No, we’re not gluttons for punishment; however, the turbulence Imagery in ForeFlight Mobile has just gotten way better! Forecasts now go out beyond 12 hours to include lead times of 15 and 18 hours. This is a significant improvement to NOAA’s Graphical Turbulence Guidance (GTG-3) product that now includes an analysis and forecast for clear air turbulence as well as turbulence from mountain wave activity with a new forecast updated every hour. Whereas the lowest altitude in the earlier version of GTG originated at 10,000 feet, the new GTG product includes low-level turbulence beginning at 1,000 feet MSL with a vertical resolution of 2,000 feet that extends to FL450. If that isn’t enough, forecasts now have a higher resolution of turbulence intensities that includes the full range of classifications from light to extreme as shown below making the product even more useful to evaluate the risk of dangerous turbulence along your proposed route.
Eddy Dissipation Rate (EDR)
On the older GTG version, the legacy terminology such as light or moderate turbulence is somewhat arbitrary and based typically on the response of the aircraft to the turbulence, not the atmospheric conditions themselves. Shown in the scale above, Eddy Dissipation Rate, or EDR, is an objective, aircraft-independent, universal measure of turbulence based on the rate at which energy dissipates in the atmosphere. In other words, it is a measure of the turbulent state of the atmosphere. According to turbulence researcher, Dr. Robert Sharman, “When the atmosphere is dissipating energy quickly (i.e the EDR is large), atmospheric turbulence levels are high.” Pilots should be keenly aware that a safe turbulence penetration airspeed varies with the aircraft’s weight which Dr. Sharman quickly points out, “The implication for aircraft bumpiness depends on the size (weight) of the aircraft.”
At the basic level EDR is really an in situ calculation. That is, it is a value determined by an aircraft while in flight. However, it is not directly measured by the aircraft like outside air temperature, for example. Instead it’s determined by using a variety of data from aircraft avionics which means aircraft can (and do) automatically report EDR in flight. Since EDR is an aircraft-independent calculation, a single engine Cessna 152 and a Boeing 747 should determine the same EDR value when flying through the same atmosphere at the same time.
It’s one thing to calculate the EDR in flight, but totally another challenge to provide a forecast for this field. That’s the job of GTG-3. Dr. Sharman points out, “From the forecasting point-of-view we cannot provide a separate forecast for every type of aircraft that is out there.” So depending on the class of aircraft you are flying, there’s a need to evaluate the EDR values properly. Below shows the EDR values that correspond to light-, medium- and heavy-weighted aircraft as they loosely relate to the vertical acceleration/deceleration (turbulence) response in that class of aircraft.
The new version of GTG includes a forecast for clear-air turbulence often referred to as CAT. Even more exciting, turbulence that is a direct result of mountain wave activity, or MTW, is also forecast separately. While these forecasts are not meant to predict turbulence associated with deep, moist convection, they will provide guidance of low-level terrain and thermally-induced turbulence sources.
To that end, in the ForeFlight Imagery view you’ll find three different forecasts that include clear-air turbulence (CAT), mountain wave turbulence (MTW) and one that combines the two (All). Each one of these is organized into low (1,000 ft – 13,000 ft), middle (15,000 ft -FL290) and high (FL310 – FL450) level collections as shown below.
“GTG is generated operationally at NOAA/NCEP which is supported 24 hours a day, 7 days a week. Its use is unrestricted (meteorologists, dispatchers, GA and commercial pilots, ATC, etc).”