Environmental Engineering Reference
In-Depth Information
21.1
Introduction
of LULC and corresponding physical properties is important for
atmospheric modeling.
Regional meteorological modeling is a prerequisite for air
quality modeling since it predicts the transport, mixing and con-
ditions for emissions, deposition and chemical reactions of trace
gases and aerosols. Particularly biogenic emission and deposition
modeling are directly influenced by LULC characterization and
enhancements of the latter for urban areas improve quantification
of those processes in air quality models. In current state of the art
atmosphericmodels the chemistry, transport andmixing is either
simulated simultaneously with the meteorology (as for example
in theWeather Research and Forecasting (WRF) - chem.Model);
or the meteorological fields are generated by a meteorological
model independently first (for example WRF), then converted
by a Meteorology-Chemistry Interface Processor as input for
the air quality model (for example the Community Multi-scale
Air Quality Modeling system CMAQ). Therefore improved rep-
resentation of meteorological fields for urban areas potentially
leads to a direct improvement of simulated trace gas and aerosol
concentrations, emissions and deposition processes.
In Section 21.2 we will give an overview of the recent develop-
ment of physical approaches for the representation of urban areas
in regional atmosphericmodels along with necessarymodel input
parameters. Section 21.3 addresses remote sensing platforms and
methods that support the acquisition and derivation of urban
model input parameters such as urban land use and associ-
ated physical characteristics. In Section 21.4 we will review the
findings of studies for several cities around the globe that have
investigated the influence of urbanization on urban meteorology
and air quality. In a specific case study for the rapidly urbanizing
Phoenix (Arizona, USA) area, we will demonstrate how remotely
sensed data are used to study the effect of historic land use and
land cover changes on near-surface air temperature during recent
extreme heat events.
Expansion of cities to accommodate increasing population has
global, regional and local effects on weather and climate due
to land use and land cover (LULC) changes and accompany-
ing effects on physical processes governing energy, momentum,
and mass exchange between land surfaces and the atmosphere
(Cotton and Pielke, 2007). Urbanization significantly impacts
regional near-surface air temperatures, wind fields, the evolution
of the planetary boundary layer, and precipitation, subsequently
influencing air quality, human comfort, and health. Increased
scientific interest in capturing the details of meteorological fields
at the urban scale results from both, the need to understand
and forecast the environmental conditions in cities where most
humans live, and by the improved computing ability to resolve
heterogeneity and physical characteristics of urban areas in
regional meteorological and air quality modeling (Brown, 2000;
Martilli, 2007).
The urban ecosystem is imbedded in, and responds to, cli-
matic conditions that vary on a wide range of temporal scales.
The earth's climate system is driven by solar forcing and global
scale forcing such as anomalies in sea-surface temperatures asso-
ciated with El Nino and the Pacific Decadal Oscillation. The local
response to larger-scale meteorological forcing is determined
in large part by land surface characteristics such as albedo,
emissivity, thermal capacity, available moisture and surface
roughness - all influencing the energy, mass, and momentum
exchange between the earth surface and the atmosphere. In urban
areas the energy balance characteristics differ as the building
materials and morphology lead to an increased surface volumet-
ric heat-storage capacity and thermal conductivity; short- and
long-waveradiationtrappingbecauseoftheverticalstructureof
buildings; and anthropogenic heat release.
The extent of many urban areas around the globe is large
enough to affect mesoscale phenomena on the 2-200 km scale
(Orlanski, 1975) such as thunderstorms, convection, complex
terrain flows, sea and land breezes. One of the well-known effects
of cities on the atmosphere is the urban heat island (UHI)
effect, i.e. a warming of the near-surface atmosphere in cities
in comparison to rural areas. Comprehensive overviews of the
specifics of the urban energy balance and the effects of cities on
weather, climate and air quality are given in Oke (1987), Arnfield
(2003), Collier (2006), Masson (2006), Cotton and Pielke (2007)
and Fisher
et al
. (2005).
Regional atmosphericmodels aim to capturemesoscale atmo-
spheric phenomena. Very good overviews of the basics of
mesoscale meteorological and air quality modeling are given
in Pielke (2002), Seinfeld (2006) and Byun and Schere (2006),
respectively. The term model, as used in the context of meteo-
rology and air quality, refers to a complex computer code that
numerically solves a set of differential equations that govern the
evolution of the state of the atmosphere in space and time in
terms of air temperature, pressure, specific humidity, chemical
constituents and wind speed. The evolution is determined in
part through the interaction between the model variables, but
also through external forcing (e.g., solar radiation) and interac-
tions with the earth's surface through fluxes of heat, moisture,
momentum and emissions. Physical properties of the earth's
surface that influence the exchange with the atmosphere depend
on LULC characteristics. Therefore the accurate characterization
21.2
Physical approaches
for the representation of
urban areas in regional
atmospheric models
In order to improve forecast model performance in urban areas
for weather, climate and air quality applications several physical
approaches of varying complexity were developed that describe
the energy and matter exchange between urban surfaces and
the atmosphere. During the past decade computer capacity has
increased significantly, allowing spatial resolutions of regional
atmospheric models as high as a few hundred meters. However,
the airflow around individual buildings and roads cannot be
spatially resolved. Therefore in analogy to vegetation canopies,
the terms ''urban parameterization'' or ''urban canopy model''
(UCM) are widely used in the scientific community tomathemat-
ically describe the average effects of a configuration of buildings
and streets on the atmosphere. ''Urban canopy parameters''
(UCPs) provide values for UCM input parameters (Ching
et al
.
2009). An excellent review of principal physical approaches and
the efforts to include them in regional meteorological models
is given by Brown (2000), while an update on more recent
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