MIMOSA PV fields : MIMOSA high resolution advection model of potential vorticity (PV) allows to determine the origin of air masses. The basic assumption is that PV and tracer mixing ratio are very well correlated on an isentropic surface. The PV field is advected using ECMWF winds interpolated on the given isentropic surface. A relaxation towards the ECMWF PV field with a 10-day time constant is applied to the MIMOSA advected PV in order to follow the diabatic evolution of air masses. PV fields are computed at 3 levels : 350K in the troposphere, 380K at the tropopause and 475K in the stratosphere (A. Hauchecorne).