Collaborative Research: Propagation, Evolution and Rotation in Linear Storms (PERiLS)

The PERiLS (Propagation, Evolution and Rotation in Linear Storms) project will bring together NSF and NOAA scientists to study severe weather in the Southeastern U.S. In the Southeastern U.S., severe weather is frequently produced by squall lines, also known as quasi-linear convective systems ('QLCSs'). QLCSs are responsible for approximately a quarter of all tornadoes in the U.S. and the majority of QLCS tornadoes occur in the Southeastern U.S. QLCS-spawned tornadoes pose a significant threat to lives and property, but forecasting QLCS tornado events continues to pose significant challenges, even more so than forecasting supercell tornadoes. The small-scale processes and precise environments leading to these hazards are not well observed by the current operational weather observing network and there is a lack of understanding of QLCS tornadogenesis processes. Complicating this further, coarse data and forecast analyses suggest large areas along QLCSs may be favorable for tornadogenesis (tornado formation), but tornadoes tend to occur only within a small fraction of that area. While many field campaigns have focused on a better understanding of supercell tornadogenesis (especially in the Great Plains), no campaigns have focused specifically on collecting data to understand QLCS tornadogenesis. Starting in 2021, PERiLS will be the first such project, and will deploy an extensive network of cuttingedge observing systems to the Southeastern U.S. to gather data to help researchers better understand how tornadoes form in QLCSs, what are the precursors for tornadoes, and what storm processes and characteristics can be used to differentiate storms that will produce severe winds from those that will produce tornadoes.

There are critical gaps in our knowledge concerning how QLCS tornadoes form, which must be addressed before forecast skill can be improved. QLCS tornadoes often form within mesovortices, but most mesovortices do not produce tornadoes. Environments with 'high-shear' and 'low-CAPE' are prevalent in the SE U.S.; storms that form in these environments account for a substantial fraction of severe wind and tornado reports in the region. How this environment specifically supports QLCS tornadogenesis is unknown. Moreover, the characteristics of the temporal and spatial variability, of both the storm and the environment, has yet to be well quantified. While there have been idealized numerical modeling studies and climatologies based on operational data of QLCS tornadoes, there is a dearth of the fine-scale observations that are necessary to characterize storm and sub-storm scale processes and the interaction of storm features with the rapidly-evolving near-storm environment. The PERiLS field campaign will obtain observations needed to address the following interrelated objectives: (1) Identify the mechanisms for low-level mesovortex formation; 2) Identify the characteristics and mechanisms that distinguish tornadic from non-tornadic QLCS mesovortices; 3) Identify the environmental variability and storm-environment interactions that are associated with QLCS mesovortex and tornado formation; and 4) Characterize the roles of cold frontal processes vs. system-generated cold pools in the evolution of strongly-forced QLCSs. This unprecedented data set will facilitate an understanding of the interplay between environmental and within-storm processes that contribute to QLCS tornadogenesis.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.