Lagrangian description of airflows using Eulerian passive tracers

A method of tracking coherent Lagrangian airmasses is presented. It is not based on the computation of individual trajectories, but rather on three Eulerian passive tracers initialized with the coordinates of each grid cell. This 'initial coordinates' method allows the later unambiguous identification of each Lagrangian air parcel by referring to its initial position. Further, the physical history of each parcel can be retrieved. The advantages of this method in the framework of mesoscale numerical modelling are discussed with respect to trajectory-based methods. Preliminary examples are given by studying idealized airflows (i) a two-dimensional stratified flow past an isolated obstacle generating gravity-wave breaking; (ii) a deep convective squall line. Then the method is applied to a heavy-rain situation over the Alps during the Mesoscale Alpine Programme (MAP) field phase, A coherent mesoscale pool of dry air is identified over the Mediterranean and tracked with the aid of the initial-coordinates method. Its influence on the rain distribution over the Ligurian Alps and Apennines is demonstrated.