ADDS - Aviation Digital Data Service

ADDS - Turbulence Help Page (2 of 2)
Back to Turbulence Page

The Graphical Turbulence Guidance (GTG-2.5) graphics are computer-generated four-dimensional forecasts of information related to the likelihood of encountering Clear-Air Turbulence (CAT) associated with upper-level fronts and jet streams, and is not intended to predict turbulence associated with convection and thunderstorm clouds or breaking mountain waves. The product provides forecasts for the 48 contiguous United States, much of Canada and Mexico, and their respective coastal waters at flight altitudes from 10,000 MSL to FL450 only, that is, it does not provide forecasts from the surface to 10,000 ft. Users should also be aware that turbulence is a highly dynamic phenomenon and in case of rapidly changing conditions the product may not accurately convey a significant hazard.

GTG-2.5 graphics may be used as a higher-resolution supplement to AIRMETs and SIGMETs, but not as a substitute for the turbulence information they provide. GTG-2.5 graphics are authorized for use as an unrestricted, supplementary weather product. The GTG-2.5 does not have the capability to be amended. See the definition of primary and supplementary weather products below.

The GTG-2.5 product consists of a 00, 01, 02, and 03 hour forecast, which are updated every hour, and a 06, 09, and 12 hour forecast, which are updated every three hours, starting at 00Z. GTG-2.5 graphics are "snapshot" graphics, intended to depict forecasted clear air turbulence conditions at the valid time (for example, at 1200Z), not for a valid time range (for example, from 1200Z to 1300Z). The GTG-2.5 graphics suite is automatically produced with no human modifications. Information on the graphics is determined from observational data, turbulence pilot reports, and automated turbulence (EDR) reports, all of which are integrated with computer forecast model output.

Due to the use of computer model output, the GTG-2.5 product issuance times are reliant upon the timing and resolution of the released underlying weather model data. GTG-2.5 uses 00, 01, 02, 03, 06, 09, and 12 hour forecast fields from the NOAA "Rapid Refresh" or "RAP" forecast model with a horizontal grid spacing of 13.5 km and 50 vertical levels. The RAP updates hourly, and the total processing time is dependant upon the amount of data in that update cycle. Due to the processing time of the model data, the GTG-2.5 will typically be released about 50 minutes after the valid time of the RAP data set. The longest update cycles occur at 00Z and 12Z due to additional RAP data ingest and processing. GTG-2.5 will normally be released 100 minutes after the valid time of the RAP data set at 00Z and 12Z. ADDS is configured so the composite graphic with the most relevant valid time should display when you initially select the GTG-2.5 product, but all graphics in the GTG-2.5 suite are available for viewing via the valid time and altitude selection interface.

NOTE: Users should pay close attention to the valid time of individual graphics due to the delay caused by RAP update cycles required for the GTG-2.5.

Selecting GTG-2.5 Graphics:

To retrieve a GTG-2.5 graphic on the ADDS turbulence page, select the specific graphic from the left-side pull-down menu. The requested graphic should appear as an image embedded directly in the turbulence page. The right-side pull-down menu allows you to select a specific altitude, with a graphic every 2000 feet, starting at 11000 feet and ascending to FL450. In addition to individual altitudes, you can select a composite, maximum value of all altitudes, labeled "max." This image provides a quick overview of the national turbulence threat.

Once an altitude has been selected, the text just above the image boundary identifies the altitude, valid time of the image, and the forecast lead time. An example is shown below. As indicated in the two lines of text just above the GTG image, this is a map of expected turbulence intensities at FL270 for (valid at) 1700 UTC on 11 Apr 2012. The map is based on a 12-hour (lead time) GTG forecast initiated from a 1400 UTC on 11 Apr 2012 model output, as indicated on the right side of the first line above the image.

Fig 1: Sample GTG Graphic

The Flight Path Tool allows access to GTG turbulence data for different altitudes in 1000 foot increments, as well as vertical cross sections for a specific route, interactive overlays of additional weather data, and a closer look at specific geographic areas. An example of the use of the flight path tool is shown in the figure below. On the left are contours of "light" and "moderate" GTG values at FL300 for a 1 hr forecast valid at 16UTC 19 Oct 2009. A flight path has been selected (indicated by the black line) from the Portland OR area to the Billings, MT area. The right image is a vertical cross section showing the GTG values at this same time along the designated flight path. According to GTG, "light" intensity clear-air turbulence could be expected eastward from eastern OR at this altitude and "moderate" clear-air turbulence could be expected near the OR-ID border and again over western MT. The moderate turbulence areas could be avoided by descending to FL200 in those areas. Note that GTG does not predict values below 10,000 ft MSL on the cross section.

Fig 2: Flight Path Tool depictions of GTG2.5 Turbulence in plan view and cross section

Overview of GTG-2.5 Display:

All graphics display atmospheric turbulence severity in three categories: none, light, moderate or greater (including severe and extreme). These are actually EDR (=ε1/3 where ε is the eddy dissipation rate in units of m2/s3) values in the ranges (0-0.14, 0.15-0.30, > 0.31), respectively. The display colors range from white for none, green for light, orange for moderate or greater. Users should always keep in mind that the three levels of turbulence severity depicted on the GTG graphic are general terms that are not specific to any particular type of aircraft and are only intended to depict general turbulence conditions for supplementing flight planning and situational awareness. The turbulence categories displayed are representative for commercial air carriers of the large (41,000-255,000 lbs maximum takeoff wt.) and heavy (weight > 255,000 lbs maximum takeoff wt.) weight classes. With this in mind users should scale the depicted turbulence levels up or down depending on the weight and airspeed of their particular aircraft relative to a large category aircraft in cruise.

The display can be manipulated using either the arrows or the pull down menu. The up and down arrows link to graphics for higher and lower altitudes, and the right and left arrows link to graphics for earlier and later valid times.

A tutorial on the Graphical Turbulence Guidance (GTG):

The GTG algorithm uses forecast fields from the RAP gridded aviation forecast model (Benjamin et al. 2006) distributed by the National Weather Service's National Centers for Environmental Prediction (NCEP). Every hour, several turbulence diagnostics (listed below) are computed from the RAP-2 forecast output. An explanation of the algorithms and the GTG formulation can be found in Sharman et al. (2006). Basically, the algorithm combines the results of several turbulence diagnostics (listed below) and weights the diagnostics based on comparisons to observations (i.e. Pilot Reports (PIREPs) or in situ EDR data). The algorithm results are interpolated to flight levels (FLs) from the native RAP grid and are then mapped to the EDR turbulence intensity scale given above.

While tuning to observations it should be remembered that PIREPs in particular are a subjective measure of the affect of atmospheric turbulence on an aircraft and have inherent uncertainties in time and location which are further discussed in Schwartz (1996). However, GTG2.5 also makes heavy use of automated in situ eddy dissipation rate (EDR) reports (Cornman 1995). Because of the high frequency of reporting, the in situ data provide a much denser set of observations when compared to PIREPs. However, it is still a problem to get turbulence observations during the overnight hours when air traffic is at a minimum. Therefore GTG forecasts initialized at night tend to have larger forecast errors than those initialized during the day.

The GTG mid-level forecasts between 10,000 feet MSL and FL200 are blended with the upper-level forecasts near the intersection at FL200. The indices used for the mid-level forecasts are different from those used at upper-levels due to different atmospheric processes attributing to the turbulence production at these different altitude bands. GTG outputs grids on 36 flight levels separated by 1000 feet intervals, regardless of the altitude. A flight level is a constant pressure surface referenced to a world-wide sea level pressure datam (1013.25 hPa). Thus the flight level is referenced to a standard atmosphere; and the actual altitude and flight level will not generally be the same, although typically the difference is small. Flight levels are a convenient way to insure adequate vertical spacing at altitudes above all terrain. At lower altitudes, terrain avoidance is of primary concern and flight altitudes are typically defined as MSL altitude. In the US & Canada the transition altitude (to the flight level regime) was set at 18,000 feet, while in Europe and other parts of the world it is implemented based on terrain height surrounding the airport, or on the runway elevation. The GTG reports all forecasts on flight levels both above and below the transition altitude of 18,000 MSL with the understanding that the difference between flight level and actual MSL altitude is small.

At upper-levels, the following 9 turbulence diagnostics are used within the GTG-2.5 combination (see Sharman (2006) for an explanation of each index):

  1. Brown2
  2. |Def|2/Ri
  3. iawind/Ri
  4. Frontogenesis function (isentropic coordinates)/Ri
  5. |DIV|/Ri
  6. LHFK/Ri
  7. -NVA/Ri
  8. Structure function derived eddy dissipation rate, EDR/Ri
  9. Structure function derived CN2/Ri

At mid-levels, the following 10 indices are used:

  1. Brown1
  2. Wind speed X horizontal deformation/Ri
  3. |DEF|2/Ri
  4. iawind/Ri
  5. |DIV|/Ri
  6. |ΔT|/Ri
  7. Structure function derived eddy dissipation rate, EDR/Ri
  8. Structure function derived CN2/Ri
  9. Frontogenesis function (pressure coordinates)/Ri
  10. Structure function vertical velocity variance, SIGW/Ri

The GTG2.5 results are shown on the ADDS displays as contour maps of predicted CAT intensity (null, light, moderate or greater). The values depicted are in terms of EDR(=ε1/3 where ε is the eddy dissipation rate in units of m2/s3) and can be interpreted roughly as follows (for large-heavy aircraft):

  • 0.0 - 0.14 = no turbulence
  • 0.15 - 0.30 = light turbulence
  • >0.31 = moderate or greater turbulence

Benjamin, S.G., D. Devenyi, T. Smirnova, S. Weygandt, J.M. Brown, S. Peckham, K. Brundage, T.L. Smith, G. Grell, and T. Schlatter, 2006: From the 13-km RUC to the Rapid Refresh. 12th Conf. on Aviation, Range, and Aerospace Meteorology (ARAM), Atlanta, GA, Amer. Meteor. Soc. CD-ROM, 9.1

Cornman, L. B., C. S. Morse, and G. Cunning, 1995: Real-time estimation of atmospheric turbulence severity from in-situ aircraft measurements. J. Aircraft, 32, 171-177.

Schwartz, B., 1996: The quantitative use of PIREPs in developing aviation weather guidance products. Wea. Forecasting, 11, 372-384.
Sharman, R., C. Tebaldi, G. Wiener, and J. Wolff, 2006: An integrated approach to mid- and upper-level turbulence forecasting. Wea. Forecasting, 21, 268-287.

Comments sent to the ADDS developers via the Feedback link will be forwarded to the developers of GTG.

Supplementary Weather Product definition

The definitions for weather product status can be found in Flight Standards Information Management System (FSIMS) Order 8900.1, paragraph 3-2073. These definitions technically refer to air carriers but Flight Standards has consistently used this definition for General Aviation as well. Similar definitions that may cover GA are contained in the Aeronautical Information Manual (AIM) section 7-1-3 (available at

For reference the relevant FSIMS paragraph is provided below:


  1. The development of new aviation weather products is an evolutionary process with distinct stages of product maturity. The growing demand for new weather products and the corresponding increase in research and development to meet that demand, along with relatively unfettered access to weather information via the public Internet, created confusion within the aviation community regarding the relationship between regulatory requirements and new weather products. Consequently, the FAA finds it necessary to differentiate between those weather products that may be used to comply with regulatory requirements and those that may only be used to improve situational awareness. To clarify the proper use of aviation weather products to meet the requirements of the regulations, the FAA developed the following definitions:
    1. Primary Weather Product. An aviation weather product that meets all the regulatory requirements and safety needs, for use in making flight-related aviation weather decisions.
    2. Supplementary Weather Product. A aviation weather product that may be used for enhanced situational awareness. If used, a supplementary weather product must only be used in conjunction with one or more primary weather products. In addition, the FAA may further restrict the use of supplementary weather products through limitations described in the product label.
    NOTE: An aviation weather product produced by the Federal Government is a primary product unless designated as a supplementary product by the FAA.
  2. In developing the definitions of primary and supplementary weather products, it is not the intent of the FAA to change or increase the regulatory burden upon certificate holders. Rather, the definitions are meant to eliminate confusion by differentiating between products that may be used to meet regulatory requirements and other products that may only be used to improve situational awareness.
  3. All flight-related, aviation weather decisions must be based on primary weather products. Supplementary weather products augment the primary products by providing additional weather information, but may not be used as stand-alone products to meet aviation weather regulatory requirements or without the relevant primary products. When discrepancies exist between primary and supplementary products pertaining to the same weather phenomena, users must base flight-related decisions on the primary weather product. Furthermore, multiple primary products may be necessary to meet all aviation weather regulatory requirements.
  4. As previously noted, the FAA may choose to restrict certain weather products to specific types of usage or classes of user. Any limitations imposed by the FAA on the use of a product will appear in the product label."

Fig 3: Example event 6hr forecast