The extratropical transition of Hurricane Ophelia (2017) as diagnosed with a generalized omega equation and vorticity equation

Abstract

Hurricane Ophelia was a category 3 hurricane which underwent extratropical transition and made landfall in Europe as an exceptionally strong post-tropical cyclone in October 2017. In Ireland, Ophelia was the worst storm in 50 years and resulted in significant damage and even loss of life. In this study, the different physical processes affecting Ophelia's transformation from a hurricane to a mid-latitude cyclone are studied. For this purpose, we have developed software that uses OpenIFS model output and a system consisting of a generalized omega equation and vorticity equation. By using these two equations, the atmospheric vertical motion and vorticity tendency are separated into the contributions from different physical processes: vorticity advection, thermal advection, friction, diabatic heating, and the imbalance between the temperature and vorticity tendencies. Vorticity advection, which is often considered an important forcing for the development of mid-latitude cyclones, is shown to play a small role in the re-intensification of the low-level cyclone. Instead, our results show that the adiabatic upper-level forcing was strongly amplified by moist processes, and thus, the diabatic heating was the dominant forcing in both the tropical and extratropical phases of Ophelia. Furthermore, we calculated in more detail the diabatic heating contributions from different model parameterizations. We find that the temperature tendency due to the convection scheme was the dominant forcing for the vorticity tendency during the hurricane phase, but as Ophelia transformed into a mid-latitude cyclone, the microphysics temperature tendency, presumably dominated by large-scale condensation, gradually increased becoming the dominant forcing once the transition was complete. Temperature tendencies caused by other diabatic processes, such as radiation, surface processes, vertical diffusion, and gravity wave drag, were found to be negligible in the development of the storm.

How Important Are Post‐Tropical Cyclones for European Windstorm Risk?

“…High‐impact European events such as the PTCs Debby (Laurila et al, 2020), Floyd (Atallah & Bosart, 2003), and Ophelia (Rantanen et al, 2020; Stewart, 2018) have revealed the potential risk associated with PTCs in Europe. In particular, Ophelia attracted widespread scientific and public attention after impacting Ireland in October 2017 as Ireland's most damaging storm in over 50 years (Stewart, 2018).…”

The numerous approaches to tracking extratropical cyclones and the challenges they present

“…Large differences between tracks can exist at the initial and final time-steps for a variety of reasons, one being that relative vorticity is more capable of identifying a system at an earlier stage than MSLP (Hoskins and Hodges, 2002;Neu et al, 2013;Grieger et al, 2018). Rantanen et al (2020) tracked Ophelia using Hodges' TRACK algorithm (1994;1995) with input data from the Open Integrated Forecast System model and compared it to the NHC best track. TRACK identified Ophelia earlier than both MCMS and Massey and had a similar dissipation location as MCMS.…”

European Windstorm Risk of Post‐Tropical Cyclones and the Impact of Climate Change

“…Because of the importance of diabatic processes during the reintensification of post‐TC as highlighted for Ophelia by Rantanen et al. (2020) it is expected that the risk of post‐TCs in terms of gale wind force and rain will increase in the future, even without an increase in post‐TCs reaching Europe, due to the fact that a warmer atmosphere can contain more moisture (about 7% per degree Celsius following Clausius‐Clapeyron).…”