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

摘要

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.