Aircraft Encounters with Mountain Wave-Induced Clear Air Turbulence: Hindcasts and Operational Forecasts using an Improved Global Model

We recently completed the first stage of a major upgrade to an operational mountain wave forecasting model, which we refer to as MWFM: The MWFM uses global forecast data to assess flow patterns over the Earth's major topographical features, in order to locate regions where mountain waves are generated. The new model (MWFM 2.0) uses numerical ray-tracing  algorithms to predict the subsequent radiation of three-dimensional nonhydrostatic mountain wave patterns from the parent orography.  The new code, while a major extension of the old, is nonetheless numerically efficient, allowing us to forecast regional or even global mountain wave distributions soon after global forecast data come on-line. 

Here we discuss MWFM forecasts of turbulence due to mountain wave breaking. A brief description of the old and new calculation methods is  provided. We have used the model previously in research campaigns using NASA's ER-2 research aircraft. This aircraft (a modified U-2) cruises at  ~60,000-70,000 feet and, due to weight constraints, does not have a wing spur, which makes it structurally vulnerable to turbulence. Though much of the weather-related turbulence found lower down is absent at these stratospheric cruise altitudes, we use in-flight ER-2 data and MWFM forecasts and hindcasts to demonstrate that breaking mountain waves are major source of hazardous turbulence for the ER-2. 

NASA's ER-2 and DC-8 research aircraft were deployed from the Arena Arctica in Kiruna, Sweden during the SAGE III Ozone Loss and Validation Experiment (SOLVE), which took place during the winter of 1999-2000. Kiruna lies downstream of large mountains that run the length of the Norwegian coast. Furthermore, research flights were also possible over southern Norway, Scotland, the Urals, Iceland, Greenland, Novaya Zemlya, Franz-Josef Land and Spitzbergen, all regions with significant orography and hence mountain wave turbulence potential. These facts, along with the hostile remote Arctic environment, meant that mountain wave-induced turbulence was a critical safety for ER-2 flight planning. Accordingly, we ran the new MWFM model in an operational forecasting configuration throughout the SOLVE mission, and ran tailored forecasts in the field during the ER-2 deployments. Some selected results from the MWFM-SOLVE forecasting effort are presented. 

Though less mission-critical, MWFM turbulence forecasts were also provided for the DC-8 at altitudes ~200-250 hPa (~35,000 feet). We investigate the potential utility of these lower-altitude MWFM turbulence forecasts for commercial aviation. We focus on two NTSB reports of  airliner encounters with unforecasted severe turbulence, one near Yakutat, Alaska, the other near Bishop, California, both of which  resulted in crew injuries. MWFM hindcasts produce zones of intense mountain wave-induced turbulence near the reported locations and altitudes  of these incidents.