Global climatology of clear air turbulence activity deduced from a numerical model index

The steady increase in transcontinental air travel requires improved knowledge of enroute hazards such as clear air turbulence (CAT). Since many air routes are lightly traveled, information from aircraft pilot reports is sparse. To improve inflight safety, climatological data on the likelihood of CAT would be useful for both long range and daily air route planning.

An operational diagnostic index for the prediction of CAT, derived daily from National Centers for Environmental Prediction Aviation numerical model data, has been averaged monthly, seasonally, and annually for the past eight years in an attempt to determine preferred locations for CAT in the Northern Hemisphere. For the past two years, this analysis has been extended to the Southern Hemisphere also. Data was save for three layers: 400-300 hPa, 300-250 hPa, and 250-200 hPa. The index is based on the product of horizontal deformation and vertical wind shear in each layer. Previous validation of the index indicated a high probability of detection (~80%) for significant turbulence episodes. The false alarm rate of ~20% can be minimized by long period averaging such as was done in this study.

The eight-year mean turbulence index shows that regions of preferred CAT occurrence can be found in elongated bands associated with major polar and subtropical jet streams and the mid-latitude storm tracks. Maximum likelihood can be found in eastern Asia over northern China and Japan, extending into the Central Pacific. Other areas prone to CAT are: the Middle East to central Asia, the Canadian Maritimes, and the central and southwestern United States. Seasonal variations show the northward retreat and decrease in regions of significant turbulence activity. Inter-annual variations also provide valuable insights. For example, the strong El Nino winter of 1997-98 produced well above normal mean turbulence index values, whereas La Nina years such as 1999-2000 were much lower than average. Results from this study for the Northern Hemisphere are consistent with previous turbulence climatology studies that used pilot reports. Preliminary results for the Southern Hemisphere will be shown.