The influence of low-level jet on canopy turbulence and CO2 flux measurements over a forest canopy

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.