Unexpected Warming Induced by Foehn Winds in the Lee of the Smoky Mountains

Abstract

On 2 January 1999, a southeasterly foehn wind produced a narrow band of temperatures up to 10°C (18°F) warmer than the surrounding area in the central Great Tennessee Valley on the northwest side of the Smoky Mountains. This particular event was interesting in comparison with other documented foehn wind events in that the dewpoint temperature rose substantially along with the actual temperature, resulting in little change in the observed surface relative humidity. A relatively warm and nearly saturated 850-hPa air mass around the highest ridges of the Smoky Mountains was found to be the source of the adiabatically compressed air observed on the northwest side of the Smoky Mountains. The observed foehn winds across the central Great Tennessee Valley were not unusually strong during the afternoon, with surface wind gusts reaching 13 m s?1 (26 kt). However, mountain waves may have been associated with the foehn winds on 2 January 1999, because a strong surface pressure gradient was in place across the mountain range, winds in excess of 15 m s?1 (30 kt) were observed in a stable layer at 850 hPa blowing nearly perpendicular to the highest Smoky Mountain ridges, and favorable vertical wind and terrain profiles were present. Any mountain wave activity likely remained close to the mountains away from any of the observation stations in the Great Tennessee Valley. Also, high relative humidity levels (>70%) throughout much of the boundary layer near the Great Tennessee Valley may have reduced the strength of the foehn winds.