Measurements of the HDO/H20 Isotopic Ratio in the Asian Summer Monsoon
Abstract: The Asian monsoon is one of the world's largest weather systems and has been thought to be one of the main pathways by which water vapor enters the UT/LS. The Chicago Water Isotope Spectrometer (ChiWIS) participated in the July/August 2017 Kathmandu StratoClim aircraft campaign, measuring water vapor and its isotopic composition between 12 and 20 kilometers to diagnose water transport processes. Water isotopic measurements can characterize the importance of overshooting convection and the extent to which convection-driven water vapor perturbations propagate to higher altitudes and contribute to the overall stratospheric water budget.
Preliminary results from StratoClim appear to support satellite measurements of the HDO/H2O ratio of UT/LS water, which show significant differences between the Asian and North American monsoons. Although we see numerous instances of detraining convection to or above 380 K, these events appear not to produce the strong isotopic enhancements seen over the North American monsoon. One potential implication is that overshooting convection may not routinely reach altitudes where the stratosphere is profoundly undersaturated, and that ice from Asian monsoon convection largely precipitates out of the UT/LS before sublimation. We discuss the instrument, its performance, and further implications of these results.