Tuesday, 29 April 2008
Floral Ballroom Magnolia (Wyndham Orlando Resort)
Features of nocturnal low-level jets observed at the Florida AmeriFlux site and their influence on canopy turbulence and CO2 flux measurements over a tall forest canopy are analyzed. Jets of diverse strength are observed during nearly 70 % of the nocturnal periods over a three-month campaign, of which almost 50 % are strong jets with speed higher than 10 m s-1 and height in the range 200 400 m. Strong jet activity contributes to weak atmospheric stabilities and higher friction velocities attributed to enhanced canopy turbulence. The canopy shear length scale exhibits a linear relationship with jet shear. Jet periods also show dominant downward transport of turbulent kinetic energy and turbulent CO2 fluxes in the range 2 to 8 µmol m-2 s-1. The difference between the net ecosystem exchange (NEE) at two levels above the canopy adds on average, a flux contribution of 1.25 µmol m-2 s-1 (18 % of the average NEE at z = 1.4h, h is the canopy height) to CO2 exchange during periods characterized by strong jets. A comparison of CO2 and wind velocity spectra and cospectra between periods with dissimilar jet activity shows larger low-frequency spectral contributions in the strong jet case, supporting the possibility of variance and flux contributions at scales comparable to the jet height.
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