A valley exit jet is a strong, down-valley, elevated air current that emerges above the intersection of the valley and its adjacent plain. These winds frequently reach a maximum of at a height of above the ground. Surface winds below the jet may sway vegetation but are significantly weaker.
The presence of these strong nighttime down-valley air flows has been documented at the mouth of many Alpine valleys that merge with basins, such as the Inn Valley of Austria, where the jet is strong enough to be heard at the ground. In the United States, exit jet signatures have been observed at the
North Fork Gunnison River at
Paonia, Colorado
Paonia is a statutory town in Delta County, Colorado, United States. The population was 1,451 at the 2010 census.
History
The area was first explored in 1853 by Captain John W. Gunnison of the United States Army. Gunnison was on an expedit ...
; the exit of
South Boulder Creek south of
Boulder, Colorado
Boulder is a home rule city that is the county seat and most populous municipality of Boulder County, Colorado, United States. The city population was 108,250 at the 2020 United States census, making it the 12th most populous city in Colora ...
;
Albuquerque, New Mexico
Albuquerque ( ; ), ; kee, Arawageeki; tow, Vakêêke; zun, Alo:ke:k'ya; apj, Gołgéeki'yé. abbreviated ABQ, is the most populous city in the U.S. state of New Mexico. Its nicknames, The Duke City and Burque, both reference its founding i ...
at the mouth of
Tijeras Canyon; and the mouth of
Spanish Fork Canyon
Spanish might refer to:
* Items from or related to Spain:
**Spaniards are a nation and ethnic group indigenous to Spain
**Spanish language, spoken in Spain and many Latin American countries
**Spanish cuisine
Other places
* Spanish, Ontario, Cana ...
in
Utah
Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...
.
Theory
Exit jets are likely to be found in valley regions that exhibit diurnal mountain wind systems, such as those of the dry mountain ranges of the US. These diurnal wind systems are driven by horizontal pressure gradients. Due to the abrupt transition over a short distance between the valley high pressure and the basin low pressure, the gradients are strongest near the valley exit, producing a jet.
Other meteorological factors acting to increase exit wind speeds are the acceleration of winds originating inside the valley as they travel to lower elevations downvalley, and the process of
cold valley air sinking and ejecting into the plain. Deep valleys that terminate abruptly at a plain are more impacted by these factors than are those that gradually become shallower as downvalley distance increases.
Impacts
Valley exit jets can play a major role in the mitigation of
air pollution
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different type ...
:
* Airflow emerging into the basin is cleaner due to lower aerosol content
* Vertical mixing resulting from directional shear and from the convergence of the jet with basin scale flows reduces ozone and other pollutants.
* Surface eddies created near canyon mouths inhibit the transport of pollution.
Methods of examining exit jets include remote sensing and direct observation.
SODAR and
Doppler LIDAR
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
have been used in numerous studies to identify, quantify and relate the jets to atmospheric transport of hazardous materials.
[Banta, R.M., L.D. Olivier, P.H. Gudiksen, and R. Lange, (1996). ''Implications of small-scale flow features to modeling dispersion over complex terrain''. J. Appl. Meteorol., 35:3, 330-342.] Detailed profiles of winds at canyon exits can be directly observed and calculated using a single or double
theodolite
A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and ...
and
tethersondes.
The identification and measurement of valley exit jets can also significantly aid in fire control, as fire often rides valley jets, as well as the development of wind energy.
References
Atmospheric dynamics
Mountain meteorology
Boundary layer meteorology
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