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 reactive minor constituents in the stratosphere and the troposphere. Flux estimates reported in the literature exhibit large discrepancies, mainly due to the application of different estimation algorithms and examination of different episodes. The different studies also focus on different exchange mechanisms. With the aim of contributing to clarification of the situation, two different methods of cross-tropopause mass transfer calculations are implemented into the mesoscale-α 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 stratospheric intrusion occurred over the North Atlantic and western Europe. Comparison of the results shows good agreement between the net flux values computed by the different methods, both yielding values near 1 × 10−3 kg m−2 s−1 for the net airmass flux from the stratosphere to the troposphere. Analyzing the tendency of potential vorticity along trajectories, it is shown that in this case turbulent processes surpass the diabatic ones and are mainly responsible for the transformation from stratospheric into tropospheric air. Employing the method of trajectory analysis, different meteorological situations are investigated in order to establish a broader range of cross-tropopause transport estimates for middle latitudes. For all analyzed cases a net downward transport from stratosphere to troposphere was found. The results vary between 0.6 and 1.0 × 10−3 kg m−2 s−1 for the airmass flux across the two potential vorticity unit surface, taken as the dynamical tropopause.