Airflow Configurations of Warm Season Southerly Low-Level Wind Maxima in the Great Plains. Part I: Spatial and Temporal Characteristics and Relationship to Convection

Abstract

A synoptic climatological approach was used to investigate 1) the airflow characteristics of southerly low-level wind maxima in the Great Plains, 2) the typical thermodynamic environments in which low-level wind maxima form, 3) the convergence fields accompanying the wind maxima, and 4) the associated patterns of cloud-to-ground lightning activity. A total of 260 low-level jet events that occurred at either 0000 or 1200 UTC during the warm seasons (Apr–Sep) of 1991–92 were employed in the analysis. The spatial configuration of southerly low-level jets was shown to be considerably more complex than previously portrayed. A subjective classification of the jet events—based on streamline curvature, latitudinal extent, and the orientation of confluence and deformation zones—resulted in 12 distinct configuration types. Only 37% of the events were broadly classified as anticyclonically curving jets, even though the majority of previous case studies focused on this type of wind maximum. Another unanticipated finding was the substantial variation in wind direction with height, usually to more anticyclonic flow, observed for over 50% of the low-level wind maxima. These events were termed multilevel jets, although it is not clear whether they are characterized by distinct, multiple airstreams separated in the vertical or a more gradual change in airflow with height. Composite temperature and humidity fields indicated that the thermodynamic conditions are fairly similar for the different jet types. All types were associated with a collocated humidity gradient and thermal ridge southwest of the jet axis. In contrast, the composite lower-tropospheric convergence fields varied considerably between types and reflect the complex airflow configurations. The spatial distribution of cloud-to-ground lightning activity corresponded well with local maxima in the composite convergence fields. This study contributes to a more complete understanding of the climatology of low-level jets. It provides a unique perspective in that no previous climatological study systematically investigated the variability in the airflow configurations of jet events or explicitly related the spatial pattern of convection to the airflow configuration and resulting convergence fields. In Part II of this study, the typical synoptic and subsynoptic-scale environments in which wind maxima with different configurations occur are described.