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Modeling turbulence far from storms

Flight paths across Kansas

Flights across Kansas on June 17, 2005, reported moderate to severe turbulence (orange and red segments) hundreds of miles north of a thunderstorm complex in Oklahoma.  (Illustration courtesy Dave Johnson, NCAR, based on NOAA Geostationary Operational Environmental Satellite data and automated flight reports.)

Several red-eye commercial flights were rocked by moderate to severe turbulence as they flew across northeast Kansas early on June 17, 2005. Thunderstorms cause at least 20% of aircraft encounters with turbulence at cruising altitudes. In this case, however, the thunderstorms responsible for the turbulence were located in Oklahoma, hundreds of miles south of the bumpy flights. The graphic below shows the storm complex and the flight paths, with orange and red segments denoting moderate to severe turbulence.

A new study by NCAR scientists Stan Trier and Bob Sharman uses modeling to connect the Oklahoma storms with the Kansas turbulence. The link involves strong upper-level winds that blow anticyclonically outward from mesoscale convective systems (MCSs) like the one in Oklahoma. Strong vertical wind shear that supports turbulence production occurs on the north side of the MCS, where its anticyclonic outflow is unopposed by the prevailing upper-level westeries.

Trier and Sharman tested this idea by performing diagnostic calculations with Rapid Update Cycle forecast model analyses and simulations with the Weather Research and Forecast (WRF) model for the case of June 17, 2005. The model showed a broad zone of turbulent kinetic energy extending across much of Kansas and Missouri. It also traced the upper-level winds in the turbulence zone back to the Oklahoma storm complex. Periods of enhanced upper-level instability and turbulence lasting several hours corresponded to the growth and decay of thunderstorm bands within the overall complex.

“We think these types of analyses could be useful in diagnosing the potential for turbulence when there are large-scale upper-level circulations produced by organized complexes of thunderstorms,” says Trier. Parts of the NASA-funded work were presented in Savannah, Georgia, in October at the American Meteorological Society’s 24th Conference on Severe Local Storms.