NOAA 95-64

CONTACT:  Patricia Viets                       FOR IMMEDIATE RELEASE
          (301) 457-5005                       10/2/95
          Gary Ellrod
          (301) 763-8251


High altitude turbulence was quite active over southern California and the Desert Southwest last fall and early winter, according to a research meteorologist at the Commerce Department's National Oceanic and Atmospheric Administration.

Gary Ellrod of NOAA's Satellite Applications Laboratory found that at 29,000 to 35,000 feet, the maximum turbulence associated with the jet stream and winter storm activity shifted north-eastward into the Plains states and Upper Midwest from mid- winter through the spring of 1995.

Ellrod found that a strong subtropical jet stream, along with occasional interaction with the polar jet, led to considerable turbulence from September to November over the southwestern United States, and in the Northeast. By winter, the jet and its associated turbulence were suppressed farther south than normal along the West Coast. Another winter peak in clear air turbulence occurred farther offshore in the central East Pacific, affecting air traffic from the West Coast to Hawaii. The Central Plains maximum in mid-winter was associated with the usual winter storm activity, coupled with periods of convergent northerly flow.

During the spring of 1995, most of the action shifted north and east into the Great Lakes, Upper Midwest, and Ohio Valley as a high pressure ridge pushed the jet stream and winter storm track northward.

These findings are based on a turbulence index derived from winds aloft once per day in the Northern Hemisphere, then averaged monthly and seasonally at the NOAA Science Center in Camp Springs, Md. Higher seasonal averages relate to a greater likelihood of significant turbulence during this period. The index calculates vertical wind shear, and horizontal shearing and compaction effects that can increase the strength of upper level fronts. The index cannot account for turbulence caused by mountain waves or thunderstorms because those features are too small in scale.

The data base that Ellrod used includes winds from balloon- borne radiosondes, aircraft, and satellite cloud motion at many altitudes. The data are merged into an "Aviation Model" analysis produced by NOAA's National Center for Environmental Prediction.