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options, the cloud radiation scheme (Dudhia, 1989) and the five-layer soil model
(Dudhia, 1996) were applied, as well as the Grell cumulus scheme (Grell, 1993)
in the outermost domain (no cumulus parameterization in the inner domain). The
model results were compared with sonic anemometer measurements of turbulence
and routine meteorological data.
10.3 Results and Discussion
For this study, the 14 September 1994, was selected from the MEDCAPHOT-
TRACE experimental campaign (Ziomas, 1998). A large set of meteorological
measurements (air temperature, wind speed and direction, heat and momentum
fluxes, etc.) were available for this day, as well as tethered balloon soundings, up to a
height of 600 m at the NOA and Marousi stations (Batchvarova and Gryning, 1998).
The surface synoptic circulation of the simulated day over the greater Athens area
was characterized by the ridge from an extended anticyclone centred over northern
Africa, and the synoptic wind was weak from the northern sector. These condi-
tions favoured the development of a local weak sea-breeze circulation, which was
observed only up to the city centre. In Fig. 10.1, the studied area is presented and
the position of three ground stations: NOA - a downtown station located on top of
a hill (107 m above sea level, asl), Peiraias - an urban station at the harbour, and
Marousi - a suburban station located 13 km inland inside a grove that is surrounded
by buildings of different heights.
During the day, the increase in diffusion coefficients (Fig. 10.2b) and air
temperature (Fig. 10.3), calculated by the modifications in the 'dynamical' part
(MRF-dyn scheme) due to the modified profile functions, is compensated by the
decrease, calculated by the modifications in the 'thermal' part (MRF-ther scheme),
resulting in a total decrease. The calculated decrease by the MRF-ther scheme is
mainly attributed to the heat storage flux which exceeds that of the anthropogenic
heat flux and acts as a sink in the thermodynamical equation at the surface layer. It
should be mentioned that the MRF-urban scheme includes the modifications of both
parts, plus the increase in the roughness length.
A decrease in turbulence and fluxes during the day is calculated by both
modifications (Fig. 10.4). In particular, the decrease in the sensible heat flux
(Fig. 10.4a, b) is related mainly to the smaller temperature gradients near the surface,
produced by the combined effect of the anthropogenic heat, the heat storage, and the
shadowing effects of buildings. The decrease in the friction velocity (Fig. 10.4c, d)
is mainly due to the modification in the convective velocity (Dandou et al., 2005)
and the reduced wind speed, because of the increased roughness length. It should
be mentioned that the increase in roughness length would lead to an increase in the
friction velocity, but the acceleration of the diffusion due to the enhancement of
the diffusion coefficients normalizes the temperature gradients. This process pushes
back the establishment of a strong instability and brings possibly the fluxes back to
lower levels. The improvement is more apparent at the NOA station, where most
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