Water Vapor Enhancement in the Polar Summer Mesosphere and its Relationship to Polar Mesospheric Clouds

Polar mesospheric water vapor exhibits a strong seasonal cycle, with summer mixing ratios dramatically higher than in winter. It is generally accepted that vertical transport from ~50 km altitude towards the mesopause is one driver behind this change, however, upwelling alone cannot explain observed H2O changes. H2O near 83 km increases from 1 ppmv in winter to over 8 ppmv in summer, and upwelling accounts for roughly half of this increase. It has been suggested that evaporation of polar mesospheric clouds (PMCs) should produce a layer of enhanced water vapor. This idea was challenged using particle measurements from the Halogen Occultation Experiment (HALOE) to derive the equivalent gas phase H2O contained in PMCs. Comparing these estimates to HALOE water vapor measurements suggests that PMC evaporation is a large component in H2O enhancement near 83 km during summer.