On the Role of Helicity in Organizing Diabatic Potential Vorticity in Rotating Convective Storms

Mesoscale convective systems (MCS) generate a vertical dipole in potential vorticity (PV) due to diabatic heating from condensation. The positive pole is often manifest as a cyclonic mesoscale convective vortex (MCV) that plays a key role in the maintenance and organization of the MCS. On the convective-storm scale, condensational heating generates a horizontal dipole in PV tendency, as described in Chagnon and Gray (2009). In the presence of helicity, convective storms are observed to rotate – a process that has been described theoretically as an accumulation of streamwise vorticity generated via a tilting mechanism (e.g., Davies-Jones 1984). This foundational theory makes no reference to moist thermodynamic processes. The purpose of this paper is to revisit this classic problem in mesoscale dynamics and describe the evolution of storm-scale rotation in the framework of diabatic PV generation. Using a simple analytical model as well as idealized WRF simulations, we show that the horizontal dipole in PV tendency associated with storm-scale diabatic heating in the presence of helicity results in an accumulation of either cyclonic or anticyclonic vorticity within the updraft core. Furthermore, most of the storm-scale rotation is associated with the diabatic PV. The significance of this finding is that it directly connects cloud microphysical processes to storm-scale dynamics – a finding of both theoretical and operational significance given the uncertainty in the representation of cloud microphysics in numerical weather prediction models.