Geoscience Reference
In-Depth Information
Data consisted of wind speeds and direction averaged over 1 hour intervals
collected at Lisbon Airport meteorological station (Figure 5.4b) for the period 1971
to 1980 [LOP 03]. WAsP transforms the original data into a set of statistical values
that contains parameters of the Weibull distribution:
A
(scale, related to “mean wind
speed”) and
k
(shape) [PET 97], taking into account the topography and urban
geometry. The accuracy of the estimations depends highly on the quality of data
related to the aerodynamic roughness of the surface or “roughness length” (z
0
). Z
0
is
defined as “the height above the surface at which the mean wind speed becomes
zero, when extrapolating the logarithmic wind-speed profile downwards through the
surface layer” [OKE 87, p.57].
The same roughness length (z
0
) values were assigned to areas that have similar
urban morphology and built density. The procedure used to obtain the roughness
map for the 1980s is explained in detail by Lopes [LOP 02] [LOP 03]. Z
0
highest
values have been assigned to the central high density areas of the city (z
0
= 1) and to
the hill of Monsanto (z
0
=0.7) (Figure 5.8). Z
0
decreases although irregularly towards
the NW, N, and NE of the city core, with low building density in the 1980s. The
lowest value (0.01) was assigned to the airport area (Figure 5.8). Z
0
was computed
for the 1980s, a period of rapid urban growth in Lisbon for which hourly wind data
was available.
WAsP was run first
for all wind directions
in the summer (June to September).
Wind profiles were estimated for 9 urban sites (representative of different urban
geometry and position in town). In Figure 5.9, profiles 'a' and 'b' show average
logarithmic wind profiles at different moments, whereas 'c', 'd' and 'e' show the
profile referring to one single point: “Baixa” [near Restauradores (R) on Figure
5.4b]. The average profile 'a' corresponds to the period before the existence of the
city, as the model was run considering only the topography, whereas profile 'b'
refers to average wind profile estimated for the 1980s. In profile 'a', Weibull A
parameter (that indicates wind speed) decreases from ca. 12 m/s at 250 m to ca. 6.2
m/s near the ground. However, building density in the 1980s caused a further
decrease of 2 m/s near the ground (profile 'b').
When considering a single location: “Baixa”, a still greater reduction in the
1980s is estimated of approximately 3 m/s wind speed (profile 'c'),
when compared
with profile
'a'. The use of WAsP provides the prediction of future wind-speed
profile modification in central Lisbon (in “Baixa”), considering z
0
increase in the
northern districts. A new dataset was prepared assuming that roughness will change
from 0.01, 0.03, and 0.5 m in Northern Lisbon (at the time the original data were
collected, Figure 5.8) to 1.5 m and to 2 m (typical z
0
of a city that increases in height
and volume) [OKE 06b; MOR 93]. If z
0
raises to 1.5 m, then a further decline of
wind speed will take place (profile 'd': minus ca. 4 m/s wind speed near the ground
than in profile 'a'). A further increase in z
0
in the northern districts will not have a
much greater impact (profile 'e'). Decreases of wind speed with increasing
urbanization are expected at least up to 250 m high (Figure 5.9).
