Joint Centre for Mesoscale Meteorology (JCMM)
JCMM Internal Report No.96:
Convective destabilisation by a tropopause fold diagnosed using potential vorticity inversion
by Morwenna Griffiths, Alan Thorpe and Keith Browning
There is observational evidence to suggest that at the time tropopause folds in extra-tropical cyclones are descending into the mid troposphere there is frequently a destabilisation of the lower troposphere to moist convection. Here we consider the dynamical reasons for such a linkage using so-called potential vorticity (PV) attribution concepts. In particular, a method is described using PV inversion to find the wind shear attributable to the tropopause fold itself. Then this shear is used to quantify the tendency to generate regions of potential instability, ie where the wet-bulb potential temperature decreases with height. The method is applied to a case of a modest tropopause fold and it is found that the potential vorticity anomaly contained in the tropopause fold can contribute substantially to the instantaneous convective destabilisation. Indeed, at some locations, it can overcome a tendency to stabilise by the atmosphere without the fold.