Turbulence

Background

Turbulence encounters by general and commercial aviation pose significant safety and flight efficiency concerns.  Almost anyone who has flown commercially has experienced turbulence and has a tale to tell about it.  In fact turbulence encounters account for well over 75% of all weather-related injuries on commercial aircraft.  Consequently, there is an urgent need to provide better turbulence information to pilots and route planners so that the number of encounters can be minimized, or at least provide adequate warnings so that passengers and crew will have time to fasten their seatbelts for an expected encounter. 

For the past fifteen years a group of scientists and engineers at the National Center for Atmospheric Research’s Research Application Laboratory (NCAR/RAL) has led the effort to address these needs.  Working under the sponsorship of the Federal Aviation Administration and the National Aeronautics and Space Administration and in collaboration with several federal laboratories and university investigators, the team has conducted research aimed at improving fundamental understanding of the nature and causes of turbulence and has developed new techniques for better observing and forecasting turbulence.

Effort has been focused in three areas: (1) Development and implementation of new techniques for obtaining automated in situ measurements and reports of turbulence encounters from commercial aircraft; (2) development of an automated detection system of in-cloud turbulence using the national NEXRAD radar network; and (3) development and implementation of an automated turbulence forecasting system called Graphical Turbulence Guidance or GTG.   While the R&D effort continues in each of these areas, the products that are being developed have reached a level of maturity that allows them to be used operationally by pilots and route planners in tactical and strategic planning for avoiding turbulence. 

A fourth long-term effort, to develop and implement a turbulence and windshear warning system for the Juneau, Alaska airport, are also discussed in this report.

Automated In situ Measurements

United Airline tracksDelta Airline tracksSouthwest Airline tracks
Fig 1.  Tracks of United Airlines(top panel), Delta Airlines (middle), and Southwest Airlines (bottom) showing automated turbulence measurements over a 24-hour period. Each line is composed of 1 minute interval measurements.

Despite the frequency and severity of turbulence encounters, our understanding of the nature and genesis of this complex atmospheric phenomenon remains limited.   Turbulence research has been limited by a lack of reliable data. Verbal pilot reports (PIREPs) have typically been the only source of information about the location and severity of turbulence at flight levels.  These reports are, unfortunately, incomplete (reporting is voluntary), and highly subjective (what one pilot views as moderate might be light or severe to another).   While computer models are very useful in forecasting other atmospheric hazards, they are of limited value here given that turbulence exists for short periods of time in small geographical areas.  In order to improve the detection and forecasting of turbulence, it is clearly essential to upgrade the turbulence observation and reporting system and to create empirical means for obtaining more abundant, reliable data. In pursuit of this goal, we are in the process of augmenting, and eventually replacing, the PIREPs with in situ measurements from selected commercial aircraft.  These measurements and dissemination are completely automated, and provide a measure of atmospheric turbulence levels as measured by the eddy dissipation rate (EDR, m 2⁄3/s). 

FY2010 Accomplishments:

Currently the in situ EDR software package is implemented on about 100 United Airlines (UAL) 757-200 aircraft and about 70 Delta Airlines (DAL) 737-800 aircraft. Initial implementation of the algorithm on the Southwest Airlines (SWA) 737–700 fleet (280 aircraft) was begun.   The DAL and SWA implementations are based on heartbeat (tied to routine Meteorological Data Collection and Reporting System [MDCRS] reporting) and threshold reports (immediate transmission when one–minute peak or mean above a certain threshold is exceeded), thus substantially reducing communication costs from previous implementations.  An example of EDR reports received from the UAL, DAL, and SWA implementations over a 24–hr time period is shown in Fig 1. The colors are peak (over 1 min flight time) EDR values, with blue smooth, yellow light turbulence).

A feasibility study using ACARS–AMDAR wind and temperature measurements to estimate EDR was completed. Initially this was used to construct a global climatology of turbulence, but with expanded data coverage it may also be possible to make real–time estimates of turbulence levels. A description of this method appeared in the Journal of Applied Meteorology and Climate (JAMC) in 2010. A follow-up study proved the correspondence between EDR statistics on constant pressure levels (flight levels) and constant altitude surfaces; these results were published in Geophysical Research Letters (GRL) in 2010. 

FY2011 Plans:

We are currently in discussions with both United and Delta Airlines to implement the algorithm on their 767 fleets which will provide expanded national coverage as well as some international coverage. We also hope to have full implementation of the in situ algorithm on the Southwest 737-700 fleet in 2011.

Remote Sensing Measurements

Experimental ADDS display of the NTDA turbulence mosaic, with convective SIGMETs and turbulence reports overlaid.
Fig. 2 Experimental ADDS display of the NTDA turbulence mosaic, with convective SIGMETs and turbulence reports overlaid.

RAL scientists have developed a product that uses ground-based radars to detect and measure turbulence within clouds.  The results can then be mapped at chosen flight altitudes and displayed for users.  This product, the NEXRAD Turbulence Detection Algorithm (NTDA), has now been adopted by the National Weather Service and implemented on all of its radar systems.  A prototype of the system (NTDA–2) continues to run in real-time at NCAR, using a data feed from 133 NEXRADs in the Continental US.  A sample of the 3-D mosaic of in-cloud turbulence produced by this system is shown in Figure 2. 

FY2010 Accomplishments:

Several updates have been made to the NTDA algorithm and software this year to accommodate recent changes to NEXRAD operational modes (super–resolution, phase coding, and new volume coverage patterns).  These changes are designed to provide greater robustness, accuracy and coverage.  In addition, the prototype NTDA processing and mosaic system was successfully delivered to the NWS Aviation Weather Center and implemented for testing.

FY2011 Plans:

NTDA turbulence measurements will be used in the Graphical Turbulence Guidance Nowcast product, currently under development. 

Nowcasting/ForecastingTurbulence

RAL has been developing and testing aviation-scale turbulence algorithms that provide forecasts out to 12 hours. These algorithms are based on operational NOW model output and are updated hourly. The version that is used over the Continental U. S. (CONUS) is termed the GTG (Graphical Turbulence Guidance product).  It relies on the Rapid Update Cycle (RUC) model output and uses what amounts to an ensemble weighted mean of various turbulence diagnostics normalized to a 0-1 scale to provide the forecasts. 

In addition to the GTG forecast system, we are currently developing a nowcast system, dubbed GTG-N, which will provide rapid (every 15 min) updates and makes heavy use of the latest available turbulence observations from the in situ measurements, PIREPS, NTDA, and other sources (e.g., satellite-based inferences) on a GTG background.  This will tremendously enhance pilot situational awareness, especially for turbulence associated with thunderstorms (convectively-induced turbulence or CIT).  Special diagnostics (termed DCIT) are being developed to predict areas of likely out-of-cloud CIT.

FY2010 Accomplishments:

The Graphical Turbulence Guidance version 2 (GTG2) which provides RUC–based forecasts of turbulence became "operational" on 11 Feb 2010 and is now available on NOAA's Operational Aviation Digital Data Service (ADDS) web site. Since the RUC is due to be replaced by the WRF RR in May 2011, work has been focused on converting the current RUC-based code to a WRF RR-based code.  Initial testing and verification have demonstrated similar verification skills. Other algorithms that make use of GTG output (GTG-N and DCIT) have been updated to use the WRF RR as well.

Work continued on the GTG nowcast (GTG–N) product, one component of which is the DCIT algorithm.  Substantial effort has been invested in developing both a better physical understanding of the mechanisms responsible for CIT (two papers were published last year on this subject) and developing a real-time diagnostic product.

FY2011 Plans:

An update to the current RUC-based GTG to use the new WRF RR will become operational when the WRF RR model replaces RUC at NCEP.  Currently this is scheduled for July 2011.  A third version of the WRF RR-based GTG, GTG3, which includes mountain-wave induced turbulence (MWT) will begin formal evaluation sometime near the end of CY2011.  The first version of GTGN (GTGN-1) will also begin formal evaluation at that time.

The GTG-like system that provides forecasts over the Taiwan airspace (for a program funded by the Taiwanese government) will be upgraded to incorporate an upgrade of the WRF system there sometime near the end of CY2011.  The Global GTG will also be updated to accommodate the recent GFS model upgrade.

Juneau Terrain-Induced Turbulence

Geographic display of Juneau Airport Wind System.  Hazard warning areas are outlined as boxes.

Fig 3: Geographic display of Juneau Airport Wind System.  Hazard warning areas are outlined as boxes.

heep Mountain anemometer site in February 2007.

Fig 4: Sheep Mountain anemometer site in February 2007.

The Juneau, Alaska, environment is characterized by rugged terrain and adverse weather that combine on occasion to produce moderate to severe terrain-induced turbulence and wind shear for flights into and out of the Juneau International Airport.  To address this problem, the FAA tasked RAL in FY1997 with the development and implementation of a new wind hazard warning system for the airport.  Over the last twelve years, RAL engineers and scientists have built a prototype warning system that relies on statistical correlations between wind-related parameters observable by the system (i.e., speed, direction, shear, variance, etc.) and the location and severity of turbulence. Since terrain is fixed and there are distinct strong-wind scenarios, correlations are expected between winds and hazards. These correlations were established using the warning system’s input measurements and hazards measured by research aircraft during three intensive field projects.  The graphical display was designed for a variety of users including Automated Flight Service Station specialists, airline dispatchers, and pilots. It contains information that depicts current alerts and conditions as well as conditions during the past hour.  The prototype warning system is used every day by airlines and pilots flying in and out of Juneau to assess current turbulence hazards in the vicinity of the airport.

FY2010 Accomplishments:

RAL continued to operate the prototype (JAWS-P) and hybrid (JAWS-H, which integrates NCAR algorithm and display software with FAA communications) systems.   A new JAWS-H software version was released and installed early in FY2010, and the JAWS-H system was evaluated in a side-by-side comparison with JAWS-P to ensure continuity of alerts.  Upon successful completion of the evaluation, the JAWS-P was shut down and JAWS-H commenced operations.  After a suitable time period to ensure JAWS-H was operating optimally, the JAWS-P was permanently shut down and dismantled.  The FAA took over anemometer site maintenance activities from NCAR.  Documentation of the JAWS-H system continued and several maintenance-related documents were delivered to the FAA for inclusion in their technical manuals.  The end-state JAWS Operational Ready Date is still on schedule for the end of FY2011.

FY2011 Plans:

RAL will continue to operate the JAWS-H warning system.  While the FAA is developing the end-state (JAWS-E) system, RAL will provide engineering support for activities such as converting from Debian to Red Hat Linux and integrating the NCAR Improved Moments Algorithm (NIMA) into the system.  At the end of FY2011 RAL will turn the JAWS-H system over to the FAA and begin the process of disposal of excess equipment not desired to be retained by the FAA.  Documentation of JAWS-H will continue, and preparation of “reference documents” such as Algorithm Design Description, NIMA Description, and Application Software Description will begin; delivery of these documents to the FAA is planned for FY2012.