Appearance in satellite images

In addition to the Waves described in this CM which show an increasing cloud bulge at the rear of frontal cloud bands, there is a sub-group of Waves which show a double structure of Wave configurations with the brighter cloud tops in the leading (usually more eastern) Wave feature and lower cloud tops and even dissipation in the rearward (usually more western) Wave feature; at the end of the process cloud upstream from the main cloud bulge has largely disappeared. In WV images there is a clear indication that upper level processes are involved. In this manual, this subtype is called "Double Structure Wave". Such situations occur quite often; the presentation below is based on an investigation of four cases of "Double Structure Waves".

It should be noted that a third type of Wave ("Upper Wave") is described in this manual (see Upper Wave ). Whether all three are different types of Waves or wether they are connected still has to be determined.

The two diagrams below show a simplified development of classical and Double Structure Waves.

In the case of a classical Wave (left) a cloud bulge increases in size and curvature. During the development a Dark Stripe seen in the WV image intensifies along and parallel to the rear side of the frontal cloud band. Quite often, convective clouds develop at the leading edge of the Dark Stripe which is also a position close to the maximum cloud bulge.

In the case of a "Double Structure Wave" two approximately parallel Wave bulges develop early in the development. During the development a Dark Stripe (in WV) overruns the rear part of the double structure leading to a further lowering of cloud tops in that region. Consequently, there mat be quite an abrupt dissipation of the rearward Wave cloud.

The 6-hourly loops below left show IR and WV images of a classical Wave, those on the right a "Double Structure Wave". The classical Wave images also show the development of Cb clouds over mid Italy moving into the northern Adriatic Sea. This cloud phenomenon should not be confused with the increasing Wave bulge which moves from Austria southeastwards to Slovenia and Croatia.

Meteorological Physical Background

WV imagery strongly shows the involvement of the upper relative stream and the dry intrusion in the "Double Structure Wave".

The following diagrams, derived from several cases, show the typical distribution of conveyor belts and the difference between the two Wave types.

In the classical Wave a Warm Conveyor Belt is the main cause of cloudiness within the cloud bulge. In the "Double Structure Wave" the Warm Conveyor Belt can only be found well in front of the Wave bulge, at the leading edge of the front. It is the upper relative stream which can be more clearly identified in the area of the Wave bulge. Also the dry intrusion overruns the rear part of the Wave, resulting in cloud dissipation.

The two images below demonstrate these findings. There is an extended Warm Conveyor Belt in the main part of the Wave bulge whilst in the second case the Warm Conveyor Belt is far in front of the "Double Structure Wave". There is an upper relative stream and a dry intrusion overrunning the more westerly Wave cloud.

Key Parameters

Surface Low

As the diagram below shows, the surface low is close to the point of inflection of the "S" - shaped rearward cloud boundary of the classical Wave (left). This is also true for the "Double Structure Wave" (right) but the low is associated with the leading Wave configuration.

Upper level trough and PVA max

In the case of a classical Wave (left) the upper level trough lies some distance from the cloud feature resulting in the PVA max lying mainly at the rear of the Wave bulge. In the case of a "Double Structure Wave" the upper level trough line is very close to the rearward Wave resulting in the PVA max being situated more above the cloud of this Wave. The images below support this configuration. In the classical Wave (left) there is an intensive surface trough immediately south of the Wave bulge; in the "Double Structure Wave" there is a closed surface low close to the leading Wave formation and a secondary trough in the surface height near to the rearward (more westerly) cloud bulge. Upper level trough and PVA max are shown in the two images below. The PVA max lies behind the frontal cloud and the Wave bulge in the case of a classical Wave but is superimposed on the lower cloud of the rearward Wave in the "Double Structure Wave".

Height of PV (PV anomalies)

When looking at the field of potential vorticity (height of PV = 2 units), differences can be seen between the two types of Wave: In case of the classical Wave (left) the gradient of height of PV = 2 units is immediately behind the frontal cloud within the area of the WV Dark Stripe, in the case of the "Double Structure Wave" it lies above the rearward Wave. In both cases, the area of lowest height of stratospheric PV is behind the whole cloud system but much further away in case of the classical Wave. All cases investigated showed a good correspondence between the rearward edge of the cloud (leading system for the "Double Structure Wave") and the 300 hPa isolines. The two examples below show these differences very well. Particularly clear in the "Double Structure Wave" is the coincidence between the high gradient of height of PV = 2 units and the dark grey cloud band to the rear.

Conclusion

For the double structure wave there seems to be a distinct influence of the upper level trough and its accompanied PVA. The rearward Wave feature seems to be the result of PVA and vorticity (preferably curvature vorticity; see Upper Wave). However, at the same time sinking dry air with high values of PV is overrunning this second cloud feature, leading to the typical lowering and dissipation of cloud (compare Rapid Cyclogenesis). Another upper level influence, running hand in hand with these processes, is a possible interaction between stratospheric PV (PV anomaly) and the surface low, which consequently can be associated with the leading Wave feature.