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Satellite-based Nowcasting and Aviation Program



Instrument

Turbulence

Clear-air turbulence remains a significant aviation hazard, yet by its nature it is very difficult to detect. One of the sources of clear-air turbulence is the dynamic instability associated with "tropopause folding", which describes the entrainment of stratospheric air into tropospheric levels at upper-level fronts. This research involves building a near real-time satellite product that estimates areas of tropopause folding in regions of strong humidity gradients in the GOES midwave infrared (water vapor) channel. Using an empirical relationship between upper tropospheric humidity gradients and tropopause breaks, the algorithm estimates that turbulence-generating tropopause folds protrude from some of these tropopause breaks. This product is being validated over the United States with manual pilot reports as well as newer automated aircraft reports of turbulence.

 
Figure Left: Modified GOES Water Vapor channel brightness temperature in the northern hemisphere. Right: Corresponding locations of tropopause folds in the original image, where upper-tropospheric clear air turbulence is more common; the colors in this product indicate the direction of flight most likely to encounter turbulence.

 

Turbulence generated by mountain waves represents another aviation hazard studied within the SNAAP. Mountain waves are caused when air flows over mountain ridges within a stably stratified atmosphere. Turbulence generated by mountain waves can be an aviation hazard due to strong vertical motions generated by these oscillating air currents. The MODIS channel 27 (6.535-6.895Ám) monitors upper tropospheric water vapor (WV) at 1 km spatial resolution, providing new details on the spatial characteristics and structure of turbulent mountain waves. Radiation at these wavelengths measured by the satellite sensor represents energy emitted by water vapor in the middle and upper troposphere, providing forecasters with useful information on flow patterns at these levels. Analysis of MODIS WV imagery and aircraft turbulence reports has shown that wave signatures on severely turbulent days have different characteristics in the imagery than the signatures on days that are less turbulent. Reports of severe turbulence are often associated with complex patterns, the appearance of interference or crossing wave fronts that extended downwind from the mountain ridge for a significant distance. Days that are less turbulent, as reported by pilots, often have wave signatures with a simpler pattern; such as linear features orthogonal to the wind flow oriented parallel to one another. The same analysis has been performed on GOES-12 water vapor imagery, but with higher temporal and lower spatial resolution.

Mountain Wave ProductFigure Left: GOES 12 water vapor channel brightness temperatures over Colorado and the surrounding states, with contours (value = 1.2) from the accompanying image. Right: "Confidence Score," a unitless measure of the strength of the wave patterns in the image. Corresponding automated aircraft reports at one-minute resolution are overlain on both images, where orange and red colors indicate moderate or greater turbulence.

Convectively-generated gravity wave features are a third source of turbulence being studied within the SNAAP. Gravity waves are formed when strong vertical motions within the primary thunderstorm updraft reach the tropopause, the interface between the troposphere and the stratosphere. The tropopause region and the stratosphere above are characterized by strong static stability, which provides resistance to the vertical motions within the thunderstorm below. As the storm updraft pushes into this stable layer, gravity waves are formed, which then propagate away from the updraft throughout the cirrus anvil. MODIS high spatial resolution multispectral imagery is used to observe these wave features within the thunderstorm top. From these MODIS observations, we can infer the wavelength, amplitude, orientation, and horizontal extent of these waves, which are then related to both manually reported and automated estimates of turbulence from nearby aircraft.