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. Results from a detailed investigation of about 45 cases are considered for this CM.

Appearance in METEOSAT imagery

• IR 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 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 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 01 October 2001 at 10.00 UTC. In the IR 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. Cold air cloudiness marks the cold air mass to the rear of the Wave bulge. The WV image (below right) also displays a white enhanced Upper Wave bulge. There is a distinct black zone behind the Wave indicating sinking dry air. 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 (20 December 2001 at 10.00 UTC). The cloud bulge extends from the Czech Republic and Poland to the Ukraine. In the IR and WV (below left and right) no distinct sharp edge can be seen to the rear of the Wave feature, whereas the VIS image (2nd row below left) clearly shows an enhanced Wave bulge of upper level cloudiness.

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 (20 December 2001 02.00 UTC to 22.00 UTC). During the whole life cycle of the Upper Wave, there is no further development, such as amplification or enhancement of the cloud bulge. A Wave and an Upper Wave often look similar in satellite imagery. In order to distinguish between them one can highlight a number of differences:

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

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