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Hells
Canyon Wilderness Area |
Site Name
IMPROVE:
HECA1
Region
Columbia Plateau
Terrain
Terrain is shown in the
2 km
terrain map and the
20 km
terrain map.
The Hells Canyon Wilderness Area, 214,933 acres, straddles the border of
northeastern Oregon and western Idaho and comprises the Hells Canyon of the
Snake River, over 2,440 m (8,000 ft) at its deepest. It includes 68 miles of
the Snake River that flows from south to north through the canyon.
Elevations range from 2,864 m (9,393 ft) at the summit of He Devil Mountain
on the Idaho side to 250 to 300 m (800 to 1,000 ft) at River level.
The IMPROVE site representing Hells Canyon Wilderness is HECA1, 15 km (10
mi) south of the southernmost Wilderness boundary. Site elevation is 625 m
(2,050 ft). It is near a hilltop west of Oxbow Dam on the Snake River, about
350 ft above river level downstream from the dam.
Representativeness
The HECA1 IMPROVE site is well within the Snake River valley upstream from
the Wilderness. Its elevation is some 800 m (2,600 ft) lower than highest
canyon rim elevations and 110 m (350 ft) higher than river level at the
bottom of the canyon. There could be times when it is more representative of
aerosol characteristics at lower canyon elevations than of regional
characteristics at higher exposed elevations. This is more likely to be true
in the winter than in the summer when convective mixing links the site to
upper air.
Nearby Population/Industrial Centers
HECA1 and the Hells Canyon Wilderness are about 100 km (60 mi) northwest and
downriver from Boise, and may be linked to Boise and Treasure Valley urban
and agricultural area emissions by down-valley drainage transport flow
within the Snake River valley. This could occur during prolonged periods of
synoptic high pressure and stagnation. There are no major area source
regions west of the site and Wilderness for some distance. Sources and
source regions in western Oregon and Washington may be significant upwind
contributors to aerosol concentrations in this relatively clean Class I
area. Highest aerosol concentration episodes may result from regional forest
fires.
Nearby Meteorological Network Monitoring Sites
Nearby meteorological monitoring network stations with respect to the HECA1
IMPROVE site are shown in the data network map and at the
RAWS station US Climate Archive
site. The Snake River Idaho RAWS site
17 km north of HECA1 is at an exposed elevation of 1,250 m (4,100 ft), 2,000
ft higher than HECA1, and would provide concurrent meteorological data
representative of higher elevation conditions including regional wind flow.
The closest and most representative upper air site is at Boise Idaho
Municipal (BOI). This and other upper air sites are accessible via the
University of Wyoming Dept of Atmospheric Science web page.
Wind Patterns and Transport Potential
Synoptic winds in the region are generally westerly in the spring and summer
when the Pacific High is off the coast of northwestern U.S., potentially
contributing to transport from sources in western Oregon and Washington and
more distant source regions. This flow is often assisted by the development
of a typical summertime heat low over the Columbia Plateau that can bring
marine air, and higher humidity, into the region via the Columbia Gorge. In
the winter with the weakening and southward retreat of the Pacific High,
interior high pressure over the Great Basin and Idaho and lower pressure
west of the Cascades can result in easterly gradient (synoptic) flow, with
potential for transport from sources to the east, including Boise and the
Treasure Valley agricultural region. These synoptic patterns are indicated
in monthly Pendleton Oregon wind roses (link to Pendleton wind roses
archived) and Boise Idaho wind roses. (link to Boise wind roses archived)
Note that these surface wind patterns may differ somewhat from upper level
winds because of terrain effects.
Hells Canyon of the Snake River is oriented south to north, generally
perpendicular to prevailing upper level winds. In the canyon, flows will be
channeled. There may be an upvalley/ downvalley diurnal pattern during
periods of weak synoptic scale pressure gradients. Downvalley flow in the
evening and nighttime would be from the south, with a potential for
transport to the HECA1 site from upstream Snake River valley sources
including Boise and Treasure Valley of southwestern Idaho.
Inversions/Trapping
The predominant inversion scenario in this deep canyon may be a surface
based valley inversion. The HECA1 site is relatively low within the canyon,
some 110 m (350 ft) above river level, and could at times be within valley
inversions that extend to this height and isolate it from regional overlying
air. This is especially true during the winter, when inversion episodes and
cold pooling can persist for multi-day periods. Mixing heights calculated
for Spokane (Ferguson and Rorig, 2003),
representative of Columbia Plateau locations, show winter heights generally
below 300 m (1,000 ft) but still higher than the HECA1 height 110 m above
river level. Note however that these figures are more representative of
flatter exposed terrain than of deep canyons. Spring and summer Spokane
mixing heights frequently reach to 1,500 m (4,920 ft). Calculated Fall
Spokane mixing heights were typically near 900 m. When the atmosphere is
well mixed, especially during warm summer days and windy periods, the HECA1
site is representative of regional aerosol characteristics over the Class I
Area.
Higher level summertime subsidence inversions associated with the
establishment of the semi-permanent Pacific high-pressure system could also
result in regional aerosol buildup over periods of days. Subsidence
inversion heights are typically at elevations of 2,000 to 3,000 m (6,000 to
10,000 ft). Highest regional aerosol concentrations may occur during
summertime stagnation and subsidence inversion periods in conjunction with
western wildland fires.
Climatological Statistics
Spokane Washington Normals Means and Extremes
are representative of low elevation locations in the central Columbia
Plateau. Other Washington Climate Summaries
and Oregon Climate Summaries
are available from the Western Regional Climate Center.
Meteorological Indicators
Keywords
References
Ferguson, Sue and Miriam L. Rorig. Regional Pollution Potential in the
Northwestern United States. USDA Forest Service General Technical Report
PNW-GTR-590, October, 2003.
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Last updated 9 December 2004
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