Improvements to an operational clear-air turbulence diagnostic index

An operational clear-air turbulence (CAT) diagnostic index has been modified to improve its performance. The Ellrod-Knapp Turbulence Index (TI) was developed in the early 1990's and is in use at many aviation forecasting facilities worldwide. It has been recognized, however, that the TI often does not sufficiently account for situations where anticyclonic shear or curvature is present. The modification to the TI is based on the addition of a proxy term for divergence tendency, appropriate to account for gravity wave generation in both anticyclonic and cyclonic flow regions. Examples will show how the modified index (referred to as DTI, for Divergence-modified Turbulence Index) led to better anticipation of significant CAT in several different scenarios where rapid divergence changes were occurring. Preliminary objective evaluation of the 6-hr forecast DTI derived from the Rapid Update Cycle-2 (RUC-2) model was completed for two months in 2007 (using more than 1,100 pilot reports). Results showed significant improvements over the TI, based on verification metrics such as Probability of Detection of turbulence (PODy), and the True Skill Statistic (TSS). Further evaluation of the DTI and other improved algorithms began at the NOAA Aviation Weather Center in late 2009, using state-of-the-art numerical models (NAM, RUC2, SREF, etc) and a larger database of pilot reports for verification.