Open AccessSeasonal variation of middle atmospheric CH 4 and H 2 O with a new chemical‐dynamical model
Author(s) -
Summers Michael E.,
Siskind D. E.,
Bacmeister J. T.,
Conway R. R.,
Zasadil S. E.,
Strobel D. F.
Publication year - 1997
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/96jd02971
Subject(s) - troposphere , stratosphere , mesosphere , atmospheric sciences , advection , atmospheric model , water vapor , lapse rate , atmosphere (unit) , gravity wave , drag , chemical transport model , mixing ratio , atmospheric models , methane , physics , environmental science , meteorology , thermodynamics , chemistry , gravitational wave , astrophysics , organic chemistry
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 CH 4 and H 2 O. This model incorporates a nondiffusive, nondispersive advection scheme, a time‐dependent linear model of planetary wave drag and horizontal mixing ( K yy ), a new parameterization of gravity wave drag and vertical mixing ( K zz ), 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 CH 4 and H 2 O 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 H 2 ” (2×CH 4 +H 2 O) below ∼50 km. However, the conservation of potential H 2 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 HO x , in particular those processes which control the partitioning of H 2 and H 2 O. At high latitudes, H 2 O 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 CH 4 mixing ratio. Finally, we suggest that the observed interhemispheric asymmetry in the seasonal cycle of mesospheric H 2 O may be linked to larger values of K zz 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.