Extra-tropical cyclones, often also called low pressure systems and mid-latitude cyclones, are the cause of most of the day-to-day variability in weather in the mid-latitudes. The over arching aim of this area of research is to fully understand the dynamics of these systems in our current climate, particularly the dynamics of extreme or unusual storms, and also identify how extra-tropical cyclones will change in the future as our climate becomes warmer.
We use OpenIFS, a state-of-the-art global numerical weather prediction model. Recently we have used OpenIFS to analyse the dynamics and evolution of two historical, unusual extra-tropical cyclones: windstorm Mauri and Hurricane Ophelia and to study how extra-tropical cyclone may change in the future.
Windstorm Mauri caused considerable damage in northern Finland on 22 September 1982 and, at the time, forecasters speculated that this windstorm was related to Hurricane Debby, a category four hurricane. Our OpenIFS simulations show that Hurricane Debby underwent extra-tropical transition which resulted in ridge building and an acceleration of the jet stream but ex-Debby did not re-intensify immediately. Instead ex-Debby travelled rapidly across the Atlantic as a diabatic Rossby wave-like feature. When ex-Debby approached the UK, it moved ahead of an upper-level trough and rapid re-intensification began. Ex-Debby then underwent 24 hours of rapid deepening before affecting northern Finland as storm Mauri.
Hurricane Ophelia was a category 3 hurricane which underwent extratropical transition and made landfall in Europe as an exceptionally strong cyclone in October 2017. We used specially developed software, consisting of a generalized omega equation and vorticity equation, to investigate the relative contribution of different physical processes (thermal advection, diabatic processes etc. ) to the transformation of Hurricane Ophelia to to an intense mid-latitude cyclone are studied. Vorticity advection, which is often considered an important forcing for the development of mid-latitude cyclones, played a small role in the re-intensification of Ophelia as an extratropical storm. Diabatic heating was the dominant forcing in both the tropical and extratropical phases of Ophelia. In particular diabatic heating from the convection scheme was the dominant forcing during the hurricane phase whereas diabatic heating from the microphysics scheme was more dominate during the extra-tropical phase.
We have also used OpenIFS to investigate how the characteristics and spatial structure of extra-tropical cyclones may change in the future as the climate warms. To do this, we have used OpenIFS configured as an aqua-planet and have run simulations where we uniformly warm the whole surface of the Planet. The results show that warming does not increase the median intensity of extra-tropical cyclones but that the strongest cyclones intensify. The amount of precipitation associated with the cyclones increases by almost 50% and the location of the precipitation moves further downstream away from the cyclone centre. These results indicate that the spatial structure of extra-tropical cyclones may change in the future as the climate.