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Over-running Troughs

In view "A", the mid-tropospheric trough (nominally around 500 hPa) is a fairly sharp, easily identifiable feature, with the trough axis to the rear of the surface location of the Occlusion/cold-front. Under PVA-maxima conditions, vertical (upward) motion is focussed just forward of the trough-axis, leading to thick cloud, high precipitation intensity (other factors being right). Given the location of the upper forcing relative to the surface trough, the front appears to be a rearward sloping/ana-frontal type.

trough not yet overrun

Rearward of the trough axis, lies the zone of negative vorticity advection (NVA) associated with descending air and the area of relatively low relative humidity conditions (at mid-tropospheric levels) shown.

In view "B", the primary forcing trough has now 'relaxed' away and is losing some of its shape, thus the dynamics (vorticity advection >> strong upward motion etc.) are also weakening, and for this reason alone, the frontal activity will start to fragment.

However, there is an element of 'de-coupling' also in play, as the upper activity moves away from the lower-tropospheric humid zone, and in satellite imagery, the cold topped cloud (seen via IR channels) will move well ahead of the low level frontal break (seen in VIS channels), and will appear to be divorced from surface discontinuities such as wind-shift, dew-point drops etc.

trough now overrun

The upper trough will still have vorticity forcing associated with it of course (albeit weaker), and may manifest itself as the upper cold front shown - this feature is found to the rearward of the IR cloud mass, with the major mid-tropospheric dis-continuity being the change from high relative humidity shown, and the now advancing dry/descending air associated with the region of NVA. The surface front, by and large, loses considerable activity. However, care should be taken in this case, as in spring/early summer in particular, as the drier air moves into what is effectively the warm sector, and over-runs humid low level air, destabilisation can lead to marked convective activity.