Physical and Dynamical Characteristics of Upper-Level Coupling in Great Plains Low-Level Jet Morphology

The U.S.’s Great Plains (GP) low-level jet (LLJ) is a ubiquitous feature of the warm season climatological southerly flow from the Gulf of Mexico to Canada contributing to a large percentage of mean and extreme rainfall, the generation of U.S. renewable wind energy, and severe weather outbreaks. It has been well documented that the GPLLJ can be 1) vertically coupled to the large-scale cyclone-anticyclone flow pattern associated with an upper-level jet stream (i.e., coupled GPLLJ) or 2) uncoupled to the large-scale flow but sustained in response to various local land-atmosphere forcing mechanisms (i.e., uncoupled GPLLJ). Analyses have predominately considered coupled- and uncoupled GPLLJs as a single phenomenon. However, through laborious manual weather map analysis approaches, recent studies have demonstrated that the mean and extreme hydrological response is considerably different between those GPLLJs that are coupled to the upper-level flow or not. For these reasons, this study treats the GPLLJ as two phenomena and exploits an automated coupled- versus uncoupled GPLLJ detection method. In short, the method dynamically classifies GPLLJ through the finite-amplitude local wave activity (LWA) diagnostic, which quantifies the strength of mid- to upper-atmospheric troughs and ridges. A GPLLJ is considered coupled (uncoupled) if the strength of the cyclonic (anticyclonic) LWA associated with troughs (ridges) exceeds a predefined threshold. The ultimate goal is to determine the defining physical and dynamical qualities that differentiate the two GPLLJ types.

We utilize the recently released Coupled ERA 20^th Century (CERA-20C; 1901-2010) reanalysis from ECMWF and the recently developed automated technique to differentiate those GPLLJs that are coupled or uncoupled to the upper-level flow. Focus is restricted to May through September due to the fact that southerly GPLLJs and their associated precipitation are largely concentrated in the warm season, both in observations and within CERA-20C. CERA-20C provides 110-years of climate quality data at ~100 km resolution and 3-hourly output in which to analyze the physical and dynamical features associated with coupled- versus uncoupled GPLLJs. These sub-seasonal to seasonal characteristics include differences in vertical wind and thermal structures, wind speed, low-level wind height, vertical wind shear, diurnal variability, frequency distribution, and contributions to extremes. Each differentiating characteristic between the two types of GPLLJs is discussed in terms of their socioeconomic impacts across the GP such as flood and drought risk, livestock and wind energy production, and severe weather outbreaks, to name a few.