Abstract
Abstract. This paper describes CHEM2D-H2O, a new parameterization of H2O 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 H2O 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 H2O measurements.
Highlights
The middle atmosphere (15–100 km altitude) is extremely dry when compared to the troposphere, detailed knowledge of the water vapor distribution in this region is important for a number of reasons
The abundance of middle atmospheric water vapor is an important factor controlling the formation of both polar stratospheric clouds in winter and polar mesospheric clouds near the summer mesopause, the former being important for heterogeneous ozone loss
The purpose of this paper is to introduce the CHEM2D-H2O photochemistry parameterization and evaluate its performance based on NOGAPS-ALPHA forecast model simulations of middle atmospheric water vapor
Summary
The middle atmosphere (15–100 km altitude) is extremely dry when compared to the troposphere, detailed knowledge of the water vapor distribution in this region is important for a number of reasons. The abundance of middle atmospheric water vapor is an important factor controlling the formation of both polar stratospheric clouds in winter and polar mesospheric (or noctilucent) clouds near the summer mesopause, the former being important for heterogeneous ozone loss. Water vapor is a fundamental prognostic variable in the dynamical cores of most numerical weather prediction (NWP) models. For these reasons, NWP and data assimilation (DA) systems whose top levels extend into the upper stratosphere and mesosphere require an accurate description of the photochemical sources and sinks of water vapor
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