by ZAMG
However, in the physical background of an Upper Wave some processes can be distinguished from "classical" theory:
The observed cloud bulge of an Upper Wave is a sign of a disturbance in the upper stream, but there is no corresponding and deepening pressure minimum at the surface.
The dominating driving mechanisms in the upper levels can also be seen from:
A sharp and pronounced trough at 300 hPa, leading to an increase of curvature vorticity. An additional reason for the release of relative vorticity is the right entrance region of the associated jet streak, supporting ageostrophic circulation at upper levels within the stream field.
The dark zone in WV 6.2 imagery to the rear of the Wave bulge is a clear sign of significant tropopause folding to the rear of the cloud band, also reflected in high values of potential vorticity lower down in the troposphere.
Seen in terms of conveyor belts, the dominating mechanism of the "classic" Wave is a Warm Conveyor Belt superimposed on the Wave bulge. In contrast, the cloud bulge of an Upper Wave usually lies completely within cold advection at lower levels, caused by the intrusion of cold air within this layer. At upper levels (above 500 hPa) regions of more or less warm advection can be seen, associated with ageostrophic circulation. At 300 hPa and higher strong warm advection appears associated with the bulge of the Upper Wave. This maximum can be related to the stable air from the tropopause which is sinking to the rear of the Wave point.
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The case of 22 August 2005 at 12.00 UTC shows such a distribution of relative streams. In the image below left the Cold Front can be seen in the vertical cross section. The relative streams on the 310K isentrope displays sinking motion to the rear, and rising above the Upper Wave feature (below upper right).
22 August 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values
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22 August 2005/12.00 UTC - Meteosat IR 10.8 image; magenta: relative streams 310K - system velocity: 222° 2 m/s, yellow: isobars 310K, position
of vertical cross section indicated
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22 August 2005/12.00 UTC - Meteosat IR 10.8 image; magenta: relative streams 316K - system velocity: 222° 2 m/s, yellow: isobars 316K
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22 August 2005/12.00 UTC - Meteosat IR 10.8 image; magenta: relative streams 324K - system velocity: 222° 2 m/s, yellow: isobars 324K
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Upper Wave | Wave |
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Moderate rising motion - mainly at upper levels | Distinct rising motion and ageostrophic circulation within the cloud band |
Narrow and pronounced trough at upper levels to the rear of the frontal cloud band | Significant surface trough or separate low; upper level trough to the rear of the frontal cloud band |
Cold advection at lower levels due to domination of the relative streams from behind the Cold Front | Dominating Warm Conveyor Belt leads to maximum of temperature advection (WA) at lower levels |
Dominating upper relative stream; rising over Wave bulge, sinking behind | Dominating rising Warm Conveyor Belt over the Wave bulge at middle to upper levels |
Sinking of potential vorticity indicates tropopause folding, no PV >=1 units at middle and low levels of the troposphere | Sinking of potential vorticity indicates tropopause folding, leading to PV > 1 units at middle and low levels causing stronger circulation at lower and middle levels. |
22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 322K - system velocity: 222° 2 m/s, yellow: isobars 316K
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16 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 304K - system velocity: 181° 13 m/s, yellow: isobars 304K
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