Airflow Configurations of Warm Season Southerly Low-Level Wind Maxima in the Great Plains. Part II: The Synoptic and Subsynoptic-Scale Environment

Abstract

Twelve different spatial configurations of southerly warm season low-level wind maxima in the Great Plains were identified in Part I, based on streamline orientation and curvature, the location of confluence and deformation zones, and the latitudinal extent of the wind maxima. Here, in Part II, composite analysis was employed to examine the typical lower, mid-, and upper-tropospheric environments for the 12 configuration types. Only two of the jet types were found to occur in association with a midtropospheric ridge, an environment that has previously been labeled as quiescent. These two jet types had a fairly simple anticyclonic airflow configuration. The remaining 10 configuration types occurred in association with a trough over the southwestern United States and a ridge to the east (i.e., an active environment). Consequently, the pronounced differences in the airflow configuration identified in this study are associated with an active midtropospheric pattern. Only 1 of the 12 configuration types appeared to be a purely boundary layer-driven wind maximum. For the remaining types, the relative influence of synoptic forcing and boundary layer forcing varied, as inferred from the relative strength of the geostrophic and ageostrophic wind components and the climatological characteristics of the wind maxima (e.g., average elevation and the typical time of day and month of occurrence). The location of the jet core for all configuration types reflected a combination of the geostrophic and ageostrophic contributions to the actual wind. Areas of large ageostrophic wind vectors were generally located to the west/northwest and to the south of the jet core, and are thought to represent transverse circulations around a confluence/frontal zone and around a dryline, respectively. The ageostrophic wind vectors also displayed a veering (i.e., anticyclonic rotation) upstream of the jet core. For most jet types, the right entrance quadrant of a 200-mb jet streak was positioned over the low-level wind maximum. The results of Part I and Part II, taken together, illustrate the considerable variability both in the physical characteristics of warm season low-level wind maxima in the Great Plains and in the large-scale environment in which these wind maxima occur. This variability must be kept in mind when forecasting convection in association with low-level wind maxima, and when interpreting previous case studies of low-level wind maxima or earlier climatological studies based on a small number of events.