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February 1999
Science

Probing the Pacific for clear air turbulence

Through early February, a team of scientists is sending probes into the jet stream over the Pacific Ocean to learn more about clear air turbulence (CAT). Research aircraft are releasing dropwindsondes to improve weather forecasts and provide insight into the sudden, invisible gusts that pose an extreme hazard to aviation. The program is a collaborative effort among NCAR, NOAA, the FAA, and the Naval Research Laboratory (NRL) in Monterey, California. At press time, researchers were planning missions with the NOAA G-IV aircraft from Honolulu between 16 January and 15 February.

Commercial pilots flying east take advantage of the fast-moving winds at about 35,000 feet, in the jet stream's central core, to gain extra momentum. However, this puts them at risk for CAT. Twenty passengers aboard a Japanese airliner were injured on 20 January as the aircraft was struck by severe turbulence at 33,000 feet above the northwest Pacific just east of Japan. Scientists want to understand why sometimes the jet stream produces severe turbulence and sometimes it does not. "The idea is to examine the core's structure to see what role it might be playing in clear air turbulence," says MMM director Bob Gall, who traveled to Honolulu in late January for the project.

The special observations are piggybacking onto the Winter Storm Reconnaissance flights being sponsored by the National Weather Service (NWS) in cooperation with NOAA's Aircraft Operations Center and the U.S. Air Force Reserve. Dropwindsondes deployed into turbulent areas of the jet stream will provide data on the structure of the jet stream between 27,000 and 45,000 feet, the flight altitude of major airlines. The project's data will be used to verify experimental turbulence prediction models at NCAR, NOAA, and NRL and to learn how operational NWS forecast models might be improved to give pilots more accurate warnings of turbulence.

Where in the nation does clear air turbulence strike most?

Although the Midwest is one of the peak areas for pilot reports of turbulence, the Rocky Mountains may actually be the nation's CAT capital. This finding comes from a preliminary climatology of upper-level turbulence compiled by Tressa Kane, Barbara Brown, and Bob Sharman (RAP). Tressa presented results at the 79th annual meeting of the American Meteorological Society (AMS) last month in Dallas.

CAT tends to be a major contributor to turbulence encounters above 20,000 feet. However, little solid data exists on the prevalence of CAT. Researchers have long depended on pilot reports, or PIREPs, but these are highly subjective and qualitative and they only reflect turbulence occurring along flight paths. By analyzing a six-year set of more than 160,000 PIREPs, Tressa and colleagues have attempted to factor out seasonal and regional biases.

Only about 0.3% of all positive turbulence PIREPs specifically identify CAT, while about 72% of PIREPs fail to specify the type of turbulence encountered. "It is likely that many of these 'unknowns' are actually CAT," says Tressa. As one might expect, most turbulence is reported during daylight hours, when flights are most frequent. The regions showing the largest number of positive turbulence PIREPs in wintertime above 20,000 feet are the Ohio Valley, the Rockies (especially Colorado), and the Pacific Northwest. Florida and Texas also have centers of action, although lightning data hints that this turbulence may be thunderstorm-related.

According to Tressa, "The high number of turbulence reports [in the Ohio Valley] may be at least partially attributable to zealous solicitation of reports in that region rather than an actual greater incidence of turbulence events." The air-traffic bias can be attacked by comparing severe turbulence reports to all turbulence reports. In this analysis, Tressa and colleagues found that the Rockies and the Gulf Coast stand out. Since the Gulf Coast reports are likely tied to convection, it appears that the Rockies are the area most likely to experience severe CAT: "Large values of all measures were found in this region, in comparison to other regions."

How persistent is CAT? Turbulence related to thunderstorms and icing is limited by the timescale of the weather event, but CAT appears to have more potential staying power. During one case in December 1997 in the southeast United States, CAT was reported for three consecutive days. However, as Tressa notes, "The characteristics of PIREPs . . . make determining persistence of turbulence difficult." For instance, if aircraft reports cease after dark, one cannot tell if the turbulence has ended or not. The RAP team hopes that an expanded data set can shed light on this question: "The addition of lightning, air traffic frequency, and in-situ turbulence data may help overcome some of the limitations of the PIREPs."


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Edited by Bob Henson, bhenson@ucar.edu

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