Influence of low-level jets on nocturnal CO2 flux measurements

Low-level jets, density currents, canopy and gravity waves are characteristic nocturnal boundary-layer features that are capable of influencing surface-atmosphere exchange. Nighttime carbon dioxide flux measurements conducted over a tall forest canopy at the Florida AmeriFlux site are analyzed in relation to the occurrence of low-level jets (LLJ). Two nights are investigated in detail: one with a strong LLJ throughout the night and the other in the absence of an LLJ. The night with an LLJ exhibits large friction velocity, deep mixing, large turbulent CO₂ flux and low canopy CO₂ storage throughout most of the night. In contrast, the second night associated with calm conditions show poor mixing, low turbulent CO₂ flux, and large storage, except during some of the early evening and intermittent turbulence periods. When an LLJ is present, CO₂-depleted sweeps contribute nearly 55 % to the nighttime positive flux, compared to 45 % during periods without an LLJ. In addition, the downward transfer of turbulent kinetic energy in the presence of LLJ is another indication that mixing takes place from higher levels down to the canopy layer. Data show that most turbulent periods with friction velocities larger than the threshold (0.2 m s^-1 for this site) are either coupled to the presence of an LLJ or early evening hours. The study draws attention to the bias herein introduced in the flux correction approach commonly used to correct eddy-covariance fluxes in calm conditions.