STORM-EX: Statistical modelling of post-tropical cyclones

Post-tropical cyclones is the name given to tropical cyclones that have completed extratropical transition (ET), a process by which their tropical characteristics are replaced by those of extratropical cyclones. These storms pose a hazard for areas outside the typical tropical cyclone extent, as they pivot to draw energy from the sharp gradients surrounding the midlatitude jet stream. They bring high winds and large amounts of precipitation to areas unused to such fury. However, the lack of data on these storms is relatively scarce due to their infrequent occurrence (in the North Atlantic basin ~50% of the ~14 tropical cyclones that form annually undergo ET). With increasing global warming, the risks to areas such as Europe will increase, as increasing sea surface temperatures allow the storms to maintain more of their tropical characteristics in areas previously incapable of supporting tropical convection.

The HURRIFIC project aims to create a model capable of generating a global dataset of transitioning storms so the risks that they pose can be accurately calculated. This model, called STORM-EX, will be built as an extension of STORM (Synthetic Tropical cyclOne generRation Model) which has had success in generating synthetic tropical cyclone tracks and pressure profiles. Using data from a combination of observations, model simulations, and literature, we will simulate ET start and end, jet stream interactions, transition pathways, wind speed, and storm lysis. As a statistical model cannot capture all the complex interactions taking place during ET, this involves creative solutions to parametrize these cyclone features. Initial work has shown the model can reproduce ET start patterns and frequencies with skill, as well as reproduce basic interactions with the jet stream.