HR: 0830h
AN: B51D-0984    [PDF]
TI: Advection and Thermotopographic Flows Near A Flux Tower: Coupled or Not?
AU: * Froelich, N J
EM: nfroelic@indiana.edu
AF: Indiana University, Dept. of Geography, Atm. Sci. 701 E. Kirkwood Ave., Bloomington, IN 47405 United States
AU: Schmid, H
EM: hschmid@indiana.edu
AF: Indiana University, Dept. of Geography, Atm. Sci. 701 E. Kirkwood Ave., Bloomington, IN 47405 United States
AB: Ecosystem-atmosphere exchange of carbon, water, and heat is routinely measured at numerous sites around the world, using the eddy-covariance technique. However, recent research has questioned the interpretation of some of these measurements, particularly at night and in areas of complex terrain. Much of this research has focused on horizontal and vertical advection, resulting from thermotopographically driven horizontal flow divergence (drainage flows). This paper examines horizontal and vertical flow patterns above and below the forest canopy at the Morgan-Monroe State Forest (MMSF) AmeriFlux site in south central Indiana. Mean vertical motions above canopy were investigated using data from a 46 m ridge-top tower. We found a tendency toward downward vertical velocities at night relative to the daytime. Some evidence suggests that this vertical convergence is driven by nocturnal thermotopographic flows. To examine sub-canopy flow through a small gully located near the main tower, a small (8 m) mast was installed. Flow direction in the gully was generally well correlated with the along-gully component of the above-canopy wind, indicating dynamic coupling. However, there were periods when wind in the gully was decoupled from flow aloft and was consistent with thermotopographic flow. Details of thermotopographic flows, in both the leaf-off and vegetative seasons, will be discussed; these include temporal patterns, relationships to likely drivers such as radiation and temperature gradients, and relationships with above canopy vertical velocities. It was found that the three-dimensional flow structure at MMSF is more complex than in non-forested terrain, particularly during the vegetative season. This research supports arguments that the inclusion of simple advection terms in calculations of NEE, based on above-canopy measurements, is not generally applicable. The results suggest a need for further investigation into the flow patterns at MMSF and at other forests in complex terrain. Furthermore, additional study of thermotopographic flows in forested terrain is needed.
DE: 0315 Biosphere/atmosphere interactions
DE: 3307 Boundary layer processes
DE: 3322 Land/atmosphere interactions
SC: Biogeosciences [B]
MN: 2003 Fall Meeting