HR: 0830h
AN: A31C-0040    [PDF]
TI: Nocturnal Katabatic Air Flow in a Forested Watershed: Measurements and Modeling
AU: Unsworth, M H
EM: unswortm@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AU: * Pypker, T
EM: tom.pypker@orst.edu
AF: Department of Forest Science, Oregon State University, Corvallis, OR 97331 United States
AU: Ocheltree, T
EM: troy.ocheltree@orst.edu
AF: Department of Forest Science, Oregon State University, Corvallis, OR 97331 United States
AU: Mahrt, L
EM: mahrt@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AU: Skyllingstad, E D
EM: skylling@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AU: Bond, B J
EM: barbara.bond@orst.edu
AF: Department of Forest Science, Oregon State University, Corvallis, OR 97331 United States
AB: Many productive forest ecosystems are in mountainous regions where their carbon and energy exchange with the atmosphere cannot be readily studied using micrometeorological methods. At night, katabatic air drainage flow commonly develops in sloping forested watersheds. Such flow is maintained by a three-way balance between gravitational forcing, turbulent mixing and non-linear advection. The carbon dioxide concentration and carbon isotope composition in nocturnal drainage flows may provide information about recent ecosystem productivity, but to interpret these data it is necessary to understand how the properties of the flow are influenced by forest structure and turbulence. We report measurements from a tower erected near the base of an incised watershed in the H.J. Andrews Experimental Forest in the Oregon Cascade Range, and include early results from a numerical modeling program. The watershed is forested with Douglas-fir about 45 years old and 25m high. The tower, 28m high, carries sonic and cup anemometers and thermistor temperature sensors at eight levels. Katabatic drainage flow occurs almost every night and persists for up to 18 hours. The depth of the flow is at least 28m on occasions, and the windspeed often peaks in mid-canopy. The in-canopy flow is well-mixed, with very small vertical gradients of carbon dioxide concentration and temperature. There are some indications that radiative cooling at the canopy top may encourage additional uncoupled drainage flow above the canopy. A numerical model of katabatic flow on slopes is being modified to include roughness elements to simulate a forest canopy. Comparisons between observations and the model will be reported.
DE: 0315 Biosphere/atmosphere interactions
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting