Near-Surface Turbulence Structure in the Presence of a Nocturnal Low-Level Jet: Results from the MATERHORN Field Experiment

The presence of Low-Level Jets (LLJs) in the nocturnal boundary layer over complex terrain has proven to generate complex interaction with turbulence. Of notice, LLJs represent one of the most common mechanisms triggering the occurrence of the so-called “up-side down” configuration of the nocturnal boundary, where turbulence kinetic energy (TKE) is transported downward to the surface instead of upwards as in a traditional boundary-layer configuration. In this context, we combined eddy-covariance measurements from flux towers with tethered balloon soundings collected in an open valley to investigate turbulence regimes and characteristics in the stably stratified layer between the nose of the nocturnal LLJ and the ground. Data for the analysis are taken from the existing database of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program, considering measurements collected within the valley floor at the Dugway Proving Ground (Utah, USA) during nights of weak synoptic forcing. The theoretical frame is given by the budget equations of the second-order moments (SOMs), taking into account explicitly the effects of the unsteadiness and the divergence of third-order moments (TOMs), under the simplifying assumption of horizontal homogeneity. The focus of this presentation is on 'simple' cases, characterized by a well-defined shape of the LLJ nose and negligible rotation of the wind with height. Although unsteady, such cases exhibit time derivatives of the normalized SOMs negligible in the relevant budget equations. The traditional resistance, or defect law is rephrased in terms of the velocity and the height characterizing the nose and the surface stress. The relationship shows a dependence on stability and quantifies the interaction between the LLJ dynamic and the turbulence below. The TKE budget turns out to be not satisfied, especially in the lower levels where the residual (i.e., the divergence of TOMs normalized on the dissipation of TKE) is often positive, suggesting a transport of TKE from the considered layer. This observation links the local budget to the vertical structure of the whole turbulent layer and is broadly connected to the vertical profile of TKE, in the flux-gradient closure frame. The ratio between the vertical velocity variance and the TKE is investigated as a function of the flux Richardson number to highlight specific features of local turbulence and their representation in closure models.