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Alpine Lakes Wilderness Area |
Site Name
IMPROVE:
SNPA1 (Snoqualmie Pass)
Region
Northern Cascade Range
Terrain
Terrain is shown in the 2 km terrain map and the 20 km terrain map.
The Alpine Lakes Wilderness Area encompasses approximately 394,000 acres on
both sides of the crest of the Cascade Range between Stevens Pass in the
north and Snoqualmie Pass in the south. On the east side Headwaters
drainages flow to the upper Columbia River via the Wenatchee and Yakima
Rivers. On the west side the North and Middle Forks of the Snoqualmie River
merge and flow to the Seattle area of Puget Sound. Lowest elevations on the
western boundary are near 1,300 m (4,300 ft) where the Middle Fork of the
Snoqualmie exits the Wilderness. This is some 1,200 m (4,000 ft) higher in
elevation than the Seattle metropolitan area 60 to 70 km (40 to 50 mi) west
of the Wilderness. Highest Wilderness elevations on the Cascade Crest that
traverses the Wilderness are near 2,400 m (7,900 to 8,000 ft).
The IMPROVE site representing Alpine Lakes Wilderness is SNPA1 located in
Snoqualmie Pass, a pass over the Cascades close to the southern Wilderness
boundary. The monitoring site elevation is 1,160 m (3,805 ft), 239 m above
the Snoqualmie Pass elevation of 921 m (3,022 ft). It is near a ski area.
Representativeness
SNPA1 is at a well-exposed ridge crest location and should be very
representative of aerosol concentration and composition at similarly exposed
locations in the Alpine Lakes Wilderness. Its elevation is at the lower end
of the range of Wilderness elevations. Its mountain pass location will be
representative of transport flow across the Cascade crest.
Nearby Population/Industrial Centers
The Seattle metropolitan area and Puget Sound source region is about 50 km
(30 mi) west of SNPA1 at its closest point, and 1,000 to 1,100 m (3,200 to
3,600 ft) lower in elevation. The city of Seattle is 70 km (40 to 45 mi)
west northwest of the monitoring site. East of the Cascades, the cities of
Wenatchee and Yakima are near 150 km (90 to 100 mi) to the east and
southeast respectively.
Nearby Meteorological Network Monitoring Sites
Nearby meteorological monitoring network stations are shown in the data
network map and at the
RAWS station US Climate Archive
site. There is a COOP site at Snoqualmie Pass, with temperature and
precipitation data, and several SNOTEL sites in the area, but there are no
identified network monitoring sites that collect hourly data including wind
speed and direction.
The nearest upper air site is at Quillayute Washington (UIL), on the Olympic
Peninsula. 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. This pattern can be seen in monthly
Seattle Washington wind
roses although 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. The SNPA1 IMPROVE site
is located near the crest of the Cascades and may be exposed to airflow over
the Cascades and to aerosols transported from upwind sources by upper level
winds. Although it is above valley elevations to the west, SNPA1 may at
times see diurnal upvalley transport from the Seattle area via the South
Fork of the Snoqualmie River. If it occurs, such flow transport would show a
diurnal pattern of aerosol characteristics.
Inversions/Trapping
Locally, the SNPA1 site is at a ridge crest location and probably above
trapping inversions that may develop at valley bottom locations east and
west of the Cascade crest. On a larger scale, inversion breakup and vertical
mixing during periods of weak synoptic forcing could at times bring urban
emissions from the Seattle and Puget Sound source region 50 to 75 km (30 to
50 mi) west of Wilderness boundaries to western Wilderness and SNPA1
elevations. Mixing heights calculated for a similar maritime location at
Salem, Oregon (Ferguson
and Rorig, 2003) show winter heights generally below 300 m (1,000 ft),
which would prevent urban emissions from reaching the SNPA1 site elevation,
but Spring and summer Salem mixing heights frequently reach to 1,500 m or
higher which could allow urban emissions to arrive at SNPA1. This could
result from concurrent afternoon upvalley flow or from entrainment and
transport by higher level flow. 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 SNPA1 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), well above the SNPA1 IMPROVE site. With weak northwesterly
winds, Puget Sound emissions can become trapped against the Cascades and
possibly impact lower crest elevations such SNPA1. Highest aerosol
concentrations may result during summertime stagnation and subsidence
inversion periods in conjunction with western wildland fires.
Climatological Statistics
Seattle
Washington Normals Means and Extremes are representative of low
elevation locations in western Washington source areas upwind of the Alpine
Lakes Wilderness. Other
Washington Climate
Summaries are available from the
Western Regional Climate Center.
Meteorological Indicators
Keywords (to be completed)
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Last updated 28 February 2005
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