Coupling of the Low-Level Jet and Atmospheric Surface Variables by Turbulent Transport during PECAN

The Great Plains Low Level Jet (LLJ) is a persistent warm-season nocturnal feature. In a general sense it is regarded as a layer of wind accelerated to supergeostrophic speed commencing with the cessation of turbulent mixing around sunset, and largely decoupled from the surface by the static stability within a nocturnal inversion. However, observations taken during a field campaign in Ellis, KS in 2015 as part of the project, Plains Elevated Convection at Night (PECAN), show that the LLJ is coupled to the surface through the vertical turbulent transport of sensible heat, moisture, and momentum, and that this transport alters the structure and evolution of the nocturnal boundary layer and feeds back to modify the LLJ.

The present research employs data from an array of instruments including SoDAR, LiDAR, Rawinsondes, and high frequency sonic anemometers on a 10-meter tower, all located at a fixed PECAN Integrated Sounding Array site (FP3), to examine the correlations between the wind speed and height of the LLJ, and the turbulent transport of heat, momentum, and moisture near the surface. Tower measurements show that the evening transition period is characterized by the growth of the surface-based inversion, sudden cessation of turbulent mixing, and negligible vertical turbulent transport, which allows the wind several 10s to 100s of m above the surface to accelerate as that layer of atmosphere decouples from the surface and begins to adjust to the mass (pressure) field. However, with this acceleration comes increasing dynamic instability in the layer below the LLJ core. Rawinsonde profiles enable the determination of the Richardson number in this layer, which exhibits sustained but variable dynamic instability and the potential for turbulent transport. Tower measurements of turbulent kinetic energy (TKE) and fluxes of heat, momentum, and moisture correlate directly with the speed of the LLJ; the greater the wind speed, the greater the turbulence intensity. In addition, embedded in this turbulent structure are oscillations in TKE and fluxes that exhibit periodicity, which appear to be associated with bursting events as turbulent eddies intrude into the surface layer and temporarily destroy lingering near-surface stability. The nature of vertical turbulent transport will be presented for three LLJ episodes: 10, 20, 22 June 2015. These episodes represent weak, moderate, and strong LLJs, respectively, and provide very different surface signatures of turbulent transport.