Medicanes as subtropical cyclones: the December 2005 case from the perspective of surface pressure tendency diagnostics and atmospheric water budget

Abstract

Mediterranean cyclones with tropical-like characteristics such as spiral cloud coverage and a central cloud-free “eye” are referred to as medicanes. These systems have been analyzed due to their relation with high-impact weather. In previous studies, the identification of medicanes has been typically performed subjectively, using satellite pictures, but also objectively through three-dimensional diagnosis of a warm core and an axisymmetric structure. Despite the presence of these characteristics, it is still unclear if medicanes show dynamical similarities with tropical cyclones. We analyse the (thermo-)dynamics of a recognized medicane that occurred in December 2005 by applying different diagnostics to a high-resolution simulation. These diagnostics are focused on the intensification, dynamical structure and water budget of this representative case, aiming to highlight extratropical and tropical cyclone characteristics. Three stages in the medicane life cycle are identified. In stage I, a potential vorticity (PV) streamer reaches the Mediterranean, triggering deep convection and deepening the medicane's central surface pressure due to diabatic heating. When the lowest central pressure is reached (stage II), the medicane presents a warm core and an axisymmetric structure. However, convection is rather weak and the PV streamer evolves into a cut-off system which contributes to the deepening of the medicane's surface pressure. Finally, stage III corresponds to the decay phase where the medicane tends to weaken and lose its axisymmetric structure. Our results highlight the detrimental role of deep convection prior to the mature stage of the medicane, as well as the possibility of positive or negative feedback of upper-tropospheric dynamics on the central surface pressure. In addition, we show that the medicane warm core might be achieved due to front seclusion, while the “eye” formation is associated with dry air intrusions. Our analysis suggests that medicanes are hybrid systems combining characteristics of both tropical and extratropical cyclones and thus they plausibly correspond to subtropical cyclones.