The Role of Upper-Level Coupling on Great Plains Low-Level Jet Structure and Variability

Abstract

The Great Plains (GP) southerly nocturnal low-level jet (GPLLJ) is a dominant contributor to the region’s warm season (May–September) mean and extreme precipitation, wind energy generation and severe weather outbreaks—including mesoscale convective systems. The spatiotemporal structure, variability, and impact of individual GPLLJ events are closely related to their degree of upper-level synoptic coupling, which varies from strong coupling in synoptic trough/ridge environments to weak coupling in quiescent, synoptic ridge environments. Here, we apply an objective dynamic classification of GPLLJ upper-level coupling and fully characterize strongly coupled (C) and relatively uncoupled (UC) GPLLJs from the perspective of the ground-based observer. Through composite analyses of C and UC GPLLJ event samples taken from the CERA-20C, we address how the frequency of these jet types, as well as their inherent weather- and climate-relevant characteristics, including: wind speed, direction, and shear; atmospheric stability, and precipitation, vary on diurnal and monthly timescales across the southern-, central-, and northern sub-regions of the GP. It is shown that C and UC GPLLJ events have similar diurnal phasing, but the diurnal amplitude is much greater for UC GPLLJs. C GPLLJs tend to have a faster and more elevated jet nose, less low-level wind shear, and enhanced CAPE and precipitation. UC GPLLJs undergo a larger inertial oscillation (Blackadar mechanism) for all sub-regions, and C GPLLJ have greater geostrophic forcing (Holton mechanism) in the southern and northern GP. The results underscore the need to differentiate between C and UC GPLLJs in future seasonal forecast and climate prediction activities.

Changes in Great Plains Low‐Level Jet Structure and Associated Precipitation Over the 20th Century

“…At decadal-to-multidecadal timescales, jet variability has been linked to atmospheric conditions in the North Pacific as indexed by the PDO, especially during May (Agrawal et al, 2021). The analysis was designed to complement Burrows et al (2020) but by adopting its Eulerian perspective inherited certain limitations. Fixed domains of comparison (i.e., NGP, CGP, and SGP) preclude the detection of shifts in the GPLLJ corridor, including a potential shift of the GPLLJ exit northward since 1979 as documented by Barandiaran et al (2013).…”

“…850-hPa ageostrophic wind is approximately the same for both jet classes in the NGP, however, uncoupled jets have moderately faster 850-hPa ageostrophic winds over the CGP and SGP (Figure 7). Campbell et al (2019) also found that drier land surfaces associated with uncoupled jets (i.e., Burrows et al, 2020; their Figure S5) act to enhance ageostrophic winds.…”

“…These regional jet event subsets include only the most persistent and spatially coherent jets (Burrows et al., 2020; their Table 1). The inclusive jets have a duration of 9‐hr or longer and cover greater than 25% of the corresponding region spatially (i.e., 11 of 42 constituent 1.125° × 1.125° grids) at 0600 UTC, when GPLLJ intensity (e.g., W max ) peaks across all three regions (Burrows et al., 2020; their Figures 5–10). The intent of the filtering approach is to build independent, representative jet type samples.…”

“…A subset of meteorological fields chosen previously by Burrows et al. (2020) is used to illustrate differences between coupled and uncoupled GPLLJs. The subset comprises fields that are relevant to wind energy resource assessment, including low‐level jet maximum (aka nose) wind speed ( W max ) and height ( H W max ), jet shear (Δ W LLJ ), and 850‐hPa wind speed (W850).…”

“…The Great Plains Low-Level Jet Occurrence and Upper-Level Coupling in CERA-20C dataset is available from https://doi.org/10.5065/KDB5-9X72. The filtered time series of NGP, CGP, and SGP jets is documented in Figure S1 of Burrows et al (2020). All jet-type regionally averaged variable time series used in this study are available upon request.…”