Topographic Amplification Factor in a Closed Basin
Location
Eccles Conference Center
Event Website
http://water.usu.edu/htm/conference/past-spring-runoff-conferences
Start Date
4-3-2009 3:20 PM
End Date
4-3-2009 3:40 PM
Description
Diurnal along-valley wind systems are driven by horizontal pressure differences caused by an overheating of the valley atmosphere during daytime and an overcooling during nighttime compared to the atmosphere over the adjacent plain at the same height. Equal energy inputs or outputs through equal areas at the tops of the valley and plain atmospheric volumes produce larger temperature changes in the smaller valley air volume. This amplification of the temperature response in the valley is proportional to the ratio of the normalized volumes which is called the "topographic amplification factor" or TAF. This factor is considered to be the primary factor responsible for the development, strength and diurnal periodicity of valley wind systems. A detailed testing of the TAF concept with observations is problematical in valleys because the pressure gradient causes a thermally driven wind to form along the valley axis that acts to equilibrate any temperature differences that develop between the valley and plain. In this paper, we provide an observational test of the TAF concept by comparing nighttime cooling inside a rather idealized bowl-shaped basin (Meteor Crater, Arizona) with cooling outside the basin over the adjacent plain. With such a topographical configuration, no thermally driven wind can develop to equilibrate the two air masses. On two undisturbed nights, the findings support the TAF concept. Cooling inside the basin is greater than over the adjacent plain by a factor that is within 10% of TAF expectations. On a third night with stronger background winds, the cold air in the basin remains largely undisturbed while the cold air above the plain is swept away, resulting in cooling differences that are not in agreement with the TAF concept. The assumptions and limitations of the TAF concept are discussed.
Topographic Amplification Factor in a Closed Basin
Eccles Conference Center
Diurnal along-valley wind systems are driven by horizontal pressure differences caused by an overheating of the valley atmosphere during daytime and an overcooling during nighttime compared to the atmosphere over the adjacent plain at the same height. Equal energy inputs or outputs through equal areas at the tops of the valley and plain atmospheric volumes produce larger temperature changes in the smaller valley air volume. This amplification of the temperature response in the valley is proportional to the ratio of the normalized volumes which is called the "topographic amplification factor" or TAF. This factor is considered to be the primary factor responsible for the development, strength and diurnal periodicity of valley wind systems. A detailed testing of the TAF concept with observations is problematical in valleys because the pressure gradient causes a thermally driven wind to form along the valley axis that acts to equilibrate any temperature differences that develop between the valley and plain. In this paper, we provide an observational test of the TAF concept by comparing nighttime cooling inside a rather idealized bowl-shaped basin (Meteor Crater, Arizona) with cooling outside the basin over the adjacent plain. With such a topographical configuration, no thermally driven wind can develop to equilibrate the two air masses. On two undisturbed nights, the findings support the TAF concept. Cooling inside the basin is greater than over the adjacent plain by a factor that is within 10% of TAF expectations. On a third night with stronger background winds, the cold air in the basin remains largely undisturbed while the cold air above the plain is swept away, resulting in cooling differences that are not in agreement with the TAF concept. The assumptions and limitations of the TAF concept are discussed.
https://digitalcommons.usu.edu/runoff/2009/AllAbstracts/26