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Three Sisters Wilderness Area |
(Also, see Mt Washington and
Mt Jefferson Wilderness Meteorological
Description Page)
Site Name
IMPROVE:
THSI1
Region
Cascade Range
Terrain
Terrain is shown in the
2 km terrain map and the
20 km terrain map.
The Three Sisters Wilderness consists of 242,400 acres abreast the crest of
the Cascade Range in central Oregon. It includes headwaters tributaries of
the McKenzie River that flow west into the Willamette Valley near Eugene and
connect the Wilderness with that source region. On the east side streams
flow east to the Deschutes River near Bend. The highest crest elevation is
3,158 m (10,358 ft) at the summit of the South Sister. Lowest elevations are
near 600 m (2,000 ft) where the South Fork of the McKenzie River exits the
Wilderness on the west boundary. This is about 500 m (1,600 ft) above the
Willamette Valley at Eugene 70 km (40 mi) west.
The IMPROVE site representing Three Sisters Wilderness, as well as Mt
Jefferson and Mt Washington Wilderness Areas, is THSI1, located outside of
the Wilderness in the upper McKenzie River headwaters basin, 15 to 20 km
north of the Wilderness boundary. The monitoring site elevation is 885 m
(2,903 ft).
Representativeness
THSI1 is in the same air basin as western Three Sisters Wilderness locations
at the same elevation and should be very representative, especially during
periods of uniform regional haze from forest fires or long distance
transport. It is also at an optimal elevation for assessing the impact and
timing of downvalley sources to the west in the direction of the Willamette
Valley. During inversion conditions it may not be as representative of
aerosol composition and concentration at higher locations near the crest of
the Cascade Range that could be seeing emissions from distant source regions
at the same time.
Nearby Population/Industrial Centers and Local Sources
THSI1 is about 60 km east of and 700 to 800 m (2,300 to 2,600 ft) higher
than the Willamette Valley, the nearest significant source region. It could
be susceptible to Valley emissions in a well-mixed atmosphere typical of
sunny summer afternoons. Highest aerosol concentrations at THSI1 may be
result from regional forest fires.
Nearby Meteorological Data Stations
Nearby meteorological monitoring network stations are shown in the data
network map and at the
RAWS station US Climate Archive
site. The Pebble
Oregon RAWS site is located in the same headwaters canyon as THSI1 at an
elevation of 1,085 m (3,560 ft) and should provide the best meteorological
measurements to represent conditions concurrent with aerosol monitoring.
Upper air data collected twice daily at the Medford and Salem RAOB should
represent regional vertical structure.
Wind Patterns
Synoptic winds in the region are generally northwesterly (from the
northwest), with an additional component of more southerly winds in the
winter with the weakening of the Pacific High Pressure System. This pattern
is indicated in monthly
Eugene Oregon Wind Roses in the Willamette Valley west of the Cascades
and in monthly Redmond
Oregon Wind Roses east of the Cascades. More locally, winds will be
typical mountain/valley upslope/downslope flow. Summertime afternoon and
evening upslope flow from Eugene and the Willamette Valley could at times
bring Willamette Valley air to the THSI1 IMPROVE site via the McKenzie River
valley, with return flow at night and early morning.
Potential local transport routes into Three Sisters Wilderness Area include
Willamette Valley emissions transported directly via diurnal upslope/downslope
flow, mixed upwards with afternoon mixing, or trapped regionally under a
persistent subsidence inversion. The McKenzie River is a potential transport
corridor for Willamette Valley emissions from the direction of Eugene into
western Three Sisters Wilderness locations including THSI1.
Inversions/Trapping
The upper McKenzie River valley where THSI1 is located may be subject to
surface inversions and trapping of pollutants during periods of regional
high pressure and stagnation, especially in the winter. Wintertime surface
based inversions are also common in the Willamette Valley, although at
heights typically below Three Sisters Wilderness elevations. Inversion
breakup and vertical mixing could at times bring valley emissions to the
site elevation. Mixing heights calculated for Salem (Ferguson
and Rorig, 2003) show winter heights generally below 300 m (1,000 ft),
which would prevent valley emissions from reaching the THSI1 site elevation
of 885 m, 700 to 800 m above the valley floor. Spring and summer Salem
mixing heights frequently reach to 1,500 m or higher, which could bring
valley level emissions to the THSI1 site elevation. Fall mixing heights are
typically 300 to 600 m, lower than in the spring and summer but occasionally
high enough to bring valley emissions to the THSI1 site elevation.
Summertime subsidence inversions associated with the establishment of the
semi-permanent Pacific high-pressure system can 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), well above the THSI1
IMPROVE site.
Climatological Statistics
Eugene Oregon
Normals Means and Extremes are representative of climate characteristics
in the Willamette Valley source region. Climate statistics accessible from
the Pebble
Oregon RAWS site are representative of the THSI1 IMPROVE site. Other
Oregon Climate
Summaries are available from the
Western Regional Climate Center.
Keywords
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Last updated 9 December 2004
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