Development mechanisms for Mediterranean tropical‐like cyclones (medicanes)

Abstract

Midlatitude cyclones with characteristics similar to tropical cyclones (also known as Tropical‐Like Cyclones, TLCs, or medicanes) are sometimes observed in the Mediterranean region. The Wind Induced Surface Heat Exchange (WISHE) mechanism has been considered responsible for their development, in analogy with tropical‐cyclone theory. However, some recent papers have proposed a different explanation, suggesting that the deep warm core in the TLC is mainly an effect of the seclusion of warm air in the cyclone core. To investigate the latter hypothesis, two case‐studies of Mediterranean TLCs are analysed here by means of high‐resolution numerical experiments. The evolution of the near‐surface equivalent potential temperature is followed along back‐trajectories around the cyclone centre, showing for both cases a strong heating when the parcel moves from the outer part of the cyclone to its inner, warmer core. Sensitivity experiments clarify the mechanism of cyclone intensification and the way the warm‐core structure is generated, showing that sea‐surface fluxes and/or condensation latent heating are fundamental to explain the intensification of the cyclones. However, the importance of air–sea interaction processes is case dependent. For the first cyclone, the intense sea‐surface fluxes, associated with tramontane and cierzo winds over the western Mediterranean Sea, transfer a large amount of energy from the ocean to the atmosphere in the area where the cyclone developed, so that the vortex is able to sustain itself in a barotropic environment and reach a tropical‐like structure at a later stage in its lifetime. For the second cyclone, the cyclone never develops a fully tropical‐like structure, evolving in the baroclinic environment associated with the potential vorticity streamer in which the cyclone formed. Based on the distinction emerging in this and other articles, a classification of medicanes in three different categories is proposed.

The Precipitation Structure of the Mediterranean Tropical-Like Cyclone Numa: Analysis of GPM Observations and Numerical Weather Prediction Model Simulations

“…Other TLC characteristics as the small horizontal extension of Numa, the low vertical wind shear, the axisymmetric shape, and the calm eye of the Medicane, were well captured by the model. These results, in agreement with previous model-based studies on Medicanes (e.g., Lagouvardos et al [2], Miglietta and Rotunno [5], Davolio et al [13], Picornell et al [21]), support the passive MW observation analysis, showing that during the TLC phase warm rain processes occur in the area surrounding the eye, while weak convective activity is observed in small portions of the eyewall. We have analyzed how DPR measurements, available over the sea, can be also used in NWP data assimilation.…”

“…From the above analysis, it follows that the differences between R16 and R17 are mainly in the tracks followed by the TLCs of the two simulations, showing the important role of initial and boundary conditions for the forecast of this kind of events, similarly to other studies ( [5,13] among Figure 14. Numa track between 12:00 UTC on 17 November and 00:00 UTC on 19 November simulated by RAMS-ISAC starting at 12:00 UTC on 16 November (asterisks) and at 12:00 UTC on 17 November (filled circle).…”

“…From the above analysis, it follows that the differences between R16 and R17 are mainly in the tracks followed by the TLCs of the two simulations, showing the important role of initial and boundary conditions for the forecast of this kind of events, similarly to other studies ( [5,13] among others). Therefore, the consequences on the precipitation field forecast over the land are significant.…”

“…The forecast of Medicane Numa was characterized by a significant change of the prediction of its position depending on the model initialization time. In an unstable flow regime, such as Numa, the sensitivity of the predicted storm features, for example the position of the cyclone eye, to the initial and boundary conditions is well known [5,13]. Nevertheless, in this section we highlight important aspects related to an operational NWP model setup [13].…”

“…An important feature of the Medicanes is the surface wind, Figure 16b shows the vertical cross-section, at the same latitude of Figure 16a of the equivalent potential temperature and of the meridional wind speed. Typical features of TLC are apparent as the state of nearly-moist neutrality of ascending air parcels, the limited horizontal extension of the TLC, the axisymmetric shape, the windless region in proximity of cyclone center (the TLC calm eye), and the low vertical wind shear up 6 km height [5,7,13].…”