Idealized numerical simulations of quasi-stationary convective systems over the Northwestern Mediterranean complex terrain

Simulations numériques idéalisées de systèmes convectifs quasi-stationnaires au-dessus du terrain complexe de la région côtière méditerranéenne du Nord-Ouest

Bresson, Emilie ; Ducrocq, Véronique ; Nuissier, Olivier ; Ricard, Didier ; Saint-Aubin, C. de

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Northwestern Mediterranean coastal regions are frequently affected by torrential rainfall associated with quasi-stationary mesoscale convective systems (MCSs). The present work examines how the characteristics of a conditionally unstable flow impinging on the coastal complex terrain of the Northwestern Mediterranean can affect the location and intensity of quasi-stationary MCSs. The study is based on idealized simulations, but including the major ingredients of Northwestern Mediterranean heavy precipitation events: a moist conditionally unstable marine flow of about 100 km width facing the true terrain, composed of the Massif Central surrounded by the Alps and the Pyrenees. We find that MCSs are located upstream of the mountain range with a slow flow, whereas with a rapid flow the heaviest precipitation is over the Massif Central slopes. In a similar way, when the lateral environment is drier the heaviest precipitation is located upstream whereas a humid environment favours precipitation over the slopes. The dominant lifting mechanism is strongly related to the location of the system: (1) direct orographic triggering for systems over mountain slopes and (2) cold-pool triggering for upstream systems. In addition, the neighbouring mountains interplay through deflection of the flow and induced low-level convergence favoured by slow or dry lateral environments and through cold-pool blocking within valleys. Copyright © 2012 Royal Meteorological Society

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