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 H 2 O changes. H 2 O 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 explored using particle measurements from the Halogen Occultation Experiment (HALOE) to derive the equivalent gas phase H 2 O contained in PMCs. Comparing these estimates to HALOE water vapor measurements suggests that PMC evaporation is a large component in H 2 O enhancement near 83 km during summer.