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and physical parameters, such as roughness length, albedo etc., while neither the
anthropogenic heat nor the heat storage are included.
Recently, several efforts have been made in order to improve the representation
of urban surface characteristics in mesoscale models (Brown, 2000). Attempts have
been made to improve either the 'dynamic' part (impact on the wind field and the
turbulent kinetic energy) or the 'thermal' part (impact on the heat fluxes) (Martilli
et al., 2002). In the present study, sensitivity tests were carried out in the 'dynamic'
and 'thermal' parts of MRF-urban planetary boundary layer (PBL) scheme (Dandou
et al., 2005) in the meteorological PSU/NCAR Mesoscale Model (MM5) (Grell
et al., 1994). The impact of both modifications seems to be important and improves
the model's results. An attempt is also made in order to examine the interaction
of the sea-breeze front with the heavily urbanized city of Athens, in terms of the
surface drag in combination with the urban heat island (Dandou, et al., 2009).
10.2 Methodology
The numerical simulations were performed by the MM5 model version V3-6-1
(Grell et al., 1994). In particular, the high resolution non-local MRF PBL parameter-
isation scheme (Hong and Pan, 1996) was applied based on the Troen and Mahrt's
(1986) representation for counter-gradients and K-profiles in the well-mixed con-
vective boundary layer. The sensitivity tests refer to the 'dynamical' and 'thermal'
part of the MRF-urban PBL scheme (Dandou et al., 2005), a modified version of
the MRF PBL scheme, whereby urban features are considered. In particular, with
respect to the 'thermal' part, the urban surface energy balance was modified by
taking into account the anthropogenic heat and the urban heat storage term to pro-
duce for urban/building mass effect, including hysteresis (the Objective Hysteresis
Model, Grimmond et al., 1991). The surface stress and fluxes of heat and momentum
were also modified in the 'dynamical' part, following recent advantages in ABL over
rough surfaces under unstable conditions (Akylas et al., 2003; Akylas and Tombrou,
2005) and stable conditions (King et al., 2001). It should be mentioned that the
whole process was supplemented by detailed information on land use cover, derived
from satellite image analysis (spatial resolution 30 m). Moreover, in order to exam-
ine topographic influences on air motions in the city, an unrealistic 'no-city' run was
also performed by the MRF PBL scheme, where the city of Athens was replaced by
dry cropland and pasture surface, as in the surrounding area (Dandou, et al., 2009).
The numerical simulations were performed by applying two-way nesting. The
coarse domain covers the extended area of Greece, with spatial resolution 6
6km,
and the second domain is centred on the Attiki Peninsula, with spatial resolution
2
×
2 km. The 25-category USGS land-use classification scheme was adopted to
provide land-cover data for the model domains. The initial and lateral boundary con-
ditions for the outermost domain were provided by the European Center for Medium
range Weather Forecast (ECMWF) numerical weather prediction (NWP) model,
together with Sea-Surface Temperature (SST) data. For the rest of the physics
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