18 October 2005
Mountains Creating Atmospheric Hotspots
by Kate Melville
Wind gusts that blow over mountains and hills can create hotspots high in the atmosphere and significantly affect regional air temperatures, say researchers writing in the Journal of Geophysical Research-Space Physics. Their study reports that such winds can create high-frequency acoustic waves that can stimulate a 1,000-degree Celsius spike in the thermosphere at altitudes of 200-300 kilometers.
Studies made previously in the Andes Mountains had found that the atmosphere directly above some peaks was approximately 100 degrees Celsius hotter than in nearby regions and that the difference occasionally reached as much as 400 degrees Celsius. The researchers said that similar effects had been observed over the Rocky Mountains in Colorado.
The researchers - Richard Walterscheid and Michael Hickey - used a 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.
The authors speculate that the waves can heat the atmosphere and could account for 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 said. "These mysterious so-called 'hotspots' observed above the Andes Mountains could be explained by such acoustic wave heating."
The study suggests that moderately strong winds, reaching speeds of 10 meters per second, can generate wave amplitudes of nearly four meters 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.
The researchers note that there are very few detailed field studies of wind field effects over hills and mountains but that previous research indicates that even weak interactions from acoustic waves can produce significant temperature effects in the atmosphere.
Source: American Geophysical Union