Improvements to an Operational Clear-Air Turbulence Diagnostic Index by Addition of a Divergence Trend Term

Abstract

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 1990s and is in use at many aviation forecasting facilities worldwide. It has been recognized, however, that TI often does not sufficiently account for situations where anticyclonic shear or curvature is present. The proposed modification to TI is based on the addition of a proxy term for divergence tendency, appropriate for both anticyclonic flow and gravity wave generation in cyclonic regions. Examples show how the modified index [referred to as the divergence-modified turbulence index (DTI)] leads to better anticipation of significant CAT for two scenarios where rapid divergence changes were occurring. Preliminary objective evaluation of the 6-h forecast DTI derived from the Rapid Update Cycle-2 (RUC-2) was completed for 2 months in 2007 (using more than 1100 pilot reports). Results showed significant improvements over TI, based on verification metrics such as the probability of detection of turbulence (PODy) and the true skill statistic (TSS). Further evaluation is planned using a larger database of pilot reports, as well as forecast data from additional state-of-the-art prediction models, altitude ranges, and forecast times.