Seasonal variation of middle atmospheric CH4 and H2O with a new chemical‐dynamical model

Abstract

A new zonally averaged, chemical‐dynamical model of the middle atmosphere is used to study the processes which control the distributions and seasonal variability of CH4 and H2O. This model incorporates a nondiffusive, nondispersive advection scheme, a time‐dependent linear model of planetary wave drag and horizontal mixing (Kyy), a new parameterization of gravity wave drag and vertical mixing (Kzz), and an explicit treatment of LTE (local thermodynamic equilibrium) and non‐LTE IR cooling. Model chemistry is calculated using a Newton‐Raphson iterative scheme, which allows consistent simulations of species with highly nonuniform chemical lifetimes. In this study we focus on the sensitivity of model CH4 and H2O to the magnitude of tropospheric latent heat release, planetary wave and gravity wave activity, and the methane oxidation rate. Model results show that in the tropical stratosphere their vertical distributions are strong functions of both the methane oxidation rate and the ascent rate, the latter driven by a combination of tropospheric latent heat release and atmospheric drag. At low latitudes HALOE observations and model results both show conservation of “potential H2” (2×CH4+H2O) below ∼50 km. However, the conservation of potential H2 from HALOE observations breaks down above ∼55 km, while the model shows conservation well into the middle mesosphere (∼70 km). This may suggest serious inadequacies in our understanding of the photochemistry of water vapor and mesospheric HOx, in particular those processes which control the partitioning of H2 and H2O. At high latitudes, H2O model/data comparisons suggest that horizontal mixing is important in determining the observed latitudinal gradient in mesospheric water vapor. We also find that inside the polar winter vortex, while the strength of tropical latent heat forcing and planetary wave drag influence the descent rate, both horizontal mixing and the methane photochemistry play important roles in determining the CH4 mixing ratio. Finally, we suggest that the observed interhemispheric asymmetry in the seasonal cycle of mesospheric H2O may be linked to larger values of Kzz in the southern winter mesosphere. This represents a key difference between mesospheric and stratospheric tracer transport. In the stratosphere, greater net unmixed descent in the southern hemisphere directly translates into lower tracer values relative to the northern hemisphere, while mesospheric tracer transport shows the opposite behavior.