SEASONAL-VARIATION OF MIDDLE ATMOSPHERIC CH4 AND H2O WITH A NEW CHEMICAL-DYNAMICAL MODEL

A new zonally averaged, chemical-dynamical model of the middle atmosph ere is used to study the processes which control the distributions and seasonal variability of CH4 and H2O. This model incorporates a nondif fusive, nondispersive advection scheme, a time-dependent linear model of planetary wave drag and horizontal mixing (K-yy), a new parameteriz ation of gravity wave drag and vertical mixing (K-zz), and an explicit treatment of LTE (local thermodynamic equilibrium) and non-LTE IR coo ling. Model chemistry is calculated using a Newton-Raphson iterative s cheme, which allows consistent simulations of 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 s how that in the tropical stratosphere their vertical distributions are strong functions of both the methane oxidation rate and the ascent ra te, the latter driven by a combination of tropospheric latent heat rel ease and atmospheric drag. at low latitudes HALOE observations and mod el results both show conservation of ''potential H-2'' (2xCH(4)+H2O) b elow similar to 50 km. However, the conservation of potential H-2 from HALOE observations breaks down above similar to 55 km, while the mode l shows conservation well into the middle mesosphere (similar to 70 km ). This may suggest serious inadequacies in our understanding of the p hotochemistry of water vapor and mesospheric HOx, in particular those processes which control the partitioning of H-2 and H2O. At high latit udes, H2O model/data comparisons suggest that horizontal mixing is imp ortant 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 i nfluence the descent rate, both horizontal mixing and the methane phot ochemistry play important roles in determining the CH4 mixing ratio. F inally, we suggest that the observed interhemispheric asymmetry in the seasonal cycle of mesospheric H2O may be linked to larger values of K -zz in the southern winter mesosphere. This represents a key differenc e between mesospheric and stratospheric tracer transport. In the strat osphere, greater net unmixed descent in the southern hemisphere direct ly translates into lower tracer values relative to the northern hemisp here, while mesospheric tracer transport shows the opposite behavior.