Linkages between boundary layer structure and evolution of the nocturnal low-level jet in the U.S. Central Plains

The nocturnal low-level jet is commonly observed in the atmospheric boundary layer over the Great Plains of the United States and in many other locations worldwide. It was found to play important roles in the transport of moisture and air pollutants, and has important implications for the regional climate, nocturnal convection, and wind-resource assessments. The nocturnal low-level jet typically begins to develop around sunset under cloud-free conditions that are conducive to strong radiative cooling and the development of a stable boundary layer near the surface. It typically reaches peak intensity late in the night, and then decays with the onset of daytime convective mixing. Despite many theoretical, numerical, and observational analyses conducted to date, which make a strong case for the nocturnal low-level jet arising from a force imbalance induced by the sudden release of the frictional constraint near sunset, many aspects of its structure and evolution are still not well understood. To better understand how the low-level jet and boundary layer evolve and interact after sunset, one aspect of the Plains Elevated Convection at Night (PECAN) experiment focused on obtaining high-resolution observations of wind, temperature, humidity, and turbulence parameters using both mobile and fixed measurement platforms. Among other instruments, the boundary-layer profiles were collected with Doppler wind lidars, Atmospheric Emitted Radiance Interferometers (AERI), and microwave profilers. Additionally radiosondes were released at mobile and fixed sites during the early evening transition and throughout the night. An overview of the PECAN low-level jet observations collected with one of the mobile platforms, the NSSL / OU Collaborative Lower Atmospheric Mobile Profiling System (CLAMPS), is presented. A number of low-level jet cases are selected, for which the mesoscale set-up, temporal evolution and spatial variability of the low-level jet and boundary-layer structure are discussed in detail using CLAMPS data supplemented by observations from other mobile platforms and fixed PECAN sites. The findings are compared to results from the Lower Atmospheric Boundary Layer Experiment and scaling hypotheses proposed in previous studies are further evaluated.