The dynamics of Storm Ciarán (2023): from diabatic Rossby wave to warm-seclusion cyclone with a sting jet

Storm Ciarán was an impactful weather system that affected western Europe in the first days of November 2023. The main threat from Ciarán were extreme winds, that particularly hit the areas around the English Channel: the highest wind gust recorded was 207 km/h at Pointe du Raz, in Bretagne (France), just as the storm was rapidly deepening. In this work, we discuss some dynamical characteristics of storm Ciarán related to the occurrence of those extreme winds: its origin as a diabatic Rossby wave, and its evolution as a warm-seclusion cyclone with the development of a sting jet.Diabatic Rossby waves (or vortices) are a particular type of cyclones driven by latent heat release along a baroclinic zone. These shallow systems have a smaller scale than usual extratropical cyclones, and are driven by the low-level vorticity generation that results from intense, localized condensational heating. Ciarán’s evolution resembled the infamous storm Lothar (1999), because both underwent explosive intensification starting from the diabatic Rossby wave stage, and both resulted in extreme winds.Rapidly deepening cyclones that develop a warm seclusion are also associated with strong surface winds, as they can provide a favorable environment to the development of sting jets. A sting jet is an airstream that accelerates as it descends out of the tip of the hooked cloud head, generating strong low-level winds into the frontal-fracture region of the cyclone. Storm Ciarán indeed originated as a diabatic Rossby wave and then developed into a rapidly deepening warm-seclusion cyclone containing a sting jet. We thus illustrate its evolution, highlighting the occurrence of a “diabatic Rossby wave-sting jet” dynamical pathway leading to the generation of high-impact winds.