Quantification of mesoscale transport across the boundaries of the free troposphere: a new method and applications to ozone

Abstract. A new Lagrangian method is proposed to quantify the transport of ozone – or any other atmospheric constituent – by objectively-defined air-masses. In the framework of mesoscale modelling, this method is an alternative to classical Eulerian or trajectory-based methods, which suffer from many drawbacks at this scale. The elementary air-parcels are tagged with their initial location (what is made possible in the model by passive transport of three tracer fields initialized with the space coordinates). This enables to retrieve not only their back-trajectories but also their physical and chemical history. This information is in turn used to relevantly define transporting air-masses along objective criteria. For instance the ozone mass that has left the planetary boundary layer (PBL) to intrude the free troposphere (FT) in a given time interval, is carried by the ensemble of air-parcels that were initially in the PBL but are finally in the FT. Such an air-mass can be characterized by e.g., a criterion on the initial and final values of the turbulent kinetic energy of the air-parcels. The last step to obtain the sought ozone mass is a simple spatial integration of the ozone concentration over the so-defined air-mass. Two case-studies are presented as illustrations with increasing complexity: (i) the downward transport of ozone accompanying a tropopause fold, across a mid-tropospheric altitude level; (ii) a case of PBL-to-FT transport, as evoked above.