Environmental Engineering Reference
In-Depth Information
800
R
G
600
R
A
L
↑
400
S
↑
200
0
0600
0000
1200
1800
2400
Time (UTC)
(a)
(b)
Fig. 6.2
Validation tests of SM2U with Marseille city centre measurements during the CLU-
ESCOMPTE campaign; (
a
) view of the central meteorology/flux measurement site (photo, CSB
Grimmond); (
b
) mean diurnal cycle of the radiation budget components (averaged over 21 days):
the measured incoming shortwave (R
G
) and longwave (R
A
) radiations are the inputs of the model;
the simulated shortwave (S
↑
) and longwave (L
↑
) outgoing radiations (
solid lines
) are compared to
the observations (
dots
)
roofs the heat storage is small and remains positive to 5 pm and is also equivalent to
the radiative loss during the night. The diurnal cycle of the streets, which includes
street floors and building walls, demonstrates that the phase shift is essentially due
to the wall behaviour: up to 8 am all the solar energy is transformed into heat by
the wall vertical surfaces and stored into the materials, reaching a maximum at 10
am. Moreover, the released energy flux after 5 pm and during the night is so large
that it compensates both the radiation loss and a positive sensible heat flux into the
atmosphere. During daytime the sensible heat flux is relatively small (compared to
that of the roofs) due to both the high canopy resistance to the aerodynamic transfers
and the high heat capacity of building materials.
Detailed modelling of the heat transfer through walls appears necessary when
rapid changes in canopy temperatures must be obtained over small time scales, e.g.
at sunrise and during morning hours (Dupont and Mestayer, 2006). Alternative sim-
ulations (Fig. 6.5) demonstrate that modelling with two layers, at least, is needed
to render both the rapid changes in surface radiation-heat conversion and the actual
limits of wall heat capacity. Curiously Fig. 6.5b shows that, for this city centre, the
radiation trapping does not significantly influence the sensible heat flux. It is also
shown that a flat canopy approach could be sufficient if only the general behaviour
without fine scale details is studied (Fig. 6.5b and d).
Evaluation with observational data for the Rezé suburban site (Fig. 6.6) demon-
strated that the evapotranspiration scheme is key in the water budget at short time
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