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Tornado researchers catch a squall line during VORTEX2

Observations taken over a three-hour period allow for an unprecedented view of the squall line’s internal structure and nearby environment.

A truck driving down an empty highway with a menacing storm in the background.

VORTEX2 vehicles stop by the side of the highway as they track a storm in June 2009. The field project, which also ran in spring 2010, is designed to improve scientists' understanding of tornado formation.

In the spring of 2009, researchers on the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) field project set out across the Great Plains to study tornadoes, but that’s not the only phenomenon they observed. They also got an exceptionally close look at a squall line, or line of severe thunderstorms that forms along or ahead of cold fronts, producing severe weather in the form of heavy rainfall, strong winds, hail, and lightning. The analysis, led by NCAR scientist George Bryan, will be published in Monthly Weather Review.

In recent decades, scientists have learned much about squall line dynamics through Doppler radar data and numerical simulations, but in situ (in place) observations of temperature and moisture in squall lines have been rare. Networks of operational rawinsondes (radiosondes designed to measure wind speed and direction) are too coarse to capture the structure of squall lines, and airplane flights in these regions are too hazardous.

During VORTEX2, the research team used high-frequency rawinsonde launches from ground-based mobile platforms to observe a squall line that passed near the project’s armada of mobile observing facilities in Oklahoma on May 15. Nine soundings were taken over a three-hour period, allowing for an unprecedented view of the squall line’s internal structure and nearby environment.

The observational data retrieved during this unique event confirms much of what scientists know about squall lines from numerical modeling. The team was surprised to find, however, that the squall line’s cold pool (relatively cold air found near the surface after the gust front passed but before precipitation began) was 4 kilometers (2.5 miles) deep—several times deeper than previous research on squall lines has indicated for cold pools.

Because this was the only squall line observed during VORTEX2’s 2009 phase, researchers stress the need for future field observations on squall lines. The research has the potential to help scientists confirm and improve weather forecasts and is also applicable to climate simulations.