Estimation of cross-tropopause airmass fluxes at midlatitudes: Comparison of different numerical methods and meteorological situations

Airmass flux across the tropopause modifies the budget of chemically reacti ve minor constituents in the stratosphere and the troposphere. Flux estimat es reported in the literature exhibit large discrepancies, mainly due to th e application of different estimation algorithms and examination of differe nt episodes. The different studies also focus on different exchange mechani sms. With the aim of contributing to clarification of the situation, two di fferent methods of cross-tropopause mass transfer calculations are implemen ted into the mesoscale-alpha European Air Pollution Dispersion model system and discussed: a trajectory-based analysis and the method developed by Wei . These methods are applied to an episode in February 1997, when a deep str atospheric intrusion occurred over the North Atlantic and western Europe. C omparison of the results shows good agreement between the net flux values c omputed by the different methods, both yielding values near 1 x 10(-3) kg m (-2) s(-1) for the net airmass flux from the stratosphere to the tropospher e. Analyzing the tendency of potential vorticity along trajectories, it is shown that in this case turbulent processes surpass the diabatic ones and a re mainly responsible for the transformation from stratospheric into tropos pheric air. Employing the method of trajectory analysis, different meteorol ogical situations are investigated in order to establish a broader range of cross-tropopause transport estimates for middle latitudes. For all analyze d cases a net downward transport from stratosphere to troposphere was found . The results vary between 0.6 and 1.0 x 10(-3) kg m(-2) s(-1) for the airm ass flux across the two potential vorticity unit surface, taken as the dyna mical tropopause.