(Also See Yosemite National Park)
The Emigrant Wilderness Area is adjacent to
Yosemite National Park to the south,
and meteorological features that affect haze and visibility in the two Class
I Areas are similar. They are represented by the same IMPROVE monitoring
site, YOSE1. Thus, also see Yosemite National Park Meteorological
Characterization page.
Site Name
IMPROVE:
YOSE1 (Yosemite National Park)
CASTNET: YOS404
Region
Sierra Nevada Range
Terrain
Terrain is shown in the 2 km terrain
map and the 20 km terrain
map.
The 113,000 acre Emigrant Wilderness Area is located on the upper western
slope of the central Sierra Nevada range. It is bordered by
Yosemite National Park on the south.
Watersheds drain to the Stanislaus via the South Fork of the Stanislaus in
the northern Wilderness, and to the Tuolumne River via Cherry Creek in the
southern Wilderness. The Stanislaus and Tuolumne Rivers flow southwest and
open up into the San Joaquin Valley of central California about 50 km (30
mi) southwest of the Wilderness boundary. The central San Joaquin Valley
area is the nearest major source region for anthropogenic emissions that
could affect visibility in the Wilderness. Wilderness elevations range from
about 1,500 m (5,000 ft) at Cherry Reservoir to 3,527 m (11,570 ft) at
Leavitt Peak on the Sierra Nevada crest.
The IMPROVE site representing Emigrant Wilderness Area is YOSE1 (Yosemite
National Park), located near the west end of Yosemite Valley in Yosemite
National Park at an elevation of 1,615 m (5,297 ft). The
CASTNET site, YOS404, is
also located there.
Representativeness
Lowest elevations in Emigrant Wilderness are higher than lowest Yosemite
National Park elevations, but are still near the YOSE1 elevation. Data from
YOSE1 should thus still be very representative of aerosol concentration and
composition in the Merced and Tuolumne River areas of central Yosemite
National Park and in the upper Stanislaus River area of the Emigrant
Wilderness Area, except when the areas are influenced by different local
sources such as wildland fires.
Nearby Population/Industrial Centers and Local Sources
The nearest major population center and source region for emissions that
could contribute to haze in the Emigrant Wilderness and measured at YOSE1 is
the San Joaquin Valley of central California, 50 km (30 mi) west of the
western Park boundary to which it is linked by the Stanislaus and Tuolumne
Rivers.
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 Yosemite
CASTNET site, YOS404, is
located next to the YOSE1 IMPROVE site and will provide meteorological data,
including hourly wind data, that can be correlated with aerosol measurements
to indicate possible aerosol transport into Yosemite via the Merced River
and, by similarity, into Emigrant Wilderness via the Stanislaus River. Data
from this site show the bimodal upcanyon/downcanyon flow within the Merced
River valley. Data from the
White Wolf
California RAWS site located near a ridge crest between the Tuolumne and
Merced River canyons, elevation 2,447 m (8,025 ft), should show wind
patterns capable of higher level transport into the Emigrant Wilderness
Area.
Upper air data is collected twice daily at the Oakland RAOB site.
Wind and Transport 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 evident in monthly Fresno California Wind Roses. The YOSE1 IMPROVE site is located in the Merced River
valley, 300 to 400 m above the Valley floor at the west end of Yosemite
Valley in Yosemite National Park. Here, there is a distinct channeled upvalley/downvalley pattern, evident in
Yosemite Wind Roses from
five years of data, with potential
for transport of San Joaquin Valley emissions. The same type of pattern
exists within upper Stanislaus River drainage in the Emigrant Wilderness
Area.
Potential local transport routes into the Emigrant Wilderness Area include
San Joaquin Valley emissions transported directly via diurnal upslope/downslope
flow and valley flow in Stanislaus and Tuolumne River valleys, mixed upwards
with afternoon mixing, or trapped regionally under a persistent subsidence
inversion. The most likely times for incursion of San Joaquin emissions into
the Wilderness is during warm months when afternoon upslope winds combined
with highest mixing heights in San Joaquin Valley provide optimal transport
conditions. Springtime incursions may be associated with agricultural and
forest prescribed burning in San Joaquin Valley and National Forest lands.
Autumn incursions are probably less frequent because of San Joaquin Valley
inversion that confines emissions to lower elevations.
Inversions/Trapping
Wintertime trapping inversions are probable in Stanislaus and Tuolumne River
canyons and could restrict local emissions such as prescribed forest
burning. Wintertime surface based inversions are also common in the San
Joaquin Valley, the principal source region for nearby emissions, although
at heights typically below Wilderness elevations. Fall and winter are less
likely to see San Joaquin Valley emissions transported to Wilderness areas
because of persistent low level San Joaquin Valley trapping inversions.
In the summer, the relatively shallow nighttime San Joaquin Valley boundary
layer is generally mixed to heights of at least 1,000 m (> 3,000 ft) above
the ground on a typical summer day, at the lower end of Wilderness
elevations but still potentially affecting aerosols at the YOSE1 IMPROVE
site. Summer inversions are usually larger scale subsidence inversions
associated with the establishment of the semi-permanent Pacific
high-pressure system and can result in 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), in the range of lower Wilderness elevations.
Climatological Statistics
Fresno
California Normals Means and Extremes are representative of climate
characteristics in San Joaquin Valley. Climate characteristics at the YOSE1
IMPROVE site can be accessed through data from the
CASTNET Yosemite NP
site, YOS404. Other
Northern California Climate Summaries are available from the
Western Regional Climate Center.
Keywords