SPADE H2O measurements and the seasonal cycle of stratospheric water vapor

We present measurements of lower stratospheric water vapor obtained during the SPADE mission with a new high precision, fast response, Lyman-alpha hygrometer. The H₂O data show a distinct seasonal cycle. For air that recently entered the stratosphere, data collected during the fall show much more water vapor than data from the spring. Fast quasi-horizontal mixing causes compact relationships between water and N₂O to be established on relatively short time scales. The measurements are consistent with horizontal mixing times of a few months or less. Vertical mixing appears to cause the seasonal variations in water vapor to propagate up to levels corresponding to air that has been in the stratosphere approximately one year.