Invigoration and capping of a convective rainband ahead of a potential vorticity anomaly

Deep convection frequently occurs on the eastern side of upper-level
troughs, or potential vorticity (PV) anomalies. This is consistent with uplift
ahead of a cyclonic PV anomaly, and consequent reduction in static stability
and increase of convective available potential energy (CAPE). Nevertheless,
the causal link between upper-level PV and deep convection has not been
proven, and given that lift, moisture and instability must all be present for deep
convection to occur it is not clear that upper-level forcing is sufficient. In this
paper we examine a convective rainband that intensified ahead of a cyclonic
PV anomaly in an environment with little CAPE (∼ 10 J kg−1
), to determine the factors responsible for its intensification. We find that the key feature was a low-level convergence line, arising from the remnants of an occluded front embedded in the low-level cyclonic flow. The rainband’s intensity and morphology was influenced by the remnants of a tropopause fold which capped convection at mid-levels in the southern part of the band, and by a reduction in upper-level static stability in the northern part of the band which allowed the convection to reach the tropopause. Ascent ahead of the trough appears to have played only a minor role in conditioning the atmosphere to convection: in most cases the ascending airstream had previously descended in the flow west of the trough axis. We conclude that simple ‘PV thinking’ is not capable of describing the development of the rainband, and that pre-existing low-level wind and humidity features played the dominant role.