December 2022 north American bomb cyclone: Insights from perturbation energetics

Explosive cyclone development has been shown to be a positive feedback process, however what causes this to end is an open question. We employ the perturbation potential energy (PPE) framework to investigate the evolution of the December 2022 North American bomb cyclone (NABC) as a case study. A layer with negative PPE anomaly separates the positive PPE anomaly generated by the December 2022 NABC from a positive PPE anomaly near the tropopause during the development phase. This layer of negative PPE anomalies acts as a barrier such that positive PV anomalies cannot pass through. At the start of the decay phase, this barrier breaks, allowing stratospheric intrusion. Here we propose a mechanism where the negative upper pole of the vertical PV dipole caused by atmospheric latent heating (the lower, positive pole is involved in the positive feedback loop of development) triggers a cascade of events, where this PV anomaly causes tropopause deformation, which causes circulation anomalies, which causes enhanced energy conversion, which causes the negative PPE anomaly barrier to break, allowing a positive PV anomaly airmass to descend (indicative of stratospheric intrusion) through the break, leading to the suppression of further atmospheric latent heat release, and therefore ending the development phase of this bomb cyclone, despite the non-depletion of moisture in the vicinity of the December 2022 NABC. While this process is shown for a specific case study, the improved understanding of the life cycle of explosive cyclones has the potential to improve forecasting of explosive cyclones in the future.

Plain language summary

Bomb cyclones over land can have large social and economic consequences. Bomb cyclones are known to get stronger in part due to a cycle involving the increased potential vorticity below a level where water vapor condenses. The reasons that explosive cyclones stop developing has yet to be determined, and finding this mechanism could improve forecasts and therefore improve preparedness. In this paper we conduct a case study on a bomb cyclone that happened over North America in late December 2022 using some traditional approaches combined with a newer energetics method focusing on the local perturbations of the potential and kinetic energies. We find that this bomb cyclone stopped getting stronger, not because it ran out of water, but because of a chain of events starting from the decreased potential vorticity above the same condensation. Further research is needed to see if this process exists in other bomb cyclones, and if it can be turned into a predictive tool for forecasting.