Environmental Engineering Reference
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residential urban categories is remarkably uniform across the
entire metropolitan area. The fractional cover of built, vegetation
and soil surfaces comprises 0.73, 0.10 and 0.17 in the xeric
residential and 0.60, 0.23 and 0.17 in the mesic residential
categories, respectively. The building morphology is very similar
in the residential classes with average building heights, roof and
roadwidthsof6m,10mand15m.
The commercial/industrial land use category is comprised
almost entirely of anthropogenic surfaces (fractional cover of
built surfaces 0.95). The average building height of commercial
and industrial buildings is 10m. The large variation in road width
(10 m to 100 m) and building area complicates the assignment of
building geometry. Here for the commercial/industrial class the
building width (10 m) and street width (10 m) of the commercial
land use class provided with WRF's urban parameter table (ver-
sion 3.0.1.1) were applied. This highlights the observation that
three urban land use classes as available with the standard WRF
might be too few to represent the heterogeneity of urban form
in high resolution WRF applications to urban areas. Similarly,
the WRF standard values for heat capacity, conductivity, albedo,
emissivity roughness length for heat and momentum of roof,
road and wall surfaces were used.
Figure 21.1 shows the 1973, 1985, 1998 and 2005 LULC
classification data as processed for the WRF model simulations.
The predominant land cover category in rural areas is shrubland
which represents desert vegetation in the USGS classification.
WRF simulations were conducted for each EHE (12-16 July
2003, 12-17 July 2005, 21-24 July 2006 and 3-7 July 2007)
with four sets of LULC: 1973, 1985, 1998 and 2005 (16 runs in
total) with a spatial resolution of 2 km. Comparison of measured
2 m air temperatures
T
2m
and 10 m wind speeds for 18 surface
stations located in rural and urban parts of the region show a
good agreement between observed and simulated data for all
simulation periods (Grossman-Clarke
et al
., 2010).
Here the focus is on the regional scale effects of LULC
changes. Hence local scale effects on a sub-neighborhood scale
might not be captured by WRF (highest resolution in this study:
2 km). Also, a dominant land use class is assigned to each model
grid cell with its fixed vegetation fraction and morphological
characteristics. It would potentially enhance the quality of the
simulations in some areas of the urban region if spatially explicit
urban vegetation cover and building characteristics are used for
each grid cell rather than urban land use classes. As discussed
previously there are limitations to the single-layer UCM because
of the assumed urban geometry of a street canyon. The single-
layer UCM was chosen for the simulations over the available
multilayer UCM option since development across the Phoenix
area is largely suburban in nature and one-story single-family
homes are the predominant building type. Therefore the vertical
extent of the city is generally low and the benefits of a multilayer
UCM such as the direct interaction with the planetary boundary
layer are relatively small. Differences in the simulated maximum
and minimum daily 2 m air temperatures,
T
2m
, between the
land use scenarios were used as a measure of LULC classification
effects on regional near-surface air temperature. To illustrate the
effects of land use changes on minimum and maximum daily
temperatures over time, the effects were averaged over the 18
simulated days that constituted the four EHEs. Difference maps
allow comparison between the regional
T
2m
simulated with the
2005 LULC classification to the
T
2m
obtained with the 1973,
1985 and 1998 LULC classifications, respectively at 0500 Local
Standard Time (LST) and 1700 LST 2005 (Fig. 21.2). Based on the
simulations, new urban development caused an intensification
and expansion of the area experiencing extreme temperatures. As
expected, 1998 with the shortest time for LULC changes, show
the smallest but still detectable differences.
For the minimum temperatures (0500 LST) Fig. 21.2 (upper)
shows that with urban development in formerly agricultural as
well as desert areas the temperatures increased on average by
5-8
◦
C, leading to an expansion of the area experiencing elevated
night-time temperatures. The largest changes inminimumnight-
time temperatures occur in the center of the formerly agricultural
areas to the southeast of Phoenix and on some nights approach
10
◦
C. Smaller changes in minimum
T
2m
of 4-6
◦
Caredetected
when desert land underwent the transformation to urban land
use. Night-time temperatures in the existing urban core show
only relatively small changes of up to 1-2
◦
C with the ongoing
LULC changes. Comparing changes in minimum temperatures
between land use scenarios showed that strong changes in
T
2m
were relatively localized where new urban development took
place. The expansion of the built-up area also increased min-
imum temperatures in the previously developed fringe region
by 1-3
◦
C, as in the center of the metropolitan region. For the
urban land use categories the night-time net radiation amounts
to up to
∼
-200 W m
−
2
leading to higher surface temperatures
and a warming of the atmosphere in the city relative to the rural
surroundings. Withmaximumnight-time values of
∼
150Wm
−
2
the ground heat flux for the urban land use types is significantly
higher than for the rural land use types. The ground heat flux for
the urban land use categories also accounts for the heat storage in
roofs and walls. Positive night-time sensible heat fluxes (QH) are
calculated for the urban categories while they are negative for the
rural ones. Heat fluxes are positive when directed away from the
surface into the atmosphere and negative when directed towards
or into the surface as for the ground heat flux.
Daytime temperatures (1700 LST) are little affected when
urban development replaces desert (Fig. 21.2 lower). In the
urban center the increase over time in
T
2m
amounted to about
0
.
5
◦
C. As the main conversion of agricultural to urban land use
concluded in themid 1990s the changes in averagemaximum
T
2m
between 1998 and 2005 were relatively small. However, when irri-
gated agricultural land was converted to suburban development,
maximum
T
2m
increased on average by 2-4
◦
C. The increase in
sensible heat fluxes with urban development led to the daytime
warming when agricultural land was replaced with urban devel-
opment. Maximum daytime latent heat fluxes (QE) for irrigated
agriculture land are of the order of 500 W m
−
2
compared to
about 20 W m
−
2
and 75 W m
−
2
for commercial/industrial and
xeric residential areas, respectively. Due to the low soil moisture
content in the natural desert QE is simulated to be
∼
20 W
m
−
2
. The magnitude of maximum daily QH was
∼
100 W m
−
2
for irrigated agricultural,
∼
450 W m
−
2
for desert and
∼
400 W
m
−
2
for commercial/industrial areas. The increase in sensible
heat fluxes with urban development led to the daytime warming
when agricultural land was replaced with urban development.
Because of the large daytime heat storage flux for the commer-
cial/industrial land use category during the hours before noon of
up to
450 W m
−
2
the simulated QH for the desert areas is
larger than for the urban area. However net radiation is signifi-
cantly larger for urban vs. desert LULC classes due to the lower
albedo and the effects of the sky view factor of the urban classes.
The influence of LULC changes on 10 m wind speed during
the EHE episodes is of interest as increased urban roughness leads
potentially to an increase in vertical momentum fluxes thereby
∼−
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