Numerical modeling of chemical-dynamical coupling in the upper stratosphere and mesosphere

A two-timescale chemical algorithm has been developed to solve photochemist ry coupled with transport in the middle atmosphere. It is suggested that tw o continuity equations for each species be solved when transport processes prevent instantaneous chemical equilibrium. The simultaneous solutions of t he two sets of equations correspond to quasi-equilibrium and transient or f orced states of all the modeled species. The chemical solver is incorporate d in a two-dimensional model to study the chemical-dynamical coupling in th e upper stratosphere and the mesosphere for different timescales in a consi stent manner. New parameterizations for calculating photolysis rates in the Schumann-Runge bands and Schumann-Runge continuum are presented on the bas is of an optimal k distribution method. Several distinct features of measur ed tracer distributions in the mesosphere can be simulated by the model. Th ese include (1) the model daytime mean OH distribution with a secondary max imum in number density of similar to 6.5 x 10(6) cm(-3) around 70 km, (2) a semiannual oscillation in O-3 mixing ratio around 85 km that characterizes the coupling effect between the OH-O-3 photochemistry and O transport, and (3) diurnal variations of O(3)in the mesosphere controlled by both fast va rying local photochemistry and slowly varying HOx transported from below. T here is no systematic underprediction of mesospheric O-3 in our model compa rison with the measurements. Our model also predicts the morphology of chem ical heating rate around mesopause by exothermic reactions. From 80 to 95 k m the dynamically controlled atomic oxygen distribution generates a latitud inal chemical heating rate that counters the radiative heating rate gradien t.