Evaluation of a modified clear air turbulence (CAT) forecast index based on deformation, vertical shear, and divergence tendency

A modified clear-air turbulence (CAT) forecast index has been evaluated using numerical model output and pilot reports (PIREPs) obtained from both the NOAA/NWS/NCEP Aviation Weather Center (AWC) and the U.K. Meteorological Office (UKMO). The original deformation-vertical shear index (or TI1) that is currently in operational use at many aviation forecast centers was modified by addition of a scaled divergence tendency term. The intent of this alteration was to produce better forecast skill in areas of anticyclonic shear or curvature, which has been recognized as a weakness of the TI1. The new Ellrod-Knox (E-K) index was generated using 6-h and 12-h forecast upper level wind data from the Rapid Update Cycle (RUC)-2 and 24-h forecast winds from the Global Forecast System (GFS) models as part of the Aviation Weather Testbed at AWC. Objective verification based on two months of data (Dec 2009 – Jan 2010) has shown that the E-K index performed with greater skill than the legacy TI1. Probabilities of Detection of light-moderate or greater CAT (PODy) for the 6-h forecasts were 30% to 50% better for E-K than TI1. Although there was a trade-off in the reduced detection of null events (leading to slightly lower PODn values), the True Skill Statistics (TSS) for the E-K index indicated marked improvement. An independent study from the UKMO based on 24-h global model forecasts and objective turbulence reports from instrumented aircraft confirmed the AWC results. Several case studies will illustrate the improvements that can be expected by use of the E-K index in various flow regimes in an operational setting.