An Objective Classification and Analysis of Upper-Level Coupling to the Great Plains Low-Level Jet over the Twentieth Century

Abstract

Low-level jets (LLJ) around the world critically support the food, water, and energy security in regions that they traverse. For the purposes of development planning and weather and climate prediction, it is important to improve understanding of how LLJs interact with the land surface and upper-atmospheric flow, and collectively, how LLJs have and may change over time. This study details the development and application of a new automated, dynamical objective classification of upper-atmospheric jet stream coupling based on a merging of the Bonner–Whiteman vertical wind shear classification and the finite-amplitude local wave activity diagnostic. The classification approach is transferable globally, but applied here only for the Great Plains (GP) LLJ (GPLLJ). The analysis spans the period from 1901 to 2010, enabled by the ECMWF climate-quality, coupled Earth reanalysis of the twentieth century. Overall, statistically significant declines in total GPLLJ event frequency over the twentieth century are detected across the entire GP corridor and attributed to declines in uncoupled GPLLJ frequency. Composites of lower- and upper-atmospheric flow are shown to capture major differences in the climatological, coupled GPLLJ, and uncoupled GPLLJ synoptic environments. Detailed analyses for southern, central, and northern GP subregions further highlight synoptic differences between weak and strong GPLLJs and provide quantification of correlations between total, coupled, and uncoupled GPLLJ frequencies and relevant atmospheric anomalies. Because uncoupled GPLLJs tend to be associated with decreased precipitation and low-level wind speed and enhanced U.S. ridge strength, this finding may suggest that support for drought over the twentieth century has waned.

Atmospheric River Signatures in Radiosonde Profiles and Reanalyses at the Dronning Maud Land Coast, East Antarctica

“…The LLJ during the AR events is accompanied by a strong increase in the upper-level jet stream at the 300 hPa level (Fig. 5b), which can be an indication of a cyclonically induced LLJ when the low-level pressure decreases below the upper-level divergence in the left-hand exit region of a jet streak, which causes development of the ageostrophic component and horizontal acceleration, in turn causing the LLJ (e.g., Burrows et al, 2019). These LLJs assist in the poleward MT towards Antarctica during the ARs.…”

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

“…In the NGP, an additional 37 days are required to exceed the median 1905-1937 May-September uncoupled jet count (Figure 5), with the greatest shortfall of events occurring in July, when coincidentally, NGP electricity demand is peak (EIA, 2020, Tables S1-S5 in Supporting Information S1). These results are noteworthy because they detail the sub-seasonal distribution underlying the May-September jet frequency declines reported by Burrows, Ferguson, Campbell, et al (2019). The results suggest that the Great Plains jet-related wind energy resource has significantly diminished over the 20th century-even while mean W850 of remaining NGP GPLLJs has increased marginally by 1 m s −1 (Table 3).…”

“…Historically, studies that performed jet type segregation were limited both in number and temporal extent due to the time demands of jet classification by expert weather map analyses (e.g., Wang & Chen, 2009; Weng, 2000). The advent of an objective dynamical GPLLJ classification methodology by Burrows, Ferguson, and Bosart (2019), combined with the availability of accurate century‐scale climate reanalyses, such as ECMWF's Coupled Ocean‐Atmosphere Reanalysis of the 20th Century (CERA‐20C; Laloyaux et al., 2018), has made long‐term jet type‐segregated analyses readily achievable (i.e., Burrows, Ferguson, Campbell, et al., 2019; Burrows et al., 2020). Initial trend assessments reveal that May–September GPLLJ frequency has significantly decreased over the 20th century in the northern (NGP: 42.75°–49.5°N, 102.375°–96.75°W), central (CGP: 36.0°–42.75°N, 102.375°–96.75°W), and southern Great Plains (SGP: 29.25°–36.0°N, 102.375°–96.75°W), owing to decreases in uncoupled jet frequency (Burrows, Ferguson, Campbell, et al., 2019; their Figure 16).…”

“…Results for May-September means, as well as for each monthly mean time series are reported and discussed. Following from Burrows, Ferguson, Campbell, et al (2019) and Burrows et al (2020), results are reported for Great Plains sub-regions: NGP, CGP, and SGP, considering the fact that jet characteristics vary substantially across these regions. The results are relevant to applications across a wide range of sectors, including agriculture, water resources, wind energy, and weather and climate prediction.…”

More rapid intensification of flash droughts with shorter onset timescales

“…The ridge brings widespread unusual warm temperatures and, by itself, would have been responsible for dry conditions as well. Meanwhile, the ridge may inhibit precipitation over the regions missed by the upper-level trough, with precipitation deficit occurring over these regions, which may cause abnormal dryness expanded and intensified or even the occurrence of flash drought events (Namias, 1983;Schubert et al, 2009;Burrows et al, 2019;Christian et al, 2020). In addition, land-atmosphere coupling also plays a crucial role in driving the occurrence of flash droughts (green boxes in Fig.…”