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C3 - Multi-scale dynamics and predictability of Atlantic Subtropical Cyclones and Medicanes

Principal investigators: Prof. Dr. Andreas Fink, PD Dr. Michael Riemer

Other researcher: Enrico di Muzio (PhD), Michael Maier-Gerber (PhD)

Preferentially during fall and winter, the Mediterranean Sea features severe storms, termed Medicanes, that exhibit phenomenological characteristics of tropical cyclones. With torrential rains and gale-force winds, these storms pose a severe threat to the densely populated shores of the Mediterranean Sea. Accurate forecasts of these treacherous storms are thus necessary to negotiate the impact of the associated severe weather.

We propose a multi-scale investigation into the dynamics and predictability of Medicanes, from the scale of the precursor breaking extratropical Rossby wave down to the organization of convection within the developing storm system. Our examination shall be compared to the process of "tropical transition" (TT) of subtropical cyclones in the North Atlantic Ocean. The overarching goal of this project is to improve our understanding of the hierarchy of predictability of relevant features on different scales.

In order to achieve this goal, an analysis of the planetary- down to mesoscale dynamics of three developing and two non-developing Medicanes will be carried out using post-2010 ECMWF operational analyses and ensemble forecasts. The precursor synoptic-scale trough will be diagnosed using an upper-level Potential Vorticity (PV) trough detection and tracking algorithm. To link the trough to a Rossby wave train and a Rossby wave breaking event, a wave activity flux diagnostic will be employed. The synoptic-scale forcing of vertical motion will be determined by inverting the omega equation under alternative balance. Contributions from upper-tropospheric balanced dynamics and from low-level frontogenesis will be distinguished.

Regarding the mesoscale forcing of convection, frictional convergence in the boundary layer will be estimated from the ECMWF data sets. It is our hypothesis that the synoptic- and mesoscale forcings of vertical motions are capable of releasing the high amount of Convective Available Potential Energy (CAPE) present in the trough area over the Mediterranean Sea, but the relative contributions will vary from case to case. The causes for the CAPE build up will be related to the synoptic-scale upper-level cold air advection in the trough region and the high surface latent and sensible heat fluxes and their convergence in the convective active regions.

The dynamic analysis will conclude with the quantification of the detrimental shear and mid-level dry air intrusion impact on Medicanes development. Applying the same sequence of diagnostics to ECMWF analysis and ensemble forecast data of three TT cases, commonalities and differences in the genesis of these storm systems will be elaborated.

An objective ensemble diagnostic of Medicane and TT dynamics will finally be developed, applied and related to their associated high-impact weather. A simple approach will be based on object-based correlation in which the overlap and amplitude of forcing objects will be correlated with the object representing storm-related deep convection.

This will be complemented by a more powerful visualization approach of coherent feature detection with the aim of identifying suitable metrics that will allow us to investigate the predictability of structural changes in the evolution of the storms in large ensembles. This will contribute to the ultimate goal to quantitatively assess the role of the various processes in re-analysis and ECMWF large ensemble forecasts on Medicane genesis and predictability. This is a prerequisite to generate grand ensembles in the anticipated second phase of W2W.