Abstract
The 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 μmol 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 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 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.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call