Abstract
Abstract. In this study, we address the question of the atmospheric processes that turn Mediterranean cyclones into severe storms. Our approach applies online potential vorticity (PV) budget diagnostics and piecewise PV inversion to WRF model simulations of the mature stage of 100 intense Mediterranean cyclones. We quantify the relative contributions of different processes to cyclone development and therefore deliver, for the first time, a comprehensive insight into the variety of cyclonic systems that develop in the Mediterranean from the perspective of cyclone dynamics. In particular, we show that all 100 cyclones are systematically influenced by two main PV anomalies: a major anomaly in the upper troposphere, related to the baroclinic forcing of cyclone development, and a minor anomaly in the lower troposphere, related to diabatic processes and momentum forcing of wind. Among the diabatic processes, latent heat is shown to act as the main PV source (reinforcing cyclones), being partly balanced by PV sinks of temperature diffusion and radiative cooling (weakening cyclones). Momentum forcing is shown to have an ambiguous feedback, able to reinforce and weaken cyclones while in certain cases playing an important role in cyclone development. Piecewise PV inversion shows that most cyclones develop due to the combined effect of both baroclinic and diabatic forcing, i.e. due to both PV anomalies. However, the stronger the baroclinic forcing, the less a cyclone is found to develop due to diabatic processes. Several pairs of exemplary cases are used to illustrate the variety of contributions of atmospheric processes to the development of Mediterranean cyclones: (i) cases where both baroclinic and diabatic processes contribute to cyclone development; (ii) cases that mainly developed due to latent-heat release; (iii) cases developing in the wake of the Alps; and (iv) two unusual cases, one where momentum forcing dominates cyclone development and the other presenting a dual-surface pressure centre. Finally, we focus on 10 medicane cases (i.e. tropical-like cyclones). In contrast to their tropical counterparts – but in accordance with most intense Mediterranean cyclones – most medicanes are shown to develop under the influence of both baroclinic and diabatic processes. In discussion of medicane-driving processes, we highlight the need for a physical definition of these systems.
Highlights
The Mediterranean basin is among the most cyclogenetic regions in the world (Petterssen, 1956; Alpert et al, 1990; Maheras et al, 2001; Neu et al, 2013)
We implemented an online diagnostic into WRF and applied it to 100 simulations of the most intense systems that occurred during a 10-year period, from 2008 to 2017
This diagnostic decomposed potential vorticity (PV) into different components, each related to a parameterised physical process, and enabled piecewise PV inversion to provide further insights into the role of different processes in cyclone development
Summary
The Mediterranean basin is among the most cyclogenetic regions in the world (Petterssen, 1956; Alpert et al, 1990; Maheras et al, 2001; Neu et al, 2013). The use of PV-budget diagnostics or piecewise PV inversion to perform a processbased classification of cyclones is still missing from the state of the art Such an approach would extend the analysis of Campa and Wernli (2012), who analysed the vertical PV profile of extratropical cyclones of different geographical origin, and would be complementary to earlier efforts by Deveson et al (2002), followed by Plant et al (2003) and Gray and Dacre (2006), where extratropical cyclones were classified into three types according to the contribution of the upper and lower troposphere to their development.
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