Appearance in Satellite Data

An Upper Wave is a smooth Wave feature accompanied by a rather flat convex bulge of cloudiness on the rearward side of a frontal cloud band.

 

Appearance in METEOSAT 8 imagery

  • IR 10.8 imagery
    • Convex cloud bulge at the rear of a frontal cloud band, often large elongation of cloud bulge
    • Grey to white due to high level cloudiness, Wave bulge appears brighter than the surrounding frontal clouds
    • Sharp edge to the cold air behind the front
    • Fibres indicating strong upper winds or jet streak
  • WV 6.2 imagery:
    • Bright grey or white due to height of cloud bulge, which appears more enhanced (brighter) in relation to surrounding frontal cloud band
    • Sharp edge to the black dry zone to the rear of the front
    • WV dark zone at rear indicating sinking of dry air from the stratosphere
  • VIS 0.6 imagery:
    • White due to thickness of cloud bulge
    • Fibre - like character due to high clouds

At first sight the upper wave seems to look similar to a classical wave but more detailed study shows differences (see Wave ):

  • Often the Upper Wave bulge as seen in satellite imagery is less pronounced than the "classical" Wave
  • There is generally less amplification than with a "classical" Wave
  • An Upper Wave propagates rapidly along the rear side of the frontal cloudband
  • In an Upper Wave there is often a fibre cloud indicating strong upper level winds (schematic below)

The set of images below shows an Upper Wave over the Atlantic on 16 March 2005 at 18.00 UTC. In the IR 10.8 image (below left) a well developed Upper Wave bulge can be seen with multilayered bright white clouds. At its edge a fibre cloud indicates strong upper winds. The WV 6.2 image (below right) also displays a white enhanced Upper Wave bulge. There is a distinct black zone behind the Wave indicating sinking of dry air.

16 March 2005/18.00 UTC - Meteosat 8 IR 10.8 image
16 March 2005/18.00 UTC - Meteosat 8 WV 6.2 image
16 March 2005/16.00 UTC - Meteosat 8 VIS 0.6 image

In the visible image, above left, the Wave bulge has fibrous characteristics. Additionally, the cloud thickness and features are well indicated.

A second case of an Upper Wave is shown below (22 August 2005 at 12.00 UTC). The cloud bulge extends over Eastern England. The presence of the jet can be seen very well in both the IR 10.8 as well as the WV 6.2 (below left and right). The Wave feature is also much distinct, whereas the VIS 0.6 image (2nd row below left) clearly shows the fibrous clouds within the enhanced Wave bulge of upper level cloudiness.

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image
22 August 2005/12.00 UTC - Meteosat 8 WV 6.2 image
22 August 2005/12.00 UTC - Meteosat 8 VIS 0.6 image

The presence of the jet and its accompanying deepening of stratospheric air to the rear of the Wave can also wel been in the Airmass-RGB as the darker area over Southwestern England. The Airmass-RGB is an artificial composite consisting of the WV 6.2 - WV 7.3 brightness temperature difference (BTD), on red, the IR 9.7 - IR 10.8 BTD on green and the WV 6.2 channel on blue. Typical features seen on the Airmass RGBs include dry intrusions, deformation zones and jet streaks. What the other colours in this RGB represent can be found out when clicking here .

A second RGB consists of NIR 1.6, VIS 0.8 and VIS 0.6. In this so-called natural colour RGB the fibrous character of the upper Wave is well observed.

22 August 2005/12.00 UTC - Meteosat 8 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i)
22 August 2005/12.00 UTC - Meteosat 8 RGB image (NIR1.6, VIS0.8 and VIS0.6)

The Upper Wave bulge usually propagates very quickly downstream along the rearward edge of the frontal cloud band, steered by upper level winds. During this propagation it generally moves "through" the jet streak (maximum of wind speed), the cloud bulge becomes less pronounced and finally vanishes within the Occlusion point of the system, or within the anticyclonic curvature of the Warm Front Band.

The loop below shows the fast propagation of the Upper Wave bulge along the frontal cloud band (22 August 2005 06.00 UTC to 23 August 2005 00.00 UTC). During the whole life cycle of the Upper Wave, there is no further development, such as amplification or enhancement of the cloud bulge. The amplitude of the upper wave even becomes less due to an oncoming depression moving in between Iceland and Scotland.

Loop: 22 August 06.00 - 00.00 UTC hourly image loop
22 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image

 

Distinction between Wave and Upper Wave according to appearance in satellite images

A Wave and an Upper Wave often look similar in satellite imagery. In order to distinguish between them one can highlight and recognise a number of differences.

Upper Wave Wave
More elongated flat cloud bulge Distinct, more amplified cloud bulge
Sharp edge to the rear; also fibre cloud Less fibre cloud
Multilayered or high non convective cloudiness Multilayered cloudiness, convective cells embedded
Distinct, relatively fast propagation along the rear of the frontal cloud band Slow propagation, possible further cyclogenesis


22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image
04 July 2005/12.00 UTC - Meteosat 8 IR 10.8 image
22 August 2005/12.00 UTC - Meteosat 8 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i)
04 July 2005/12.00 UTC - Meteosat 8 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i)

Both waves are marked by deepening of stratospheric air behind the cloudbulge. A feature both common for the Waves, however it is more pronounced and better observed in the case of the Wave (above right). This has to do with the fact that normally no further development is expected within an upper wave in contrast to the wave (further development into Occlusion). Other differences in appearance are the clouds itself. Whereas the Wave consist of thick clouds which appear as firm structured and white in the RGB, the upper wave is more translucent and consistent of high fibrous clouds appearing more bluish (due to the cold polar air mass origin). More on the different colours observed in the Airmass RGB can be found out here