Interaction between turbulence bursts and low-level jet in a nocturnal boundary layer flow over complex terrain

Intermitted turbulent events in a nocturnal downvalley flow over complex terrain are investigated throughout boundary layer flow measurements made during the 2013 campaign of the Mountain Terrain Atmospheric Modeling and Observations (MATHERHORN) Program. Turbulent and meteorological data were continuously measured at five different heights of three different towers located in the north-north west direction along a wide, gently sloped valley of Dugway Proving Ground in Utah. A case study of intermittent turbulent events in a nocturnal downvalley flow was detected and analyzed for the night of the 12th of May 2013 when tethered balloon measurements were performed to support tower observations. The low-level jet behavior of the nocturnal downvalley flow follows somehow the inertial oscillation motion modeled by Wiel et al. (2010), which was developed for flat terrain low-level jet. Thermal stratification indeed confines the low-level jet in the lower portion of the boundary layer, while the gravity wave behavior of the flow modifies its structure with respect to a flat terrain jet. Gravity waves within the motion was found to have a period of 15 minutes. Gravity waves can develop in a nocturnal boundary layer motion as a result of a flow perturbation, which can be induced by a hydraulic jump, an intrusion of mass and momentum or an intermittent turbulent event. A gravity wave generation event can also perturb the inertial oscillation responsible of the mean motion behavior. In this study, we found a correlation between pulsed turbulent events, detected as turbulence bursts, and the appearance of a double nose shape in the low-level jet. The double nose shape was found to be the result of a mutual decrease of the speed of the low-level jet maximum and widening of the jet depth, and the development of a second jet at higher levels. This second jet is located at about 150-200 m as jets occurring over great US planes while the lower one occurring at about 50 m appears as a local feature. The impulsive increase of the momentum eddy diffusivity coefficient related to turbulence burst is supposed to be the mechanisms that induce the double nose formation, since a variation on the coefficient in time and space determines a shape and intensity modification of the jet behavior.