Contrasting Characteristics and Evolution of Southerly Low-Level Jets During Different Boundary-Layer Regimes

Nocturnal low-level jets (LLJs) have been documented at many places around the world, particularly across the Great Plains of the United States. Nocturnal LLJs significantly affect the regional weather and climate, as well as several industrial activities. While the LLJ has been extensively studied, little attention has been paid to the relationship between the evolution of the LLJ, its properties, and the boundary-layer structure. Here, observations collected in northern Oklahoma, USA are used to analyze the characteristics and evolution of the LLJ during conditions of weak and strong turbulence, respectively, coinciding with strongly and weakly stable nocturnal boundary layers. Additionally, the relationship between the LLJ and the temperature profile under these different conditions is examined. Three case studies are used during conditions of weak and strong turbulence, followed by statistics computed from a larger dataset of LLJs. When turbulence is weak, the LLJ is shown to reach peak intensity later near sunrise, while a maximum in the intensity is reached about six hours after sunset in the highly turbulent boundary layer. Furthermore, the LLJ tends to stay constant with height when turbulence is maintained throughout the night, while the LLJ tends to follow isentropic surfaces when vertical mixing is suppressed. When the LLJ follows isentropic surfaces, changes in the wind speed are related to the stability modification.