Seasonal variability of water vapor in the LMS during SPURT

During the SPURT projects, airborne high-resolution in situ measurements of a large set of trace gases such as water vapour and ozone in the UTLS were performed in a wide latitude region (20-80°N) covering all seasons. Based upon this data set, we derived a seasonal climatology and characteristics of tracer distributions, their gradients and wind velocities in that region around the polar jet. Elevated amounts of total water in the UTLS have been detected in all seasons with a pronounced maximum in late spring/early summer. The seasonal and latitudinal variability of extratropical tropopause temperatures determines the transport of water vapour into the LS. A mixing layer with H2O mixing ratios well above 5 ppmv, indicative for significant tropospheric contribution, ranges several ten degrees latitude polewards or, in potential temperature, several ten K above the tropopause. To identify the effects of cross-tropopause transport by advection and mixing processes in the observations, the Chemical Lagrangian Model of the Stratosphere (CLaMS) is used for high-resolution hemispherical simulations based on trajectory calculations with mixing between the considered air parcels. Comparisons between the CLaMS results and the observations show good agreement for both large and small scale features, e.g. the occurrence of clouds and filamentary structures, as well as for the seasonal variability.