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Model Description

A stratospheric nudging Chemical Transport Model has been developed in NIES based on CCSR/NIES AGCM (Center for Climate System Research, University of Tokyo/National Institute for Environmental Studies Atmospheric General Circulation Model), which has been developed by Numaguti (1993), Numaguti et al. (1995), and Numaguti et al. (1997). A detailed description of the dynamical, radiative, and chemical component of a privious version of our chemistry coupled GCM was given in Takigawa et al. (1999). The model has 30 atmospheric layers, as shown in Table 1 of Takigawa et al. (1999). The top level is located around 70 km. The new nudging CTM was developed by incorporating a nudging module into the model and by replacing the chemical scheme with a more sophisticated one that has been developed in NIES and used in a 1-D coupled chemistry-radiation model (Akiyoshi, 2000).

The new model includes BrO_X chemistry and heterogeneous reactions on NAT/ICE clouds in the stratosphere as well as the O_X, HO_X, NO_X, hydrocarbons, and ClO_X gas phase chemical reactions for the stratosphere. The chemical species and the families predicted numerically in this model are O_X (O(¹D) + O + O₃), NO_X (N + NO + NO₂ + NO₃), ClO_X (Cl + ClO + 2Cl₂O₂ + ClOO + OClO), BrO_X (Br + BrO), CH₄, CO, N₂O, CCl₄, CFCl₃, CF₂Cl₂, CH₃CCl₃, CH₃Cl, CClF₂CCl₂F, CHClF₂, H₂O, HF, H₂O₂, HNO₃, HNO₄, N₂O₅, ClONO₂, HCl, HOCl, CF₂ClBr, CF₃Br, CF₂Br₂, CHBr₃, CH₃Br, HBr, HOBr, BrONO₂, Cl₂, Br₂, NO_y (NO_X + HNO₃ + HNO₄ + 2N₂O₅ + ClONO₂ + BrONO₂), Cl_y (ClO_X + HCl + HOCl + ClONO₂ + BrCl + 2Cl₂), and Br_y (BrO_X + HBr + HOBr + BrONO₂ + BrCl + 2Br₂). CH₃O₂, CH₃OOH, CH₂O, OClO, and BrCl were also predicted, but photochemical equilibrium concentrations were assumed during daytime. HNO₄ was also predicted, but photochemical equilibrium concentrations were assumed in the troposphere during daytime. The HO_X family (H + OH + HO₂) and following species are assumed to be in photochemical equilibrium; O(¹D), O, O₃, H, OH, HO₂, HO_X, N, NO, NO₂, NO₃, CH₃, CHO, Cl, ClO, Cl₂O₂. ClOO, Br, and BrO. The concentrations of these species were calculated by partition equations in the families. Nighttime consentrations of O(¹D), O, H, OH, N, NO, CH₃, CH₃O, CHO, Cl, and ClOO were assumed to be zero.

Thirteen heterogeneous reactions were considered in the model. These reactions were tabulated in Table 2 of Sessler et al. (1996). The code for a box model version of the SLIMCAT model was incorporated into the nudging CTM. In this work, only NAT and ICE were considered as PSCs.

Photolysis rates of chemical species were calculated directly from the outputs of the solar radiation fluxes in the model. The solar energy absorbed by all radiatively active chemical species, which was calculated by the convergence of solar radiation fluxes in an atmospheric layer, was distributed into the energy absorbed by each chemical species, weighted by absorption cross sections of chemical species (Akiyoshi, 2000).

The photolysis processes in the Schumann-Runge bands of H₂O₂, N₂O, NO₂, HNO₃, HNO₄, N₂O₅, ClONO₂, HCl, HOCl, Cl₂O₂, CCl₄, CFCl₃, CF₂Cl₂, CH₃CCl₃, CH₃Cl, CClF₂CCl₂F, BrONO₂, BrCl, HBr, CF₂ClBr, CF₃Br, CF₂Br₂, CH₃Br, CHBr₃ were not included in the previous versions of our chemical 3-D models. The CCSR/NIES AGCMs, which are the basic frame of the nudging CTM, do not include the ultraviolet radiation less than 200 nm. Thus in the new CTM, the photolysis rates in the Schumann-Runge bands were calculated separately by using the Schumann-Runge band radiation flux parameterization developed by Minschwaner et al. (1993), and added to the photolysis rates at wavelengths more than 200 nm that were computed by the radiation code of the CCSR/NIES AGCM itself. The Schumann-Runge photolysis of NO was calculated by using Allen and Frederick (1982)'s parameterization.

The zonal wind velocity, the meridional wind velocity, and the temperature of ECMWF data were input at 0:00 UT every day, interpolated linearly with respect to the time step of the model, which is variable between 20 minutes and several minutes according to computation stability. Then the interpolated values were assimilated into the model with the nudging method,

dx/dt = - (x-x_obs)/?Ó, x = u, v, T,

where u is zonal wind velocity, v is meridional wind velocity, and T is temperature, x is GCM values of u, v, and T, x_obs is the ECMWF data values (observation values), ?Óis the time scale of nudging. The time scales of 1 day and 5 days were used for this study. Above 10 hPa, where no ECMWF data exist, monthly, zonal-mean CIRA temperature data were input into the model every month, and interpolated linearly with respect to the model time step, and nudged to the zonal-mean values of the model temperature. Thus vertical wind velocity was calculated in the model by the continuity equation.