Meso to sub-mesoscale origins of strong surface winds and influence of surface waves in an extra-tropical cyclone

Abstract. Extra-tropical cyclones can bring extreme surface winds. Several mesoscale jets can be associated with these winds originating from the upper, mid, and lower troposphere. Sub-mesoscale processes have been shown to play a major role in the downward transport of jets, but few modeling studies represent them explicitly. This study sheds light in the relative contribution and storm-relative location of jets present in the explosive cyclone Alex which made landfall in southern Brittany in October 2020 causing extensive wind damage in Belle-Ile and further inland. A series of stand-alone atmospheric simulations and coupled wave-atmosphere simulations using the Meso-NH and WAVEWATCH-III^® models were run. Their horizontal grid spacing was incrementally halved by two going from 1.6 km (numerical weather prediction mode) down to 100 m (large eddy simulation mode). The simulated storm is consistent across resolutions at the mesoscale with little differences in extreme wind values between the kilometric and hectometric resolutions. Waves coupling is shown to decrease surface wind extremes by 0.5 to 1.5 m s^-1. Impacts on the surface drag coefficient and winds are not uniform spatially with winds decreasing ahead of the cold front and within the high wind core while decreasing elsewhere. Online trajectory calculations allow for a Lagrangian air mass tracking in Meso-NH. In the kilometric simulation, four distinct jets are shown to be responsible for the strongest winds. The evolution of state parameters along these trajectories helps match the jets to the classical conceptual model for extra-tropical cyclones. Evidence hints to the presence of a Sting Jet associated with Alex’s extreme winds, along with the more common Cold Conveyor Belt, and Dry Intrusion. The fourth jet is labeled as Potential Sting Jet on account of its similarities with the Sting Jet prior to subsidence. The Cold Jet is shown to be responsible not only for the majority of the strong wind trajectories in the lower 500 m, but also for the strongest wind speeds. Spatial distributions of these jets is further investigated in the hectometric simulation. These reveal that the Sting Jet descends towards the surface at the rear of the high wind core in the frontal fracture region. This location differs from that reported for previously studied storms where the sting jets are typically found at the forefront of the high wind core. Two distinct Dry Jets are shown to coexist, one descending to the surface around the Sting Jet and another one remaining above the Cold Jet. The Potential Sting Jet is located above the cold conveyor belt and shown to be brought down towards the lower troposphere by coherent structures invigorated by moist processes and spanning several kilometers in width and height at the outer edge of the cloud head. Unlike in previous high-resolution numerical studies, boundary layer rolls are not found to play a key role at the studied instant. Different sting jet location and processes associated with potential sting jet subsidence compared to literature, encourage more systematic studies of extra-topical cyclones through common diagnostics adapted for both kilometric and hectometric resolutions.