Conclusion

In the urban canopy, the atmospheric circulations are mostly impacted by mechan- ical effects. There are also thermal effects which can influence more or less turbu- lence generation. According to Santiago and Martilli [2010] the size of turbulent eddies inside the canopy is limited by the presence of buildings and they showed that these eddies can be considered to have a constant height inside the canopy.

Inside the urban canopy, mechanical production of turbulence (proportional to the size of the eddies) are pre-dominant. In the Monin-Obhukov Similarity Theory, it is assumed that after a height, L (often above the height of the urban canopy), the buoyancy effects becomes much greater than the mechanical effect. It can thus be seen that there is a transition zone, which happens to be between two different scales, which is not often easy to grasp and take into account in models.

Moreover, an enhancement of the boundary conditions in models (both meso- scale for the surface layer and in micro-scale for actual boundary condition) is

2.6 Conclusion needed to improve simulations and also to include the spatial and chronological history of the weather variables.

Britter and Hanna [2003] pointed out that there is still a gap into how the neighborhood scale should be addressed and how it should be connected to the city and street scale. We proposed here to develop a canopy model, that will be at the interface between these two scales, and can thus be used to connect meso- scale models and micro-scale models. The aim of this canopy model is to use data from meso-scale models as input so as to calculate new profiles for the various variables which can then be used as input for urban parameterization schemes or micro-scale models. The model also aims at addressing the limits mentionned in Section2.5. The canopy model will be able to provide an improved profile for the micro-scale models where the history of these variables are taken into account with data coming from the meso-scale models. In return, the meso-scale models will get more precise information concerning the surface layer as more precise fluxes will be calculated in urban areas and hence the impact of obstacles and buildings will be properly described.

Besides the fact that meso-scale models can now interact directly with micro- scale models, it will not be necessary to increase the vertical resolution of the meso-scale models to improve simulations. With the use of a canopy model the first level of the meso-scale model can thus be increased as the use of the canopy model is expected to improve the calculation of more precise and accurate vertical meteorological profile for the meso-scale grid. It is hence expected that computer processing time will be reduced with the use of a canopy model as compared to highly resolved meso-scale model simulations.

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Chapter 3

Development of a 1D-CANOPY model. Part I: Neutral case and comparison with a C.F.D

This chapter corresponds to “Mauree, D. et al. 2014b, Development of a 1D Canopy Interface Model. Part I: Neutral case and comparison with a C.F.D, in preparation, 2014”

Abstract

A new Canopy Interface Model (CIM) is developed to evaluate the influence of obstacles on the atmosphere in the boundary layer. The objective is to analyze ur- ban parameterizations and guarantee the coherence between these propositions to simulate their influence on spatially averaged variables (wind speed, temperature, humidity and turbulent kinetic energy).

CIM development is presented through the main governing equations, with a specific focus on the coherence with past propositions and the modification brought to these equations. Compared to previous studies, obstacles characteristics are computed using surface and volume porosities in each cell of the model domain.

These porosities are used to weight several terms in the Navier-Stokes equations and have been introduced to prepare a coupling of the model with micro-scale model including the modeling of different kind of obstacles. A 1.5 order turbulence closure using the Turbulent Kinetic Energy (T.K.E) is used in the model. The mixing length is computed to take into account the obstacle density in the canopy layer as proposed by Santiago and Martilli [2010].

Results are compared with analytical solutions obtained in neutral atmospheric conditions, and also with data collected from a C.F.D experiment. When no obsta- cles are present, the comparison of results from CIM with the analytical solutions shows that CIM is able to reproduce the surface layer processes over a plane sur- face. We show that over such a surface, a constant turbulent kinetic energy profile is obtained. With the presence of obstacles, few scenarios are performed in order to analyze the effect of obstacles on wind and turbulent kinetic energy profiles.

The results show that fluxes from vertical surfaces have the most important effect.

CIM is also able to reproduce an Inertial Sub-layer as described by the Prandlt or constant-flux layer theory above a displacement height over a homogeneous canopy. The comparison of CIM with the C.F.D results show good agreements.

Keywords: atmospheric boundary layer, turbulence parameterization, turbu- lent kinetic energy, surface layer theory, urban canopy.

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