Turbulent versus non-Turbulent Low-Level Jets – Differences in their Characteristics and Evolution

Nocturnal low-level jets (LLJs) have been observed over many parts of the world and are particularly prevalent over the Great Plains region of the United States. They can play an important role in moisture transport and convection initiation in this region and were also shown to affect the long-range transport and mixing of ozone and its precursor pollutants. LLJs often form around sunset, concurrent with the transition from a convective boundary layer to a stable boundary layer (SBL). Due to the large wind shear, LLJs often interact and modify the structure of the SBL. The SBL is often classified into weakly stable boundary layers with sustained turbulent mixing throughout the night or strongly stable boundary layers where turbulence becomes weak and intermittent. Previous studies have found that the transition from weakly stable boundary layers to strongly stable boundary layers occurs for wind speeds below a critical value and this critical wind speed was found to increase with height. However, it remains unclear how the structure of the nocturnal SBL and turbulence characteristics in the SBL affect the development and evolution of the LLJ itself.

Understanding these relationships was one of the primary goals of the Lower Atmospheric Boundary Layer Experiment (LABLE-I), which was conducted from 18 September to 13 November 2012. During this field campaign, several instruments including multiple Doppler lidars were deployed at the Southern Great Plains Atmospheric Measurement Radiation site to supplement existing observation systems. Vertical profiles of horizontal wind, vertical velocity variance, temperature, and humidity were available from a suite of instruments at high temporal and vertical resolution. These observations were used to investigate the relationship between the structure of the SBL and the LLJ evolution. The LLJ tended to evolve very differently depending on the magnitude of turbulent mixing within the SBL. For strongly stable boundary layers, the LLJ tended to reach a maximum wind speed shortly before sunrise, and the height of the LLJ followed isentropic surfaces. In contrast, LLJs associated with weakly stable boundary layers generally had a wind speed maximum seven hours after sunset, and the height of the LLJ remained roughly constant overnight.