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San Jacinto Wilderness Area |
Site Name
IMPROVE:
SAGO1 (San Gorgonio Wilderness)
Region
Southern California Ranges
Terrain
Terrain is shown in the 2 km terrain
map and the 20 km terrain
map.
The San Jacinto Wilderness Area is part of the San Jacinto Mountains in
southern California, essentially adjacent to the Los Angeles Basin to the
west, which can be seen from its higher elevations. It is one of the
Peninsular Ranges that extend south from the Los Angeles Basin to the tip of
the Baja Peninsula and separate the Los Angeles Basin from the Mohave Desert
to the east. It occupies 20,564 acres and is split into a north Wilderness
and a south Wilderness, separated by the Mount San Jacinto State
(California) Park and Wilderness. It is separated from the San Bernardino
Mountains and San Gorgonio Wilderness by San Gorgonio Pass. Elevations range
from less than 600 m (2,000 ft) on the north edge within San Gorgonio Pass
to almost 3,350 m (11,000 ft) at its higher peaks. The highest peak in the
area is San Jacinto Peak located between the north and south Wilderness
sections, elevation 3,294 m (10,804 ft).
The IMPROVE site representing San Jacinto Wilderness is SAGO1 (San Gorgonio
Wilderness Area) located north of San Gorgonio Pass in the upper
(headwaters) Santa Ana River Valley at an elevation of 1705 m (5592 ft) and
~ 35 km (20 mi) north of the Wilderness boundary across the San Gorgonio
Pass. It is also separated from the San Jacinto Wilderness by the San
Gorgonio Wilderness that includes the so-called “Ten Thousand Foot Ridge”,
with elevations in excess of 10,000 ft.
Representativeness
The SAGO1 IMPROVE site is near the bottom of the Santa Ana River valley and
separated from the San Jacinto Wilderness by the San Gorgonio Wilderness
that presents a massive intervening obstruction. It should be representative
of lower Wilderness elevations when the atmosphere is well mixed, but may
not be as representative when it is within a local trapping inversion in the
Santa Ana River Valley, or beneath a regional inversion between the SAGO1
elevation and Wilderness elevations. The San Gorgonio Pass, a potential air
pollution corridor between the Los Angeles Basin and the Mohave Desert to
the east, also lies between SAGO1 and the San Jacinto Wilderness and could
at times create a gradient in concentrations between the SAGO1 monitoring
site and San Jacinto Wilderness locations. There could also be a difference
in aerosol composition if and when the SAGO1 site is influenced by local
sources such as wildland fires.
Nearby Population/Industrial Centers and Local Sources
The Wilderness is just above the eastern foothills of the sprawling and
heavily populated and industrialized South Coast Air Basin immediately to
the west and is very subject to its influence.
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. There are no identified network
meteorological monitoring sites within Wilderness boundaries. There are
three RAWS stations at lower elevations to the west, the
Cranston
California RAWS Site, elevation 596m (1,950 ft), the
Keenwild
California RAWS site, elevation 1,500m (4,920 ft), and the
Anza California
RAWS site, elevation 1,195m (3,920 ft), that may provide useful
information on vertical wind and temperature structure between the Los
Angeles basin and the San Jacinto Wilderness. There are numerous surface
meteorological monitoring sites in the Los Angeles basin that should provide
data that is regionally representative for lower elevations.
The San Diego/Miramar and Vandenburg AFB RAOB sites in southern California
both conduct atmospheric soundings twice daily. Data from these sites is
regionally representative.
Wind Patterns
Overall, the prevailing wind direction in the region surrounding San Jacinto
Wilderness is south to southwesterly. This is evident in
Los Angeles wind roses. There are seasonal phenomena
that modify this overall pattern. In the summer a thermal low develops over
the Mohave Desert. During the winter, higher pressure over the Mohave Desert
results in winds blowing from the desert towards the California south coast.
This frequently takes the form of Santa Ana foehn desert winds, which may
carry large amounts of dust (PM10 into the South Coast Air Basin
Locally and in the absence of synoptic forcing, winds at the SAGO1 site
should be characteristic of mountain/valley circulation, with easterly (from
the east) nighttime drainage flow and westerly daytime upslope flow in the
valley. Upslope flow will be from the direction of the greater Los Angeles
area and could bring pollutants into the area. Drainage flow will be the
reverse, return flow. There may be a microscale north/south component at the
monitoring site proper, resulting from its location near the southerly
Hamilton Creek drainage. Within the San Jacinto Wilderness, local upslope/downslope
winds on the west side should be generally easterly and westerly
respectively, which could bring Los Angeles basin air into the wilderness
during daytime upslope conditions, with return flow at night.
Potential local transport routes into the Wilderness include Los Angeles
Basin emissions transported directly via upslope winds, mixed upwards with
afternoon mixing, or trapped regionally under a persistent subsidence
inversion.
Inversions/Trapping
The steep Santa Ana River valley, in which the SAGO1 IMPROVE site is
located, is subject to inversion and trapping of pollutants during periods
of high pressure and stagnation over southern California. During strong
early morning shallow inversion conditions over southern California, the
site may be well above the inversion height, with associated good
visibility. Under some conditions there may be a diurnal pattern to
extinction, when afternoon vertical upward mixing results in pollutants
being transported into the valley and associated increase in extinction at
the site, followed by return drainage transport at night and decrease in
light extinction. Such a pattern could persist until the region is scoured
of visibility degrading particulate matter by synoptic events. This pattern
is most likely to occur during summer, when strong surface heating results
in afternoon upward mixing to valley elevations. During strong trapping
inversion conditions, aerosol concentration and composition observed at the
SAGO1 site may differ from aerosol conditions at higher elevations of the
San Jacinto Wilderness, although the diurnal pattern observed at SAGO1
should also be seen within the San Jacinto Wilderness, especially on west
facing slopes.
Wang and Angell,
1999 describe a region of the U.S. with high frequency of regional
stagnation events centered over southern California. According to long term
data the region typically has two or more stagnation episodes per month from
May to October, where an air stagnation episode is defined as stagnation
conditions that persist for 4 days or longer. During this period, pressure
and temperature gradients in the region are weakest, and wind circulations
weakest. Subsidence inversions during these conditions may trap regional
haze with buildup over periods of several days. These buildup periods should
be evident in data from the SAGO1 IMPROVE site.
Climatological Statistics
In addition to Climate data from the
Cranston
California RAWS Site, the
Keenwild
California RAWS site, and the
Anza California
RAWS site, long term
San Bernardino
Climate Data and
Lake Arrowhead
Climate Data should be representative of lower and intermediate
Wilderness elevations, respectively. These and other
Southern California
Climate Summaries are available from the
Western Regional Climate Center.
Keywords
Valley
Inversion
Channeling
Santa Ana
Foehn
Mountain/valley circulation
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Last updated 13 November 2004
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