Characteristics of Nocturnal Low Level Jets and their influence on the eddy covariance fluxes at the Florida slash Pine AmeriFlux site

Stably stratified nocturnal boundary layers are often associated with Low Level Jets (LLJ) formed due to either one or a combination of mechanisms such as inertial oscillations, horizontal temperature gradient associated with weather patterns or sloping terrain, land and sea breezes and advective accelerations. LLJs appear to play an important role in the nocturnal transport of gases and other atmospheric quantities and generates shear and turbulence between the level of the jet nose and the surface, in an otherwise weak and intermittent turbulent stable boundary layer. Depending on the strength and height of the jet maximum, the role of LLJs in enhancing the turbulence, mixing and fluxes between the atmosphere and surface can vary. Characteristics of the nocturnal LLJs and their influence on eddy covariance fluxes within and above a forest canopy are investigated using the data collected at the slash pine AmeriFlux site, Gainesville, Florida. The wind profile measurements are carried out using a boundary layer sodar up to a height of 1000 meters. LLJs are observed over the measurement location during many of the nights with the nose height between 200- 400 m and maximum wind speeds up to 15 ms-1. The effects of LLJ on turbulence characteristics and fluxes within and above the slash pine canopy are studied using eddy covariance measurements on a 30 m tall tower instrumented with sonic anemometers and CO2 analyzers at several levels within and above the canopy. The study underlines the importance of understanding flow characteristics in order to correctly interpret the flux measurement results.