Lagrangian analysis of three heavy precipitation events during MAP

The origin and the physical history of the air-masses involved in severe orographic precipitation events are crucial features in the various mechanisms observed during the MAP IOP's (stationary orographic convection, frontal precipitation, etc.). The transport of coherent Lagrangian air-masses is studied with the aid of three Eulerian passive tracers initiated with the initial coordinates of each grid-cell. This allows the later identification of each air-parcel by referring to its initial position. This method is carried out in the framework of high-resolution nonhydrostatic numerical simulations of the MAP IOP's #2, 3 and 14, with the model Meso-NH. The simulations are performed with three two-way interactive nested models, with a maximum horizontal resolution of 2.5 km over the MAP target area in order to simulate explicitely the convection. They have been validated with respect to operational analyses. The aim of the study is to point out the analogies between the mechanims of alpine heavy precipitation generation for these three IOP's. Particular attention is given to low-level humid air-masses advected over the Mediterranean, the Adriatic Sea and the Po-Valley, but also to the interaction with high-tropospheric air-masses (e.g dry and cold advection). For example during the IOP #2, a cold front extending from the North Sea to Gibraltar is associated with a deep low. The warm conveyor belt ahead of the cold front is well visible with a Lagrangian air-masses based visualisation (figure). The selected ensemble of parcels is coherently taken away by the southerly wind jet and stretched into a thin belt, with uplift up to 9000 m while they are advected northwards (with diabatic heating of about 20 K - not shown).