Vertical Cross Section Wave A: 2nd July 2004:00UTC

Isentropes
The isentropes show a configuration which is typical for an occlusion or a system with a pronounced thickness ridge in front of the cold front. The coldfront part in the isentropes shows two phenomena:

1). A downward inclined zone of high gradient as it is necessary for a frontal surface

2). A bulge close in lower layers which is characteristic for a highly unstable situation. The unstable situation leads to several showers which are indicated in the cross section.

The high values in the IR image are clearly reflected in the pixel values in this cross section. Generally the values of WV are lower than the IR values which reflects the fact that this is an area of dry values in upper layers.

The VCS was laid from position A to B.

Temperature Advection
The field of TA in relation to the frontal surface is very close to the typical one would expect for an occlusion cloud band. There is WA in the center of the isentropic trough which is in front of the coldfront zone. The whole cold front zone is under CA.

Convergence
The field of divergence shows a double structure. The cold front part is accompanied by a (backward inclined) zone of convergence. But there is also a second maximum of convergence only in upper levels which corresponds to the high cloud part in the frontal band.

Vertical Motion (Omega)
There is intensive upward motion on top of the frontal surface and especially in middle and upper layers. This corresponds to the field of convergence. Both systems, the frontal cloud band and the upper part within the frontal cloud band are reflected in the field of upward motion.

Vorticity
The frontal zone is connected to a zone of high cyclonic values for vorticity. This is a consequence of the fact that there is a trough in low and high layers.

Vorticity Advection
Corresponding to the vorticity field an intensive field of positive vorticity advection is in front and on top of the frontal surface. The maximum in upper levels is situated above the highest cloud part in the leading part of the cloud band.

PV
The lowest part in the WV signal is connected to a small PV anomaly. High values of PV representing either stratospheric values or at least a transition from tropospheric to stratospheric values are down to 500 hPa.