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Turbulence Help

GTG Graphics

To retrieve a GTG-3.0 graphic, select "Turbulence" under the "Weather Products" navigation menu. Then click on the Graphical Turbulence graphic to access plots. The plot menu will show the plot type including Clear-Air, Mountain-Wave and Combined plots. The vertical level and forecast time are also available from pull-down menus. Altitudes are available for every 2000 feet, starting at 1000 feet and ascending to FL450. In addition to individual altitudes, you can select a composite, maximum value of all altitudes, labeled "Maximum." This image provides a quick overview of the national turbulence threat. The forecast time menu will show times from 00hr (the analysis) to 18 hours. Arrow buttons are available to clickly navigate through levels and forecast times.

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 in Figure 1. 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) 0700 UTC on 01 Jul 2015. The map is based on a 0-hour (lead time) GTG forecast initiated from a 2200 UTC on 28 Sep 2015 RAP model output.

Sample GTG-3 Graphic
Figure 1: Sample GTG Graphic

The legend will show the relative intensities for each type of aircraft. The aircraft weight (Heavy, Medium, Light) can be selected from the drop down menu. The legend will show the turbulence intensity based on EDR value as either light, moderate, severe or extreme in the color bar. The bar for each aircraft weight is shown below:

Heavy Aircraft: Heavy legend
Medium Aircraft: Medium legend
Light Aircraft: Light legend

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 Figure 2, below. On the left are contours of "Light" and "Moderate or Greater" GTG values at FL300 for a 1 hr forecast valid at 16 UTC 19 Oct 2009. A flight path has been selected from the Portland, Oregon area to the Billings, Montana area. The right image (Figure 3) 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 Oregon at this altitude and "Moderate or Greater" clear-air turbulence could be expected near the Oregon-Idaho border and again over western Montana. 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.

Flight Path Tool GTG-2 Plain View
Figure 2: Flight Path Tool depictions of GTG
Turbulence in plain view
Flight Path Tool GTG-2 Cross-Section
Figure 3: Flight Path Tool depictions of GTG-2
Turbulence in cross-section

Overview of GTG Display

All graphics display atmospheric turbulence intensity as energy (or eddy) dissipation rate to the 1/3 power, i.e. EDR =ε1/3 where ε is the eddy dissipation rate in units of m2/s3). Typically EDR varies from close to 0, "smooth", to near 1, "extreme for most aircraft types. The display colors of EDR range from white near 0 to violet near 1.

Note: Actual EDR display contours are multiplied by 100 for ease of interpretation and colors are provided in intervals of 10 starting at 5 as shown in the color scale in the lower right. It is important to note that EDR is an atmospheric turbulence intensity metric and individual aircraft response to this depends on the type of aircraft, its weight, airspeed, altitude and configuration. Therefore, the EDR color bar is augmented with a second scale which provides the range of aircraft-dependent nominal expected response in the standard turbulence categories of "null"ΒΈ "light", "moderate", and "severe". The ICAO definitions used for this translation are:

  • Light < 15,500 lbs maximum takeoff weight (examples are Cessna 320, Piper Cub, LJ23)
  • Medium (or large) 15,500 - 300,000 lbs maximum takeoff weight (examples are A320, B737, Gulfstream, MD80)
  • Heavy > 300,000 lbs maximum takeoff weight (examples are A330-A380, B747, B777)

The aircraft class-specific thresholds are defined in the following table:

AC weight class Estimated EDR Threshold * 100
Light Moderate Severe Extreme
Light 13 16 36 64
Medium (Large) 15 20 44 79
Heavy 17 24 54 96