Geoscience Reference
In-Depth Information
Figure 5.7.
Monthly frequency of wind
direction in Lisbon [ALC 92]
In addition to a greater reduction of wind speed near the ground, the airflow is
disturbed up to a higher altitude in an urban area that on rural landscape or over the
ocean. The reduction of wind speed near the ground can be positive in cold climates
or cold periods of the year in “temperate” climates, but it has several disadvantages
for the city dweller. First, removal of air pollutants is less effective in calm weather
(Figure 5.2a), and second, in warm periods, particularly during heat waves, the
absence of wind aggravates the heat load of the individuals and can lead to
discomfort; both situations may cause health problems. In most cities the knowledge
of wind systems and patterns is, therefore, important for planners, decisions-makers,
and the population [ALC 09]. Due to great temporal and spatial variability of wind
speed and direction, to the fact that meteorological stations do not supply spatially
continuous data, and owing to the complexity of urban areas, it is particularly
difficult to understand spatial variation of wind patterns in cities. Numerical
modeling can be a powerful tool to solve this problem. A study has been carried out
in order to understand the summer winds in Lisbon.
3D meteorological models that can simulate and predict airflow and 3D
temperature fields are suitable for meteorological simulation on particular
conditions. Urban canopy parameterization was attempted in PSU/NCAR mesoscale
model (MM5], but there are still several unsolved problems due to the complexity of
the city geometry and heat fluxes within the urban fabric [RIG 08]. For the present
purposes, a simple airflow model, such as WAsP, was considered appropriate.
WAsP is a piece of software developed by Riso National Laboratory in Denmark,
which estimates both profiles and spatial data taking into account the effect of
terrain and surface roughness. It was shown that the model has good agreement with
real wind climatology, confirmed by wind tunnel experiments [LOP 03].
