(Also, see Three Sisters Wilderness Meteorological Description Page)
Site Name
IMPROVE:
THSI1 (Three Sisters Wilderness)
Region
Cascade Range
Terrain
Terrain is shown in the
2 km terrain map and the
20 km terrain map.
The Mt Washington Wilderness occupies 52,516 acres on the crest of the
Cascade Range in central Oregon. Like the Three Sisters Wilderness that it
borders to the south, 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 eastern slopes of the
Cascades descend to the Deschutes River near Bend. The highest Wilderness
elevation is 2,376 m (7,794 ft) at the summit of Mt Washington. Lowest
elevations are near 900 m (3,000 ft) in the upper headwaters basin of the
McKenzie River.
The IMPROVE site representing Mt Washington Wilderness, as well as the Three
Sisters and Mt Jefferson Wilderness Areas, is THSI1 (Three Sisters), located
outside of the Wilderness in the upper McKenzie River headwaters basin, 7 to
10 km west of the Wilderness boundary. The monitoring site elevation is 885
m (2,903 ft).
Representativeness
THSI1 is in the same air basin as western Mt Washington Wilderness locations
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 Mt Washington 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 Mt Washington 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