Land and Atmosphere Conditions prior to Extreme Great Plains Low-Level Jets

摘要

The warm season in the United States Great Plains (GP) is characterized by frequent nocturnal low-level jets (LLJs). The GPLLJ serves as a major mechanism of atmospheric moisture transport, contributing to severe weather and pre-cipitation in the region. A combination of synoptic and regional forcing modulates GPLLJ frequency and intensity. The GPLLJ has primarily been studied at the diurnal scale. We hypothesize that, due to the memory of the land surface, longer time scale variability associated with surface moisture also modulates GPLLJ intensity. This work identifies GPLLJ days from ECMWF Reanalysis v5 (ERA5) wind data and isolates extremes using a peaks-over-threshold approach. Extreme GPLLJs are classified by geographic region and synoptic state. Composites of daily soil moisture anomalies show a preference for extreme GPLLJs to occur over anomalously dry soil. Critically, antecedent soil moisture anomalies emerge weeks before the extreme jet occurrence. The dry soil moisture signal coexists with clear skies and drying of the surface at the syn-optic time scale. A diurnal PBL heat accumulation, which intensifies the buoyancy oscillation, is also present. The identification of a subseasonal dry anomaly suggests that, although the GPLLJ is generated by diurnally varying oscillations and intensified by synoptic-scale processes, the memory of the land surface can modulate the GPLLJ far beyond the diurnal and synoptic scale. Additionally, the location of the antecedent soil moisture anomalies corresponds with the eventual GPLLJ. The spatiotemporal characteristic of these antecedent anomalies suggests the potential for improved prediction of the GPLLJ activity.