Observations and Numerical Simulations of Winds within a Broad Forested Valley

Abstract

The Tennessee River Valley in the eastern part of Tennessee is a broad valley with a moist climate and extensive forest cover. A series of 50?100-m-high parallel ridges forms corrugations along the floor of the valley. Tower measurements and numerical simulations are used in this paper to study the channeling of the winds in this valley over the diurnal cycle. At night, pressure-driven channeling caused by geostrophic imbalances is often present, but this channeling tends to be concentrated on the leeward side of the valley (relative to the winds aloft). The channeling is significantly weaker during the afternoon, so the winds on top of the corrugations are more closely aligned with those aloft. In the bottomlands between the corrugations, however, the daytime winds are more effectively channeled parallel to the valley axis. Although thermally driven winds are not dominant in the Tennessee Valley, a discernible pattern of upvalley daytime winds and downvalley nighttime winds is observed in the tower measurements. Nighttime drainage flows are particularly evident in the bottomlands between the corrugations. Climatologically, the transition from typical nighttime to typical daytime flow conditions occurs around 0900?1100 LST, and the transition back to nighttime conditions occurs around 1700?1900 LST. Separate sets of numerical simulations were performed for typical summer and winter conditions. These sets differed in their temperature and humidity profiles, surface vegetation characteristics, and soil water. Little systematic difference is observed in the two sets of simulations, suggesting that seasonal variations in surface properties play a relatively minor role in the observed channeling. Overall, the wind channeling can be explained by individually considering the flow deflections caused by the mountain ranges on each side of the valley. Each of these ranges can produce leftward deflections of the oncoming flow depending on its bulk Rossby and Froude numbers. The two ranges appear to interact in that the range on the windward side of the valley tends to reduce the Rossby and Froude numbers of the leeward range, thus allowing the leeward range to be more effective in deflecting the oncoming flow.