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Mount Hood Wilderness Area |
Site Name
IMPROVE:
MOHO1
Region
Cascade Range
Terrain
Terrain is shown in the
2 km terrain map and the
20 km terrain map.
Mt Hood Wilderness is 47,100 acres on the slopes of Mt Hood in the northern
Oregon Cascades. Wilderness elevations range from 3,426 m (11,237 ft) on the
summit of Mt Hood down to almost 600 m (2,000 ft) at the western boundary.
It is almost adjacent to the Portland Oregon metropolitan area; the
westernmost boundary is about 20 km east of the Portland Oregon suburb of
Sandy and 40 km from the heavily populated metropolitan center, elevation
100 m (300 ft).
The IMPROVE site representing Mt Hood Wilderness is MOHO1, located just
south of the Wilderness boundary near Government Camp. The site elevation is
1,341 m (4,398 ft).
Representativeness
MOHO1 is well-exposed and representative of Wilderness locations generally.
When it is above the mixed boundary layer over the Portland and the northern
Willamette Valley it may be less representative of Wilderness locations at
lower elevations that are within the boundary layer. Because of the wide
range of Wilderness elevations it may at times be similarly isolated from
highest elevations closer to the summit of Mt Hood that are above summertime
subsidence inversion heights.
Nearby Population/Industrial Centers and Local Sources
MOHO1 is roughly 40 km east of the Portland Oregon metropolitan area, the
nearest significant source region. The Wilderness Area may also subject to
emissions from point sources along the Columbia River to the north and east.
High aerosol concentrations at MOHO1 may also 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
Government Camp Oregon COOP site is close to the MOHO1 monitoring site
at an elevation of 1,213 m (3,980 ft), very close to the MOHO1 site
elevation. The
Blue Ridge Oregon RAWS site is located north of the Mt Hood Wilderness
but its elevation, 1,152 m (3,780 ft), is also close to the MOHO1 site
elevation and site data includes hourly wind data that may be representative
of MOHO1 meteorology concurrent with aerosol monitoring. Long term and
current data for the Portland metropolitan area are available from the first
order Portland
Intl Airport Oregon station. This is also a RAOB site where regionally
representative upper air data is routinely collected that could be used to
correlate Portland mixing heights with MOHO1 aerosol measurements.
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. With buildup
of the high pressure center over the Great Basin during winter, easterly
transport flow from that region is also common. This pattern is indicated in
monthly Portland Oregon Wind
Roses although these represent near surface conditions where wind
directions may be modified by Columbia River and Willamette Valley terrain
features. Near the MOHO1 monitoring site, local winds in the absence of
synoptic forcing will be typical upslope/downslope on the slopes of Mt Hood,
with the possibility for afternoon upslope pollutant transport from the
Portland area, with return flow at night. Transport from the Portland area
may also be enhanced during this typical summer condition when urban
emissions are mixed upwards and transported into the Wilderness Area by the
prevailing upper level westerly flow.
Potential local transport routes into Mt Hood Wilderness Area include
Portland and Willamette Valley emissions transported directly via diurnal
upslope/downslope flow, mixed upwards with afternoon mixing and carried by
prevailing westerly winds, or trapped and accumulated regionally under a
persistent subsidence inversion.
Inversions/Trapping
The MOHO1 IMPROVE site is probably above the height of typical surface based
inversions that develop locally and over the northern Willamette Valley and
Portland area at night, but inversion breakup and vertical mixing could at
times bring valley emissions to the site. Mixing heights calculated for
Salem (Ferguson and Rorig, 2003) should be reasonably representative of the
area. During the winter Salem mixing heights generally remain below 300 m
(1,000 ft), which would prevent valley emissions from reaching the MOHO1
site elevation of 1,341 m, 1,200 to 1,300 m above Portland metropolitan area
ground level. Spring and summer mixing heights frequently reach to 1,500 m
or higher, which could bring valley level emissions to the MOHO1 site. 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 MOHO1
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 MOHO1
IMPROVE site.
Climatological Statistics
Portland Oregon
Normals Means and Extremes are representative of Portland and the
northern Willamette Valley.
Government Camp
Climate Data are representative of the MOHO1 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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