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Sierra Ancha Wilderness Area |
Site Name
IMPROVE:
SIAN1
Region
Mexican Highlands
Terrain
Terrain is shown in the 2 km terrain
map and the 20 km terrain
map.
The Sierra Ancha Wilderness Area is in central Arizona, 90 to 100 km east of
the Phoenix/Mesa area. Terrain consists of box canyons, vertical cliffs, and
pine-covered mesas. Intermittent drainages flow into Cherry Creek, which
flows from north to south along the eastern edge of the Wilderness into the
Salt River just south of the Wilderness. The Salt River flows westward
towards Mesa via Roosevelt Reservoir. Wilderness elevations range from 1,200
m near Cherry Creek at the southeastern boundary of the Wilderness to 2,250 m
at several high peaks. The highest elevation is 2,358 m at the crest of Aztec
Peak. Chaparral covers lower elevations along with turbinella oak, manzanita,
and mountain mahogany. Piņon and juniper cover the east side of the
Wilderness, dropping to semidesert brush and grassland below. Snow falls in
the highland in winter.
The IMPROVE monitoring site representing the Sierra Ancha Wilderness Area is
SIAN1, located outside of the Wilderness Boundary about 20 km to the north
near the town of Young, Arizona. It is at an elevation of 1,595 m, midway
within the range of elevations of the Sierra Ancha Wilderness, and is midway
between the Wilderness Area and the Mogollon Rim to the north. Access to
Young is via gravel road.
Representativeness
SIAN1 should be very representative of aerosol concentration and composition
across long site paths from view locations for the Sierra Ancha Wilderness
Area. It may not be as representative of aerosol concentration and
composition at interior valley and canyon locations if and while they are
within strong shallow surface based inversions.
Nearby Population/Industrial Centers and Local Sources
Nearby Meteorological Data Stations
Nearby meteorological monitoring network stations are shown in the data
network map and at the
US Climate Archive and RAWS
station US Climate Archive sites.
The Cherry Creek
RAWS site is within the northern Wilderness boundary at an elevation of
1,495 m. Meteorological data including wind data have been collected at this
site since August, 2001.
The Sierra Ancha
COOP station, with temperature and precipitation data, is nearby at an
elevation of 1,555 m. These stations should provide the best representation
of climate data within the Wilderness, although wind direction data may have
some terrain influence. Also, wind speed data from the
Cherry Creek RAWS
site show a high frequency, ~ 80 % of calm wind with speeds < 1.3 mph.
The nearest upper air site is the Tucson RAOB site, which conducts
atmospheric soundings twice daily. Sounding data from Phoenix should be
representative of regional upper air structure.
Wind Patterns
Wind patterns of the region are characterized by predominantly westerly
winds except during spring and summer.
Phoenix wind roses show the
prevailing westerly to southerly wind direction predominance on an annual
basis. A seasonal shift in wind from a westerly direction to a more
southerly direction, not clearly evident in the Phoenix wind roses which may
be steered locally, typically establishes itself in early July and persists
through mid-September and is associated with the
North American
Monsoon. The wind direction shift is more evident in wind data
from the Cherry
Creek RAWS site, which shows the summertime shift to southerly
winds. During this period moisture bearing winds move into the region from
the southwest at the surface, from the Gulf of California, and aloft from
the southeast, from the Gulf of Mexico. Convective instability associated
with surface heating of the moist air, combined with orographic uplift as
air is transported towards higher terrain in the direction of the Mogollon
Rim, produces frequent convective precipitation events often associated with
intense rainfall, lightning, hail, and damaging winds. Strongest
thunderstorms are often found in the mountainous regions such as the Sierra
Ancha Wilderness. These thunderstorms are often accompanied by strong winds
and brief periods of blowing dust prior to the onset of rain. South and
southwesterly regional transport winds may also carry emissions from nearby
sources and source regions west of the Wilderness, such as the Phoenix/Mesa
metropolitan area, into higher terrain of the Sierra Ancha Wilderness and
Mogollon Rim.
Transport flow into the Wilderness may also occur with more quiescent
regional flow conditions as a result of mesoscale upslope flow in the Salt
River valley and Cherry Creek drainage and surrounding terrain from nearby
sources and source regions to the west including Phoenix/Mesa. Locally
within the Wilderness, upslope/downslope patterns will be steered according
to orientation of local terrain.
Potential local transport routes into Sierra Ancha Wilderness include
low-level mesoscale upslope flow from sources and source regions to the
west, perhaps along the Salt River valley and mixed upwards to high
elevations of the Wilderness, and long distance transport via upward mixing
from more distant source regions in the southwest and transport into the
region via upper level flow.
Inversions/Trapping
The southwestern U.S. region that includes the Sierra Ancha Wilderness is
subject to frequent stagnation events. In the central Arizona long-term data
indicate an average of at least one stagnation episode per month from May to
October, where an air stagnation episode is defined as stagnation conditions
that persist for 4 days or longer (Wang
and Angell, 1999). During this period, pressure and temperature
gradients in the region are weakest, and wind circulations weakest.
Subsidence inversions during these conditions may trap regional haze with
buildup over periods of several days.
Surface based radiation inversions in the Phoenix/Mesa valley can occur,
especially in the wintertime. Wilderness locations are likely above this
surface inversion layer most of the time, but may be impacted if mixing
caused by surface heating lifts trapped pollutants to Wilderness elevations.
At lower canyon elevations within the Wilderness, locally generated aerosols
may be trapped within smaller scale radiation inversions.
Climatological Statistics
Phoenix Normals
Means and Extremes are representative of regional climatology at lower
elevations. Within the Sierra Ancha Wilderness, the
Cherry Creek
RAWS site, or
Sierra Ancha
COOP sites should provide long-term data more representative of Sierra
Ancha Wilderness locations.
Meteorological Indicators for Local Sources
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Last updated 13 November 2004 |