The Graphical Turbulence Guidance (GTG-2) 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 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 graphics are authorized for use as an unrestricted, supplementary weather product. The GTG-2 does not have the capability to be amended. See the definition of primary and supplementary weather products below.

The GTG-2 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 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 graphics suite is automatically produced with no human modifications. Information on the graphics is determined from observational data, pilot weather reports, upper air soundings, satellite soundings, automated aircraft reports, and surface weather reports, all of which are integrated with computer model output.

Due to the use of computer model output, the GTG-2 product issuance times are reliant upon the timing and resolution of the released underlying weather model data. GTG-2 uses 00, 01, 02, 03, 06, 09, and 12 hour forecast fields from the Rapid Update Cycle (RUC) model with a horizontal grid spacing of 20 km and 50 vertical levels. The RUC 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 will normally be released 50 minutes after the valid time of the RUC data set. The longest update cycles occur at 00Z and 12Z due to additional data ingest and processing. GTG-2 will normally be released 100 minutes after the valid time of the RUC 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 product, but all graphics in the GTG-2 suite are available for viewing via the valid time and altitude selection interface.

To retrieve a GTG-2 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 10000 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 FL350 for (valid at) 0000 UTC on 20 Oct 2009. The map is based on a 9-hour (lead time) GTG forecast initiated from a 1500 UTC on 19 Oct 2009 RUC model output, as indicated on the right side of the first line above the image.

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.

All graphics display turbulence severity in three categories: none, light, moderate or greater (including severe and extreme). The display colors range from white for none, green for light, yellow for moderate or greater. Users should always keep in mind that the three levels of turbulence severity depicted on the GTG-2 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 based on best fits to PIREPs data for all altitudes above 10,000 ft MSL. The majority of these PIREPs are from 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. Therefore a qualitative frame of reference is large aircraft in cruise conditions for the GTG-2 values on the displays. 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.

The GTG algorithm uses forecast fields from the Rapid Update Cycle (RUC-2) gridded aviation forecast model (Benjamin et al. 2004) distributed by the National Weather Service's National Centers for Environmental Prediction (NCEP). Every hour, several turbulence diagnostics (listed below) are computed from the RUC-2 forecast output. An explanation of the algorithms and the GTG formulation can be found in Sharman et al. (2006). The algorithm results are interpolated to flight levels (FLs) from the native grid and mapped to a common turbulence intensity scale of 0 to 1. Each of these values is then compared to the available observations (i.e. Pilot Reports (PIREPs) or in situ) within ±90 minutes of the analysis time, which have also been mapped to a 0 to 1 scale (where threshold values for none, light, moderate (including severe and extreme) for large aircraft are associated with the values 0.0, 0.3, 0.475 respectively). The algorithm values are weighted according to the best agreement to those observations (mainly from large and heavy aircraft) and the sum of the weighted algorithm value forms the GTG combination. The weights formed for the 00 hr forecast are then applied to each of the diagnostics computed from the RUC-2 forecast files (1, 2, 3, 6, 9, and 12 hour) to create the GTG forecast.

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). More recently, automated in situ reports of atmospheric turbulence measured in terms of eddy dissipation rate (EDR), a metric independent of aircraft type, have become available (Cornman 1995). These automated reports are made every minute during cruise and downloaded in four minute bundles. 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 GTG2 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 10 turbulence diagnostics are used within the GTG combination:

1. Colson-Panofsky index
2. Richardson number
3. DTF3
4. Frontogenesis function (isentropic coordinates)
5. Unbalanced flow diagnostic
6. Horizontal temperature gradient
7. TI1
8. NCSU1
9. Structure function derived eddy dissipation rate, EDR
10. Structure function derived sigma vertical velocity, SIGW

At mid-levels, the following 9 indices are used:

1. TI1
2. Wind speed X horizontal deformation
3. ABSIA
4. Horizontal temperature gradient
5. Wind speed
6. NCSU1
7. Structure function derived eddy dissipation rate, EDR
8. Structure function derived sigma vertical velocity, SIGW
9. Frontogenesis function (pressure coordinates)

The GTG results are shown on the ADDS displays as contour maps of predicted CAT intensity (null, light, moderate or greater). The prediction can be interpreted roughly as follows:

• 0.0 - 0.3 = no turbulence
• 0.3 - 0.475 = light turbulence
• 0.475 - 1.0 = moderate or greater turbulence

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

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 http://www.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/).

For reference the relevant FSIMS paragraph is provided below:

"3-2073. CLASSIFICATION OF AVIATION WEATHER PRODUCTS.

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."

An example 6 hr (Fig. 3) and 3 hr (Fig. 4) forecast at FL300 valid at 12 UTC on April 24, 2006 are shown, with a GOES 12 IR satellite image from 1215 UTC shown in Fig 5. There is a large area of moderate turbulence forecast from northern Illinois southeastward across the Ohio Valley. This area is associated with many moderate turbulence PIREPs in a swath of clear air between two cloud banks. In a different meteorological situation, there are several moderate PIREPs evident across the Midwest in an area of null to light turbulence forecasts. However, large thunderstorms are evident in the satellite imagery in this region. Finally, in the northwest there are also a few moderate PIREPs in the area of a moderate turbulence forecast, also in clear air. This example illustrates the skill of GTG with turbulence forecasts in clear air; however, the current version does not explicitly forecast turbulence associated with convection.