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Joshua Tree Wilderness Area |
Site Name
IMPROVE:
JOSH1 (Joshua Tree National Monument)
CASTNET: JOT403 (Joshua Tree National Monument)
Region
Southwest Deserts
Terrain
Terrain is shown in the
2 km terrain map and the
20 km terrain map.
The large Joshua Tree Wilderness consists of 429,690 acres within
Joshua Tree National Park
located in the eastern extent of the Mohave Desert of southern California,
with the eastern portions also within the Sonoran Desert Physiographic
province. It occupies a portion of the Little San Bernardino Mountains.
Elevations range from just under 200 m (650 ft) in easternmost portions to
near 1600 m (3,150 ft) at highest peaks that include Quail Mountain in the
west and Monument Mountain in the central portion. The eastern portion of
the National Park consists of the dry Pinto Wash that drains to the east.
Just to the west is the Whitewater River valley that includes the city of
Palm Springs and urban areas near Banning. San Gorgonio Pass is also just
west of the Wilderness and National Park. San Gorgonio Pass forms a break
between the San Bernardino Mountains to its north and the San Jacinto
Mountains to its south and is a natural corridor for air transport between
the Mohave Desert and the eastern portions of the South Coast Air Basin.
The IMPROVE site representing the Joshua Tree Wilderness is JOSH1, located
near the northwestern Wilderness Boundary at an elevation of 1,228 m (4,028
ft). The Joshua Tree CASTNET site,
JOT403, is also located here.
Representativeness
The Joshua Tree Wilderness IMPROVE site, JOSH1, is close to the wilderness
boundary on the west side and is at an elevation near the midrange of
wilderness elevations. It should be very representative of aerosol
characteristics within the Joshua Tree Wilderness Area.
Nearby Population/Industrial Centers and Local Sources
Nearby population centers include the Palm Springs area to the west and
developed land near the northern boundary. Joshua Tree Wilderness is also
near San Gorgonio Pass, which presents a potential corridor for emissions
from the eastern South Coast Air Basin to the west.
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. In addition to the
Joshua Tree CASTNET site
JOT403, the Lost
Horse California RAWS site and
Covington
California RAWS site are within Joshua National Park boundaries on the
west side. All of these sites collect Meteorological data that should be
representative of Joshua Tree Wilderness. There are several other long-term
meteorological monitoring stations in the region, including Surface Airways
Observations (SAO) sites at Palm Springs, Twenty-Nine Palms, and Beaumont.
Beaumont, in the San Gorgonio Pass, should have valuable data to
characterize seasonal flow through the pass.
The Mercury/Desert Rock RAOB site in southern Nevada and the San
Diego/Miramar RAOB site in southern California both conduct atmospheric
soundings twice daily. The Mercury/Desert Rock site is probably most
representative of upper air structure in Joshua Tree Wilderness.
Wind Patterns
Overall, the prevailing wind direction in the region surrounding Joshua Tree
Wilderness is southwesterly, from the direction of the California South
Coast. This is evident in 1995-2001 Joshua Tree CASTNET wind roses from the JOT403 site. There are
seasonal phenomena that modify this overall pattern. In the summer a thermal
low develops over the Mohave Desert. During this period winds blow into the
area from the Gulf of California to the south and across passes in the
Sierra Nevadas, including San Gorgonio Pass, a pollutant transport corridor
located directly west of Joshua Tree Wilderness. This may be the mostly
likely period for transport of southern California urban emissions into the
area. During the winter, higher pressure over the Mohave Desert causes the
opposite effect with 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.
The area is always susceptible to windblown dust from exposed desert
surfaces during high wind periods, especially in the Spring.
Locally, in the absence of strong regional pressure gradients, flows in the
Joshua Tree Wilderness will be characteristically upslope/downslope winds
typical of mountainous areas. This is evident in day/night
Joshua Tree CASTNET wind roses as
southerly (downslope) nighttime winds and northerly (upslope) winds at this
site located on northerly sloping terrain. In the eastern Wilderness, the
upslope/downslope flow will be along the Pinto wash that drains eastward.
Potential transport routes into the Joshua Tree Wilderness Area include long
distance transport via upward mixing from more distant source regions and
transport into the region via upper level flow. Possible source regions
include the South Coast Air Basin to the west, and surrounding desert
terrain, especially to the north and east, as a source for windblown dust.
Inversions/Trapping
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 JOSH1 IMPROVE site that is well within the
representative elevation range for the Joshua Tree Wilderness.
Climatological Statistics
Long term
Palm Spring California Climate Data including temperature and
precipitation data should be very representative of Joshua Tree Wilderness.
This and other
Southern California Climate Summaries are available from the
Western Regional Climate Center.
Meteorological Indicators for Local Sources
Keywords
Thermal Low
Heat Low
Mohave Desert
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Last updated 13 November 2004
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