Model simulation of convectively lofted ice contribution to stratospheric water vapor

Changes in the amount of stratospheric water vapor can affect both the chemistry and climate in the stratosphere and troposphere. One important process that can affect stratospheric water vapor is convectively lofted ice near and above the tropopause, which can evaporate and contribute to stratospheric water vapor. Here we conduct several experiments using a trajectory model driven by two chemistry-climate models (CCMs) to study the contribution of lofted ice to tropical lower-stratospheric water vapor. We show that the largest amount of evaporation of convectively lofted ice occurs above the Lagrangian cold point. This occurs in two key regions: the Asian monsoon region during June, July, and August and the tropical western Pacific during December, January, and February, regions where convection frequently occurs and the air is subsaturated. The net contribution of lofted ice is then transported to the rest of the stratosphere by the general circulation. Over the 21st century, the increase of the degree of subsaturation makes it possible for more lofted ice to evaporate, thus contributing to the increase of stratospheric water vapor over the 21st century.