A long way of water vapor from the Asian Summer Monsoon into the stratosphere

During the StraoClim Geophysica campaign, moist air with total water mixing ratios up to 200 ppmv was observed within the Asian Summer Monsoon anticyclone, above the local cold point tropopause (CPT). High ozone mixing ratios of up to 250 ppbv suggest substantial stratospheric moistening. We used 60-day back- and forward trajectories to classify the observations into two groups based on their distance to the Lagrangian dry point (LDP): those where the LDP has just occurred or is still expected to occur (type A, 0-3 days from LDP), and those where the LDP was passed 15-35 days before (type B). We applied a microphysical box model (CLaMS-Ice) and a simple freeze drying model (FDM) to simulate the evolution of ice mixing ratios along the trajectories. Type A air masses, with ice mixing ratios larger than 1 ppm, underwent multiple transitions between the solid and gas phase, in good agreement with CALIPSO ice and MLS water vapor observations of around 5 ppm. In contrast, type B air masses showed less agreement with CALIPSO ice and significantly overestimated MLS observations when CLaMS-Ice or FDM were applied. However, water vapor reconstructed from the LDP of the merged back- and forward trajectories agreed much better with MLS, indicating that the wet air masses of type B, observed up to 1.7 km above the CPT, are not representative of the large-scale water vapor distribution detected by MLS. Our results suggest that the full backward and forward evolution of the sampled air masses needs to be considered when inferring stratospheric moistening in the Asian monsoon region. Water vapor concentrations set by LDPs seem to be a better proxy for the stratospheric water vapor budget than rare observations of enhanced water mixing ratios above the local CPT. These observations call into question their applicability to quantify long-term stratospheric water vapor trends.