For a cyclone to develop, the upper low must be to the west of the surface low (divergence aloft coinciding with max PVA overhead surface low).

As the cyclone evolves, it becomes occluded, warm air is lifted off the surface and cold air surrounds it, equalising the temperature differences.

Now the question: why do the upper low and surface low become vertically stacked at the occlusion process? what creates this?

Many thanks,

Stephane