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Influence of nocturnal low‐level jet on turbulence structure and CO2 flux measurements over a forest canopy

Journal Article · · Journal of Geophysical Research
 [1];  [2];  [2];  [3];  [3];  [4];  [5]
  1. Laboratory for Environmental Physics University of Georgia Griffin Georgia USA; The University of Georgia
  2. Laboratory for Environmental Physics University of Georgia Griffin Georgia USA
  3. Environmental Research and Technology Division Brookhaven National Laboratory Upton New York USA
  4. School of Earth and Environmental Science Queens College CUNY Flushing New York USA
  5. School of Forest Resources and Conservation University of Florida Gainesville Florida USA
+ Show Author Affiliations
he present study analyzes features of nocturnal low-level jets observed at the Florida AmeriFlux site and their influence on CO2 flux measurements over a tall forest canopy. At that location, two categories of nocturnal flow are commonly observed, one with a strong low-level jet throughout the night and the other without. Jets of diverse speed and height are observed during nearly 70% of the nocturnal periods over a 3-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 with gradient Richardson numbers lower than 0.2 and higher friction velocities (0.2 to 0.6 m s-1) 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 mmol m-2 s-1. The difference between the net ecosystem exchange (NEE) at two levels above the canopy adds on average, flux contribution of 1.25 mmol 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 Fourier 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.
Research Organization:
Univ. of Georgia, Athens, GA (United States)
Sponsoring Organization:
USDOE - Office of Energy Research (ER)
DOE Contract Number:
FG02-00ER63024
OSTI ID:
929556
Report Number(s):
ER63024-15
Journal Information:
Journal of Geophysical Research, Journal Name: Journal of Geophysical Research Journal Issue: D10 Vol. 113; ISSN 0148-0227
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English

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