Parameterization of middle atmospheric water vapor photochemistry for high-altitude NWP and data assimilation

This paper describes CHEM2D-H2O, a new parameterization of H<sub>2</sub>O photochemical production and loss based on the CHEM2D photochemical-transport model of the middle atmosphere. This parameterization accounts for the altitude, latitude, and seasonal variations in the photochemical sources and sinks of water vapor over the pressure region from 100–0.001 hPa (~16–90 km altitude). A series of free-running NOGAPS-ALPHA forecast model simulations offers a preliminary assessment of CHEM2D-H2O performance over the June 2007 period. Results indicate that the CHEM2D-H2O parameterization improves global 10-day forecasts of upper mesospheric water vapor compared to forecasts using an existing one-dimensional (altitude only) parameterization. Most of the improvement is seen at high winter latitudes where the one-dimensional parameterization specifies photolytic H<sub>2</sub>O loss year round despite the lack of sunlight in winter. The new CHEM2D-H2O parameterization should provide a better representation of the downwelling of dry mesospheric air into the stratospheric polar vortex in operational analyses that do not assimilate middle atmospheric H<sub>2</sub>O measurements