Influence of nocturnal low-level jets on eddy-covariance fluxes over a tall forest canopy

Observations of low-level jets (LLJs) at the Howland AmeriFlux site and its impact on nocturnal turbulent exchange over a tall forest canopy are discussed. Low-frequency wave type motions and microfronts characterize moderate LLJs, whereas low-frequency motions are suppressed during strong LLJs with enhanced wind shear. The analysis is carried out in light of shear sheltering hypothesis to elucidate the effect of large-scale motions on the eddy covariance measurements. Characteristic length scales are considerably higher than the height of canopy in the absence of shear sheltering. The wind shear is lower and large eddies penetrate the roughness sublayer and reduce concentration gradients considerably, while during the case where shear sheltering is effective, large concentration gradients are observed in the roughness sublayer. The ‘upside-down' eddies characterize downward transport of turbulent kinetic energy and introduce microfronts, which appear to be primarily responsible for the enhanced velocity variances and scalar and momentum fluxes.