Three-Dimensional and Trajectory Chemistry-Transport Modeling

1. results from the Goddard global three-dimensional (3-D) model of stratospheric chemistry and transport (CTM),
2. calculations of chemistry along back trajectories from the aircraft and balloon flight tracks, and
3. expert analysis for mission planning, flight forecasting, and data interpretation.

The CTM [Douglass and Kawa, 1999] utilizes winds and temperatures from the GEOS-2 meteorological data assimilation system. The resolution is 2 degrees latitude by 2.5 longitude by 28 vertical levels between the surface and 1 mbar, with spacing of about 1 km up to 20 km and 2.7 km above 21 km. The advection scheme is semi-Lagrangian [Lin and Rood, 1996] which maintains sharp gradients, does not produce unrealistic maxima or minima, and maintains appropriate correlations for long-lived constituents. A time step of 15 minutes is used. Seventeen species and four families are transported in the model. An additional twenty species are inferred. All gas-phase reactions thought to be important in the stratosphere are included; rate constants are taken from [DeMore et al., 1997]. A zonal mean distribution for water vapor is specified based on observations from the Limb Infrared Monitor of the Stratosphere. The sulfate aerosol surface area is specified based on SAGE II data and the WMO climatology. PSC volume growth occurs at vapor pressure equilibrium controlled by the 3-D temperature and condensibles distribution. The photolysis rate routine is based on the radiative transfer calculations of Anderson and Lloyd [1990] and temperature-dependent cross sections [DeMore et al., 1997] using a table-lookup method. Photolysis rates calculated in this way compare favorably with the AEAP photolysis benchmark [Stolarski et al., 1995]. The CTM initialization is based on MLS ozone, Cryogenic Limb Array Etalon Spectrometer (CLAES) N_2O, and an updated version of the Goddard 2-D CTM. For SOLVE we will initialize around September 1, 1999 and run continuously through the experimental time period.

The model for chemistry along trajectories uses the same photochemical scheme as the CTM, except that mixing ratios of the longer lived species are held constant during the trajectory. Initial mixing ratios are prescribed as much as possible from measurements and correlations with measured species. Position, pressure, and temperature as a function of time are determined from the back trajectory information. Albedo and total column ozone along the trajectories are obtained from TOMS data and scaled to the CPFM measurements on the ER-2. The heterogeneous chemistry scheme in the model, constrained by observed particle distributions, has shown good success at simulating ER-2 observations of chlorine activation [Kawa et al., 1997]. Particle composition and volume in the model evolve according to equilibrium vapor pressures [Carslaw et al., 1995], but the details of the size distribution are not simulated. Output from these models will be available in the field for flight planning and data analysis. We are focussed on the science issues of simulating observed ozone loss, modeling rates of heterogeneous processes including particle formation and chlorine activation, deactivation of chlorine following cold temperatures, evaluating observational techniques for estimating ozone loss, validation of SAGE III measurements, and effects of aircraft traffic in the upper troposphere and lower stratosphere in winter.