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.

Development mechanisms for Mediterranean tropical‐like cyclones (medicanes)

“…The eastward movement of the medicane close to the northern coast of Africa (Libya, Egypt) was characterized by significant dust advection (T. Giannaros, personal communication; fig. in Fita and Flaounas, ). The cyclone kept moving eastward on 15 December and progressively weakened while the upper‐level cut‐off was absorbed by the main zonal circulation, appearing as the extreme tip of the trough extending southward from Russia.…”

“…The second cyclone is described in Fita and Flaounas (). On 9 December 2005, an elongated trough extended toward the western Mediterranean from Scandinavia forming an upper‐level cut‐off.…”

“…In the following days, the cut‐off remained trapped in the western Mediterranean, in between the Azores and the Siberian Highs. At low levels, on 12 December a weak pressure minimum, rapidly moving northward, appeared over western Libya (Fita and Flaounas, , fig. ).…”

“…). On 13 December, the cyclone was over the Mediterranean Sea, where it rapidly intensified, remaining nearly stationary close to the east coast of Tunisia, and started to show a symmetric deep warm core (Fita and Flaounas, , fig. ).…”

Assessing the predictability of Medicanes in ECMWF ensemble forecasts using an object‐based approach

“…). In fact, a unique, objective definition of what constitutes a Medicane has not yet been established in the literature (Fita and Flaounas, ). However, all storms analysed in this study share some distinctive traits, in that at some point during their life cycle they shrank notably, acquiring a highly axisymmetric circulation with strong MSLP gradients while quickly moving towards positive values of upper‐level thermal wind (i.e., building a warm core).…”