Sensitivity Tests in the Dynamical and Thermal Part of the MRF-Urban PBL Scheme in the MM5 Model - Meteorological and Air Quality Models for Urban Areas - page 83

ditions (King et al., 2001), also lead to an increase in the diffusion processes at
the surface layer (Fig. 10.2a). The increase in the wind speed values, calculated by
the MRF-urban scheme in the lower atmosphere at the city centre, compared to the
unrealistic no-city run (Fig. 10.5a), could be directly related to the influence of the
urban heat island, which is more intense during the night and exceeds the expected
frictional retard due to the increased roughness length.
10.4 Conclusions
Modifications, both in the 'dynamical' and 'thermal' parts, seem to play an
important role and improve the model's results. In particular, the MRF-urban
scheme calculated a decrease in the air temperature amplitude wave, and is in a
better agreement with the measurements. During the day, the decrease in air tem-
perature, diffusion coefficients and sensible heat flux is mainly attributed to the
modifications in the 'thermal' part and, in particular, to the heat storage flux, plus
the increase in the roughness length. The modifications in the 'dynamical' part are
significant in the calculated decrease of the friction velocity.
The MRF-urban scheme calculated a slowing of the sea breeze front and a
reasonable frictional retard concerning its penetration, during the day. This can
mainly be attributed to the higher roughness of the urban canopy compared to the
surrounding area. In addition, the developed urban-heat island was displaced inland.
The unrealistic 'no-city' MRF run revealed the existence of other important mecha-
nisms (e.g. sea-breeze) which coexist with urban influences, when an urban area is
surrounded by complex topography.
During the night, the calculated total increase in air temperature, diffusion coef-
ficients, turbulence and fluxes is due to the calculated increase by both 'dynamical'
and 'thermal' parts. The anthropogenic heat flux and the heat storage flux are both
released into the lower troposphere, producing a temperature increase which is
enhanced by the increase in the diffusion processes at the surface layer due to the
modified diffusion coefficients under stable conditions. The calculated increase in
the wind speed, by the MRF-urban scheme, could be directly related to the influence
of the urban heat island, which is more intense during the night.
Acknowledgments We are grateful to the COST Action 715 and 728 activities which actually
inspired this study.
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