Global Graphical Turbulence Guidance (G-GTG) for World Area Forecast System (WAFS) Upgrade

Unexpected turbulence encounters in the upper troposphere and lower stratosphere (UTLS) are dangerous for cruising aircraft, especially since in-flight crews and passengers are more likely to be unbuckled. The International Civil Aviation Organization (ICAO) has tasked with two World Area Forecast Centers (WAFC Washington at NOAA/Aviation Weather Center, and WAFC London at the Met Office in the UK) to provide automated grid information of en-route turbulence and other weather hazards to global aviation users for strategic flight planning (06-36 hrs ahead). For consistent information, the two WAFC centers provide blended World Area Forecast System products from both centers. The WAFS upgrade plan is to improve the current WAFS products by replacing uncalibrated hazard potential forecasts with calibrated severity forecasts and probabilities of exceeding specific hazard severity levels.

This study describes the development of the Global Graphical Turbulence Guidance (G-GTG) for the WAFS upgrade. The strategy is to compute a set of diagnostics that identifies regions of strong spatial gradients, and using an empirical mapping technique, convert these indicators into an equivalent Energy Dissipation Rate (EDR) to the 1/3 power. This metric is a physically sound severity metric of atmospheric turbulence, and is the ICAO standard for reporting turbulence, and thus provides a convenient basis for verification. Deterministic forecasts are provided using an ensemble mean of the individual turbulence diagnostics from two global NWP models (NOAA’s Global Forecast System model and The MO’s Unified Model). Probabilities for different levels of turbulence intensities can be provided using the diagnostic ensemble from two NWP models. In this way clear-air turbulence sources and mountain wave turbulence sources are captured, although convective turbulence sources are more problematic in the short-term perspective. The research-to-operations (R2O) procedures for getting G-GTG into the WAFS system will be described. Verification results based on case studies and statistical evaluations against aircraft-based EDR observations will also be presented.