Mountain winds may create atmospheric hotspots

October 17, 2005

WASHINGTON - Rapidly fluctuating wind gusts blowing over mountains and hills can create "hotspots" high in the atmosphere and significantly affect regional air temperatures. A research paper to be published this month in the Journal of Geophysical Research-Space Physics reports that the actions of such winds can create high-frequency acoustic waves and could stimulate a 1000-Kelvin [1,000-degree Celsius; 2,000-degree Fahrenheit]spike in a short period of time in the thermosphere, at an altitude of 200-300 kilometers [100-200 miles]. Such exceptional temperature increases would require continuous waves, and the heating rate would likely be diminished with intermittent winds.

Richard Walterscheid and Michael Hickey used a theoretical model of the interaction between rough terrain and wind eddies to suggest that high winds may represent a previously unknown source of acoustic waves in the environment. Ocean waves and earthquakes are known to produce similar waves, which strengthen as they propagate higher in the atmosphere. The authors speculate that the waves can heat the atmosphere at prodigious rates and could account for a large part of the unusual and unexplained high-altitude background heating seen above the mountainous landscape in parts of South America.

"We show that that the acoustic waves generated by gusty flow over rough terrain might be a significant source of heating in the upper atmosphere," Hickey says. "These mysterious so-called 'hotspots' observed above the Andes Mountains could be explained by such acoustic wave heating."

Previous observations near the Andes Mountains in Peru had found that the atmosphere directly above some peaks was approximately 100 Kelvin [100 degrees Celsius; 200 degrees Fahrenheit] hotter than in nearby regions and that the difference occasionally reached as much as 400 Kelvin [400 degrees Celsius; 700 degrees Fahrenheit]. Other research had recorded similar effects near the Rocky Mountains in Colorado. After comparing simulations of atmospheric gravity waves and acoustic waves, the researchers found that the acoustic waves reached higher altitudes than the gravity waves, leading them to speculate that the acoustic waves constituted a far more plausible source of the observed hot spots. They then identified wind fluctuations as the most likely source of the heating, noting that the upwind waves could only be generated by unsteady wind flow.

They cite further evidence indicating that the high- frequency acoustic waves in the thermosphere originated from the ground, including proof that nighttime atmospheric motion (convection) is not a plausible source of the persistent heating. In addition, they note that only high-frequency acoustic waves could cause the thermospheric heating, as the slower-speed gravity waves are not fast enough to reach the higher altitudes and therefore could not produce the substantial effects at that height in the atmosphere.

The paper indicates that moderately strong winds, reaching speeds of approximately 10 meters [30 feet] per second, can generate wave amplitudes of nearly four meters [10 feet] per second above rough terrain. In addition, the authors found that steeply sloping terrain further enhanced the waves, which are generated by rapid variations in the up-and-down turbulence in the air. Wider hills and those spaced further apart can also have a similar wave- generating effect, but the authors found that the wind effects typically do not propagate vertically near isolated hills as they do around rougher terrain.

The researchers note that there are very few detailed field studies of the wind field over hills at present. They report, however, that models and previous research indicates that even weak interactions from acoustic waves can produce significant effects in the thermosphere.
-end-
The research was funded by the Aerospace Corporation and through NASA and NSF grants.

American Geophysical Union

Related Atmosphere Articles from Brightsurf:

ALMA shows volcanic impact on Io's atmosphere
New radio images from ALMA show for the first time the direct effect of volcanic activity on the atmosphere of Jupiter's moon Io.

New study detects ringing of the global atmosphere
A ringing bell vibrates simultaneously at a low-pitched fundamental tone and at many higher-pitched overtones, producing a pleasant musical sound. A recent study, just published in the Journal of the Atmospheric Sciences by scientists at Kyoto University and the University of Hawai'i at Mānoa, shows that the Earth's entire atmosphere vibrates in an analogous manner, in a striking confirmation of theories developed by physicists over the last two centuries.

Estuaries are warming at twice the rate of oceans and atmosphere
A 12-year study of 166 estuaries in south-east Australia shows that the waters of lakes, creeks, rivers and lagoons increased 2.16 degrees in temperature and increased acidity.

What makes Saturn's atmosphere so hot
New analysis of data from NASA's Cassini spacecraft found that electric currents, triggered by interactions between solar winds and charged particles from Saturn's moons, spark the auroras and heat the planet's upper atmosphere.

Galactic cosmic rays affect Titan's atmosphere
Planetary scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) revealed the secrets of the atmosphere of Titan, the largest moon of Saturn.

Physics: An ultrafast glimpse of the photochemistry of the atmosphere
Researchers at Ludwig-Maximilians-Universitaet (LMU) in Munich have explored the initial consequences of the interaction of light with molecules on the surface of nanoscopic aerosols.

Using lasers to visualize molecular mysteries in our atmosphere
Molecular interactions between gases and liquids underpin much of our lives, but difficulties in measuring gas-liquid collisions have so far prevented the fundamental exploration of these processes.

The atmosphere of a new ultra hot Jupiter is analyzed
The combination of observations made with the CARMENES spectrograph on the 3.5m telescope at Calar Alto Observatory (Almería), and the HARPS-N spectrograph on the National Galileo Telescope (TNG) at the Roque de los Muchachos Observatory (Garafía, La Palma) has enabled a team from the Instituto de Astrofísica de Canarias (IAC) and from the University of La Laguna (ULL) to reveal new details about this extrasolar planet, which has a surface temperature of around 2000 K.

An exoplanet loses its atmosphere in the form of a tail
A new study, led by scientists from the Instituto de Astrofísica de Canarias (IAC), reveals that the giant exoplanet WASP-69b carries a comet-like tail made up of helium particles escaping from its gravitational field propelled by the ultraviolet radiation of its star.

Iron and titanium in the atmosphere of an exoplanet
Exoplanets can orbit close to their host star. When the host star is much hotter than our sun, then the exoplanet becomes as hot as a star.

Read More: Atmosphere News and Atmosphere Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.