(Also See
Emigrant Wilderness
Area)Site Name
IMPROVE:
YOSE1
CASTNET: YOS404
Region
Sierra Nevada Range
Terrain
Terrain is shown in the 2 km terrain
map and the 20 km terrain
map.
Yosemite National Park consists of
some 750,000 acres in the central Sierra Nevada range, west of the crest. It
includes headwaters of the Tuolumne River in the north National Park area,
and the Merced River in the south National Park area, that flow west and
open up into the San Joaquin Valley of central California about 30 km (20
mi) west of the National Park boundary. The central San Joaquin Valley area
is the nearest major source region for anthropogenic emissions that could
affect visibility in the National Park. Park elevations range from about 600
m where the Tuolumne River exits the Park and 1,000 m where the Merced River
exits the Park, to up to 4,000 m at the Sierra Nevada crest that forms the
Park’s eastern boundary. Lowest elevations are thus 500 m (1,500 ft) or more
above the San Joaquin Valley floor. The Tuolumne and Merced Rivers form
steep canyons, the Grand Canyon of the Tuolumne River and Yosemite Valley,
respectively, oriented east to west in the heart of the National Park.
The IMPROVE site representing Yosemite National Park is YOSE1, located near
the west end of Yosemite Valley at an elevation of 1,615 m (5,297 ft). The
CASTNET site, YOS404, is
also located there.
Representativeness
Data from YOSE1 should be very representative of aerosol concentration and
composition in the Yosemite Valley and Merced River areas of central
Yosemite National Park. It should also be representative of the Tuolumne
River area except when the two areas are influenced by different local
sources such as wildland fires. YOSE1 is at an elevation of 1,615 m, 300 to
400 m above the canyon floor, so there could be times when canyon bottom
locations are within a surface inversion that does not extend upward to the
monitoring site elevation.
Nearby Population/Industrial Centers and Local Sources
The nearest major population center and source region for emissions that
could contribute to haze measured at YOS1 is the San Joaquin Valley of
central California, 30 km (20 mi) west of the western Park boundary to which
it is linked by the Tuolumne and Merced River valleys.
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.
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
Yosemite National Park.
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. 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 the
Tuolumne River valley in the north National Park area.
Potential local transport routes into
Yosemite National Park include San Joaquin Valley emissions transported
directly via diurnal upslope/downslope flow and valley flow in Merced 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 National Park 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 Merced and Tuolumne River
canyons and could restrict local emissions such as prescribed forest burning
and woodsmoke in Yosemite Valley. Wintertime surface based inversions are
also common in the San Joaquin Valley, the principal source region for local
emissions, although at heights typically below National Park elevations.
Fall and winter are less likely to see San Joaquin Valley emissions
transported to National Park 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 National Park
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 National Park 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