Model Urbanization Strategy: Summaries, Recommendations and Requirements - Meteorological and Air Quality Models for Urban Areas - page 142

For down-scaling, a chain of urban models of different scales with sub-domain
nests using finer grid sizes is applied. A common approach is to use outputs of large-
scale models as boundary condition inputs to domains employing smaller grids
successively from global to urban and street scales. It is well recognized that trans-
port and transformation are nonlinear in scale (especially for reactive and rapidly
deposited species), and parameters controlling atmospheric processes are typically
grid-size dependent. Usually, the microscale (street canyon) models are obstacle-
resolving and consider the detailed geometry of the buildings and UC, whereas the
up-scaled city-scale (sub-meso) or mesoscale models consider parameterizations of
urban effects or statistical descriptions of the urban building geometry. FUMAPEX
(Baklanov, 2006) is an example of model down-scaling with integration of urban
meteorology, air pollution, and population exposure modeling. Downscaling from
regional (or global) meteorological models to the urban-scale meteorological mod-
els, with statistically parameterized building effects, and further downscaling to
microscale obstacle-resolved, CFD-type models was included in the methodology.
Likewise, methods are needed for regional- and global-scale models to properly
account for downwind transport of pollutant species from urban sources in regional-
and global-scale contexts. This is because the modeled composition by species is
grid-size dependent. Thus, for global and climate change models, the mesoscale
model can provide a proper pollutant species accounting from biogenic and urban
sources ranging from small urban areas to megacities to regional and global scales.
It serves investigations of the evolution of pollutants from large urban plumes (e.g.,
Sarrat et al., 2006) or from major industrial and power-generation point sources.
Such plumes are subgrid phenomena for the regional-global models that have the
highest resolution (between 10- and 100-km grid sizes) in the focus areas. Therefore,
urban-scale models can provide appropriate composition mix for the regional-global
model. Currently, to understand the impact of aerosols and gas-phase compounds
emitted from local/urban sources on regional and global scales, at least three scales
of the integrated atmosphere-chemistry-aerosol and general circulation models are
being considered: (1) local, (2) regional, and (3) global. Note that two-way nesting
approaches are ideal for situations in which the scale effects in both directions (from
the mesoscale on the microscale and from the microscale on the mesoscale) are
important. However, such approaches are difficult to implement.
15.4.4 Urban Pollution and Climate Integrated Modeling
Integrated air quality modeling systems are tools that help in understanding impacts
from aerosols and gas-phase compounds emitted from urban sources on the urban,
regional, and global climate. The integration of urbanized NWP and ACT mod-
els is a strategic approach to providing the science-based tools for assessments of
urban air quality and population exposure in the context of global to regional to
urban transport and climate change. This is reasonable because meteorology gov-
erns the transport and transformations of anthropogenic and biogenic pollutants,
drives urban air quality and emergency preparedness models; meteorological and