The Spatial and Temporal Variations in the Nocturnal Low-Level Jet and Its Role in the Initiation and Maintenance of Mesoscale Convective Systems

A nocturnal maximum in thunderstorm activity occurs over the Great Plains during the summer. These nocturnal convective systems are associated with the presence of flash floods, high winds, hail, lightning, and even tornadoes. This study investigates the spatial and temporal variations in the nocturnal low-level jet (NLLJ) over the Great Plains in an effort to understand why a maximum in deep convection occurs at night during the summer over this region. The observations utilized in this study are the 3-h radiosonde launches made from five sites during the International H2O Project (IHOP_2002) field campaign. The measurement period extended over a period from 25 May to 15 June 2002. Consistent with recent theoretical and observations studies, our investigation found that the NLLJ was characterized by a westerly wind maximum located above the well-known low-level peak in the southerly winds. Our investigation, however, showed that the horizontal and vertical structure of the NLLJ was strongly modulated by the interactions between the regional terrain and variations in the synoptic flow. Our analyses also raises new questions about the behavior of the NLLJ environment as, for example, the thermodynamic profile above the stable nocturnal boundary layer frequently lacked the expected presence of a residual layer.

The spatial and temporal variations in the structure of the NLLJ also caused systematic changes in the Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN). One critical aspect of these variations was that these convective stability parameters supported the general trend of a west-to-east progression of convection during the night as conditions become less favorable over the western portions of the domain. This study also suggests that other variations in CAPE and CIN in NLLJ environment appear to be related to the presence of mesoscale ascent and the impacts of differential advection. These variations help to explain instances of convective initiation during the night. Since the NLLJ in our study appears to be highly ageostrophic, the ascent is unlikely to be explained by quasi-geostrophic theory.