhaps, urban canopy. With this idealization in mind, it might be spoken about easily penetrable roughnesses (EPR). 

Figure 2.1 Characteristic features of canopy flows (especially urban canopies). Lo is outer-or meso- (regional) scale on which the canopy affects the dispersion. Lc and L/ are the canopy and inner length scales, respectively. Note that UG is the approach geostrophic wind speed above the boundary layer, UB is the typical wind speed associated with local buoyancy effects, e.g. downslope winds from nearby mountains. Hc is the canopy height and Hc is the standard deviation of building height.

Table 2.3 Typical configurations of obstacles (e.g. in urban canopies).

Fires in porous media natural and urban canopies... 

Again the reader is referred to earlier chapters in this monograph related to air movement within urban canopy layers. Only those details related to the growth and spread of mass fires within a suburban, urban or wildland/urban interface will be discussed below. 

A number of authors, however, have represented forest or urban canopy layers by porous regions of distributed force (or drag) [206, 213, 217, 318, 320, 576, 662], The advantage of such an approach is that it permits inclusion of a canopy sublayer without the use of excessive and costly grid resolution. Yamada [662] and Shaw and Schumann [576] introduced the approach in order to add vegetation to meso-scale models of complex terrain. Jeram et al. [320] used the concept in 2d calculations for inviscid flow and constant eddy diffusivity flow estimates of the up and downwind penetration of flow within simple urban areas. 

The development and validation of these SEB models brought to light and helped to quantify several specificities of the urban canopy energetics ... 

Simulating urban canopy effects in urban-scale NWP and meso-meteorological models can be considered with the following two main approaches (Baklanov et al., 2005 [38]) ... 

This formulation is suitable for detailed urban canopy models (see e.g. Sections 9.2.4 and 9.2.5), when the surface is just millimetres above ground and the canopy layers are within the simulation domain. For meso-scale meteorological and NWP models in which the surface may be high above the urban canopy (average roughness level or displacement height), the SEB can be rewritten in the following form ... 

The wind velocity is mainly affected by the buildings walls drag force, which produces a negative momentum flux. Consequently, highest are the buildings, lower is the wind velocity inside the urban canopy (see Figure 9.10a). 

Figure 9.10 Sensitivity of the simulated meteorological fields using the BEP module to urban parameters a) wind profile simulated with 40 m (black line) and 10 m (red fine) building height b) Temperature evolution inside the urban canopy for three different street heat capacities 1.4 (basecase), 14 (fine ), 0.14 (line ) MJm 3K 1 (EPFL contribution in Baklanov et al., 2005 [38]).

The radiation trapping between building walls and streets stores the energy inside the urban canopy. Consequently, when buildings are high and streets are narrow the decrease of the night temperatures is low. 

The parameterisation has been tested on the city of Basel (Switzerland), Mexico City (Mexico), Copenhagen (Denmark), and verified versus the BUBBLE experiment (Basel Urban Boundary Layer Experiment Rotach et al., 2005 [549]). The verification results (Figure 9.11) show that the urban parameterization scheme is able to catch most of the typical processes induced by an urban surface Inside the canopy layer, the wind speed, the friction velocity and the atmospheric stability are reduced. In the other hand, even if the main effects of the urban canopy are reproduced, the comparison with the measurement seems indicates that some physical processes are still missing in the parameterization. In most of the cases, the model still overestimates the wind speed inside the canopy layer and it can have difficulties to simulate the maximum of the friction velocity which appears above the building roofs. 

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