Geoscience Reference
In-Depth Information
5.6. Modeling
As there is a large spatial and temporal variability of urban climate parameters
and phenomena, it is nearly always impossible to make use of a high-resolution
network of meteorological stations and to monitor every single street, square, or
park. Models have the ability to reproduce the spatial variation of meteorological
features in urban areas, and sometimes even help to make predictions for the future.
Furthermore, “their use opens new perspectives for example in the mitigation of
UHI, or assessment of the role of air conditioning systems or the impact of urban
dynamics on air pollution” [MAS 06]. Moreover, models are less costly and hence
more adapted to the budget of most projects than intensive measuring campaigns
and keys to interpretation of phenomena are supplied in several cases. However,
models are not a substitute for measurements; data are necessary, not only to
construct the models, but also to validate them. Several papers describe different
types of models, give a number of examples, and refer to the potentialities and
limitations of the different types of models [OOK 07; MAR 07; KAN 06; RAT 06].
Several models are described in http://www.stadtklima.de/EN/E_1tools.htm.
There are different types of models that can be applied to the urban climate:
mathematical/physical numerical models, scale models, and empirical (statistical)
models [HEL 99, simplified].
Mathematical numerical models are based on equations that reproduce the
energetic processes that occur in urban areas and are able to simulate climate
behavior in present and altered conditions. According to Pearlmutter [PEA 07,
p.1877] they also “offer the flexibility to evaluate a wide range of urban
configurations”. However, the need of validation is not always considered by users
of the different models. ENVImet (BRU 99) is an example of mathematical model
that has been used in Lisbon's urban climate study. It is based on 3D “computer
fluid dynamics” (CFD) and energy balance models, and it is widely used not only by
urban climatologists, but also by architects. According to Martilli and Santiago [in
BAK 08], CFD are “numerical models that solve the Navier-Stokes equation over
small domains (few hundreds of meters at maximum), at high resolution (meters or
less), and explicitly resolve the buildings”. In the
Cost Report
edited by Baklanov
et
al.
[BAK 08], several models applied to urban areas are described.
Scale models are considered valuable tools for characterizing the effects of
detailed urban features and also for validating the predictions made by mathematical
models. Kanda describes the major contribution of scale models (to study the
statistical characteristics of turbulence and dispersion in neutral flows, the radiation
balance, the nocturnal UHI, among others [KAN 06, p. 31]). However, according to
Pearlmutter they cannot “replicate the complex interactions with other processes
such as radiation and heat storage” or establish “the linkages between canopy-layer
climate and the atmosphere above” [PEA 07]. Kanda also points out that this method
“must be complemented with numerical models and field observations to overcome
the mismatch between the thermal inertia of models and the real world” [KAN 06].
The wind tunnel model that we used is a scale model. It has helped us to understand
