Site Name
IMPROVE:
PASA1
Region
Cascade Range
Terrain
Terrain is shown in the 2 km terrain map and the 20 km terrain map.
The Pasayten Wilderness Area comprises 529,850 acres on the crest of the
Cascade Range in northern Washington, bordered on the north by 80 km (50 mi)
of the Canadian border. Terrain of the western Wilderness area is a series
of high ridges that flatten out in the eastern Wilderness area to high
plateaus. Almost 150 peaks in the Wilderness have elevations above 2,300 m
(7,500 ft), close to the 760 to 770 mb standard pressure height. The western
Wilderness Area, west of the Cascade Crest, is in the upper Skagit River
basin and drains into Ross Lake and the Skagit River to northern Puget
Sound. (See North Cascades National Park Meteorological description page).
From the eastern Wilderness, streams flow north into British Columbia, or
southeast into the Central Columbia Plateau. Lowest Wilderness elevations
are around 1,000 m (3,000 ft) at the western boundary near Ross Lake and the
southern boundary near Lost River Gorge.
The IMPROVE site representing Pasayten Wilderness is PASA1, located near the
crest of Little Buck Mountain, 50 km (30 mi) south and east of the
Wilderness boundary, at an exposed elevation of 1,634 m (5,360 ft), near the
830 mb standard pressure height.
Representativeness
The PASA1 IMPROVE site is at a well-exposed ridge top location and should be
very representative of regional conditions including high elevation
locations in the Pasayten Wilderness. It is also representative of upper
level (850 mb) aerosol characteristics of the central Columbia Plateau and
Basin. The North Cascades National Park site, NOCA1, may be better
representative of low elevations east of the Cascade crest. See North
Cascades National Park Meteorological description page.
Nearby Population/Industrial Centers
Seattle and Puget Sound source regions are ~ 200 km (120 mi) west of the
PASA1 site, on the other (west) side of the Cascade crest. Aerosols may be
transported to the monitoring site from the Puget Sound region by upper
level (850 mb) westerly winds. Columbia Plateau sources and the Spokane
Washington area are close to the PASA1 site. Columbia Plateau sources
including agricultural and crustal (dust) components may impact the site
during regional summertime stagnation periods when lofted to upper levels on
hot afternoons.
Nearby Meteorological Network Monitoring Sites
Nearby meteorological monitoring network stations with respect to the PASA1
IMPROVE site are shown in the data network
map and at the RAWS station
US Climate Archive site. The
Starvation
Washington RAWS site is about 15 km north of PASA1 at an elevation of
2,070 m (6,789 ft). The
First Butte
Washington RAWS site with a more complete period of record is 30 km (20
mi) northwest of PASA1 at a similar elevation of 1,677 m (5,500 ft). Data
from either of these sites should be representative of concurrent
meteorological conditions at PASA1.
The most representative upper air site is at Spokane Washington (OTX). This
and other upper air sites are accessible via the
University of Wyoming Dept
of Atmospheric Science web page.
Wind Patterns
Synoptic winds in the region are generally westerly, with more northwesterly
flow during the summer when the Pacific High is off the coast of
northwestern U.S., and more westerly flow during the winter when the Pacific
High has retreated southward. Monthly Spokane Washington wind roses (link to
Spokane wind roses archived) indicate this pattern, although wintertime wind
directions are more northeasterly, bringing continental air southward with
high pressure over the Canadian interior. Note that these surface wind
patterns may differ somewhat from upper level winds because of terrain
effects. During the winter, with high pressure over the Great Basin and
Idaho and low pressure west of the Cascades easterly gradient (synoptic)
flow is common. Upper westerly flow may bring Puget Sound area emissions to
the monitoring site. With weaker summertime regional pressure gradients, a
diurnal pattern may allow Columbia River Basin and Plateau emissions to be
lofted to upper levels, including the PASA1 site, during the day and return
with downslope flow at night (Ferguson,
1998)
Inversions/Trapping
The PASA1 site is at a ridge top location and should be above local surface
based trapping inversions. On a larger scale, persistent low level
temperature inversions over the Columbia Basin keep pollutants trapped at
low elevations during most of the winter. Mixing heights calculated for
Spokane Washington (Ferguson
and Rorig, 2003), show winter heights generally below 300 m (1,000 ft),
which would prevent urban emissions from reaching the PASA1 site elevation,
but spring and summer Spokane mixing heights frequently reach to 1,500 m
(4,920 ft), allowing Columbia Basin emissions to reach the PASA1 site
elevation of 1,634 m (5,360 ft). Calculated Fall Spokane mixing heights were
typically near 900 m, lower than in the spring and summer but occasionally
high enough to bring valley emissions to the PASA1 site elevation.
Regionally, summertime subsidence inversions associated with the
establishment of the semi-permanent Pacific high-pressure system could
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), near or above the PASA1 site elevation. 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 are available from the
Western Regional Climate Center.
Meteorological Indicators
Keywords (to be completed)
References
Ferguson, Sue. 1998. Air Quality Climate in the Columbia River Basin. USDA
Forest Service General Technical Report PNW-GTR-434. August 1998.
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.