Site Name
IMPROVE:
SAGO1
Region
Southern California Ranges
Terrain
Terrain is shown in the 2 km terrain
map and the 20 km terrain
map.
The San Gorgonio Wilderness Area occupies 34,644 acres of the San Bernardino
Mountains of southern California approximately 120 km (75 mi) east of Los
Angeles. Elevations range from 1,340 m (4,400 ft) to 3,506 m (11,499 ft) at
the crest of Mt. San Gorgonio. Eleven of the 12 peaks in the Wilderness are
above 3,050 m (10,000 ft).
The San Gorgonio Improve site SAGO1 is located at an elevation of 1705 m
(5592 ft) within the upper (headwaters) Santa Ana River valley to the north
of the northern Wilderness boundary. (The Wilderness boundary does extend to
the bottom of the Santa Ana River valley). The orientation of the Santa Ana
River valley is west to east, with its mouth to the west, exiting into the
LA basin. The valley bottom location nearest the site is about 1650 m (5400
ft) just south of the monitoring site. Slope aspect at site is towards the
south, hence subject to midday solar heating. Elevations rise to ~ 2350 m
(7700 ft) at the ridgecrest ~ 2 miles north, and to ~ 3000 (9800ft) at the
ridgecrest ~ 7 miles south of the site.
Representativeness
The SAGO1 IMPROVE site is near the bottom of the Santa Ana River valley at
an elevation well below typical Wilderness elevations that extend to over
10,000 ft on some of the peaks. It may thus be at times contained within a
low inversion with a cap that below Wilderness elevations. During such
periods aerosol composition and concentration measured at SAGO1 may not be
representative of higher Wilderness elevations. When the atmosphere is well
mixed to Wilderness elevations the SAGO1 site should be representative.
Nearby Population/Industrial Centers and Local Sources
The Wilderness is almost adjacent to the sprawling and heavily populated and
industrialized South Coast Air Basin 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. The nearest RAWS site that is at a representative
elevation is the Burns Canyon California RAWS
site
located 10 to 15 km northeast of the Wilderness boundary at an elevation of
1,829 m (6,000 ft). There are numerous 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 Air Force Base 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
Gorgiono Wilderness is southwesterly 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.
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
This steep valley 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,
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. Most likely
to occur during summer, when strong surface heating results in afternoon
upward mixing to valley elevations.
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 the
Burns Canyon 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