Contribution of deep convection in the intensification of Mediterranean Tropical-Like Cyclones.
Directeurs.rices de thèses : Claud C.
Composition du jury
Phillippe Drobinski (LMD), President
Nadia Fourrié (CNRM), Reviewer
Jean-Pierre Chaboureau (OMP), Reviewer
Heini Wernli (ETH), Examiner
Mario-Marcello Miglietta (ISAC-CNR), Examiner
Konstantinos Lagouvardos (NOA-IERSD), Examiner
Chantal Claud (LMD), Thesis Director
The role of deep convection in the intensification of Mediterranean tropical-like cyclones (MTLCs) is examined in this thesis. While most of the Mediterranean cyclones present a common baroclinic life cycle where cyclogenesis is mainly triggered by upper tropospheric systems, the role of deep convection on cyclones development has been addressed only by a few studies in the recent past. In order to investigate the contribution of deep convection in the intensification of MTLCs between 2005 and 2018, the emphasis has been put on the Central and Eastern Mediterranean basin where these cyclones have received less attention than those in the Western Mediterranean.
First, the relation of deep convection with MTLCs’ formation and intensification is investigated using remote sensing techniques, through a multi-satellite approach, with observations in the infrared and microwave spectrum. In addition, the vertical wind shear and vortex tilt are calculated by ERA5 reanalysis data to study the cyclone structure evolution. Results show that only a fraction of the studied cyclones experience intense convective activity close to their centers and persistent deep convection in the upshear quadrants leads to intensification periods. Convective activity solely in the downshear quadrants is not linked to intensification periods, while short-lived hurricane-like structures develop only during symmetric convective activity.
As a second step, in order to address the impact of fine-scale thermodynamics related to deep convection and explain the observed convective activity, atmospheric modeling is employed, using the WRF model with a fine spatial resolution (3 km). To account for the effects of latent heat release during deep convection, online potential vorticity (PV) tracers are used at every model time step. In addition, a modified version of the classical pressure tendency equation (PTE) is used to post-process the numerical results to study the atmospheric dynamics related to the MTLCs. Results show that cyclone intensity changes are only partly explained by deep convection activity, with an emphasis given on the diabatically-induced low-level PV fields and diabatic heating. Finally, a possible definition and a two-fold classification of the MTLCs is proposed.