Transverse cirrus bands: a satellite signature of potential turbulence.
06 June 2022
08 November 2009
By Jochen Kerkmann (EUMETSAT) and Gerhard Venter (South African Weather Service)
Air turbulence can be a serious hazard for air navigation. Because aircraft move so quickly, they experience sudden accelerations or 'bumps' as they rapidly cross bodies of air which are moving vertically at different speeds. Cabin crew and passengers on airliners have been injured, and, in a small number of cases, killed. Of particular importance for air traffic is so-called Clear Air Turbulence (CAT], which is defined by Wikipedia as the "erratic movement of air masses in the absence of any visual clues, such as clouds". CAT is impossible to detect either with the naked eye or with conventional radar, meaning that it is difficult to avoid. However, it can be remotely detected with instruments that can measure turbulence with optical techniques, such as scintillometers or Doppler LIDARs.
There are various mechanisms that can lead to (clear air) turbulence in the atmosphere. The most common mechanisms are:
1) Shear instability along jets and upper fronts
2) Flow over obstacles (mountains, thunderstorms)
3) Convection (thunderstorms, dry thermals)
4) Low level "mechanical" (strong winds over rough terrain)
5) Wake turbulence from other aircraft
As indicated above in the first bullet, (clear air) turbulence typically occurs near jet streams and other regions of significant wind changes in the vertical and/or horizontal direction (see conceptual model). Jet streams can easily be identified using NWP model data or MSG water vapour or Airmass RGB images (see e.g. case study: Jets in the South Atlantic), but it is difficult to know where exactly turbulence may occur in the area of a jet stream as usually the flow in jet streams is quite smooth. However, often there are satellite signatures that suggest the presence of (clear air) turbulence, although the altitude cannot be determined accurately. One of the indications is the presence of well-defined cirrus bands oriented nearly perpendicular to the upper winds (see conceptual model). The most intense turbulence is related to wide, thick bands easily seen in Meteosat imagery.
An example of 'transverse cirrus bands' is given in the Airmass RGB image shown below. Large transverse cirrus bands can be seen over southern Namibia and smaller ones over the centrals parts of South Africa (see Interpretation). As not much flying occurs in the southern part of Namibia or the north-western part of South Africa, it is difficult to verify if there was turbulence in the area of the transverse cirrus bands or not. However, looking at the forecast winds and the significant weather (SIG WX) charts it is likely that there was turbulence in the suspect area.
It should be noted that transverse cirrus bands (sometimes called radial cirrus clouds) indicating turbulence can also be observed above or around deep convective clouds. An example is described in the case study of 25 May 2009. In the morning of that day, Meteosat-8/-9 observed an interesting convective system over northern France, with strong overshooting tops, a high-level plume and radial cirrus clouds. The Meteosat-8 rapid scan loop (HRV channel) from 6:00 to 7:00 UTC shows the high-level plume and the radial cirrus clouds very nicely. Gordon Bridge, who was flying through the area of radial cirrus clouds from London over Belgium to Frankfurt noted some moderate turbulence at around 35,000ft in the area of Brussels, which interrupted the serving of refreshments for a short while. This moderate turbulence could well have been so-called 'convectively induced turbulence', as described in a presentation from Wayne Feltz (2007).
GOES-8, IR, 24 November 1995 (source: G. Ellrod, NOAA)
Met-9, Airmass RGB, 30 August 2009 (source: SatRep Online)
Met-9, Airmass RGB, 13 September 2009 (source: SatRep Online)
Hong Kong Observatory: Presentation on turbulence
Annelise Lenz: Identification of transverse band signature in satellite imagery
Kristopher Bedka et al.: Satellite signatures ass. with significant convectively-induced turbulence events