In Situ Turbulence


For both research and operational purposes, there are currently an insufficient number of reliable, accurate, and timely measurements of atmospheric turbulence locations and intensities. Turbulence "measurements" in the form of pilot reports (PIREPs), although useful, do not comprise a satisfactory turbulence detection system. NCAR, under FAA sponsorship, has developed in situ algorithms for inferring and reporting turbulence encountered by commercial transport aircraft. The algorithms compute an eddy dissipation rate, or EDR (m 2/3 s –1), from available on–board flight parameters. EDR is truly a "state–of–the–atmosphere" turbulence metric; it is widely used in the research community as a measure of turbulence intensity, and has been adopted as the ICAO standard for aviation turbulence. About 100 757 aircraft from United Airlines are presently equipped with software which downlinks EDR reports recorded at one–minute intervals (in cruise) In order to reduce communication costs, an improved algorithm has been developed that downlinks turbulence encounter information immediately if the EDR is above a certain threshold, and still provides routine reporting but at less frequent intervals. This improved algorithm has been recently implemented on about 70 Delta Air Lines 737–800s. The algorithm will also soon be deployed on DAL’s 767 fleet.  As the airline industry becomes more aware of the benefits of this program, it is anticipated that some form of in situ EDR observation systems will ultimately operate on the majority of commercial aircraft flying both continental U.S. (CONUS) and international routes. Although EDR is the preferred prediction metric for forecasting purposes, some users may also require the aircraft acceleration loads associated with an atmospheric EDR measurement. This is easily provided by an aircraft/configuration dependent linear mapping factor.

Coverage provided in a 24 hour period of in situ EDR measurements (colored code in units of m 2/3 s -1) from UAL mostly 757–200 aircraft (top panel), with one–minute routine reporting and DAL 737–800 aircraft (bottom panel) with event–based EDR reporting plus 15–minute routine reporting.

Remote sensing

The NEXRAD Turbulence Detection Algorithm is described in the Radar section. The feasibility of using other remote sensing techniques to detect turbulence for tactical avoidance is also being studied. These include airborne forward–looking Doppler radar, lidar, GPS scintillation, and forward–looking infrared interferometer methods.