SOCRATES: Chapter 1

1. Overview

The NCAR interactive chemical dynamical radiative two-dimensional (2-D) model has undergone substantial changes since its description was last published (Brasseur et al 1990). This report presents the details of the most recent version SOCRATES (Simulation of Chemistry, Radiation, and Transport of Environmentally important Species). Major changes in SOCRATES from the previous version include :

Model domain extended up to 120 km altitude to include mesospheric and thermospheric processes ( molecular diffusion, simplified ionic productions, tidal wave forcing etc.).
UV Radiative transfer that accounts for multiple scattering by air molecules, aerosols and clouds.
Parameterization of aerosol and cloud radiative effects in the infrared.
Non-LTE infrared radiation for the mesosphere.
Planetary wave model to parameterize interactions between planetary wave and mean flow.
Update of the gravity wave forcing parameterization.
Option of imposing QBO forcing.
Circulation boundary condition moved down to 2 km and determined from physical principles.
Tropospheric winds and temperature calculated interactively with specified forcing.
Semi-Lagrangian transport scheme used to solve for chemical transport.
Tropospheric hydrocarbon chemistry.
Diurnal averaging scheme that allows for diurnal variation for chemical species.
Parameterization of convective and frontal induced vertical transport in the troposphere.

The model domain extends from the surface to 120 km with a 1 km vertical resolution, and from -85^o to 85^o latitude with a 5^o latitudinal resolution. Time step for the thermodynamic and the chemical transport equations can be varied, but for a general run, a time step of 1 day is being used. The radiative heating is calculated every 5 days. Different time steps can also be assigned for the time integration of the planetary wave model. Currently, a time step of 1 day for the wave model is used. In order to describe approximately the diurnal variation of chemical species, the chemical equation without the effect of transport is time integrated with 8 timesteps per day, with 4 timesteps per daytime, and 4 timesteps per nighttime.

SOCRATES has the capability of accounting for multiple-scattering of air molecules, aerosols and clouds in the calculation of the solar heating rate and photolysis rates. This is performed by solving a multiple-scattering radiative transfer equation using the two-stream delta-Eddington method. Absorption cross sections of chemical compounds have been updated to more recent data, and improved parameterization of the Schumann-Runge bands for O2 photolysis rates and the delta bands for NO photolysis rates were implemented.

In order to facilitate mesospheric studies of the model, a non-LTE CO2 infrared radiative code of the mesosphere is incorporated in the model in place of the simple Newtonian cooling formulation. Solar heating from chemical recombination and solar energy loss to airglow process which are important in the mesosphere are taken into account. To consider the effect of molecular diffusion on the chemical species and heat budget, molecular diffusion and thermal conductivity are implemented in the chemical transport and the thermodynamic calculations.

New methods to estimate the dynamical forcing from planetary and gravity waves have been implemented. Planetary wave momentum forcing is calculated from a quasi-geostrophic wave model that takes into account dissipation caused by Newtonian cooling, Rayleigh friction, and wave breaking. For grravity wave forcing, the standard run utilizes the Lindzen (1981) formulation, although another option of a parameterization scheme that utilizes the observed energy spectral characteristic of gravity wave motions is available. Another update in the dynamic aspect of the model is the option of including a quasi-biennial oscillation (QBO) type forcing (deduced from observed zonal wind oscillation) in the temperature and circulation fields.

The lower boundary of the circulation and temperature has been moved from the tropopause level down to 2 km altitude. In addition, the lower boundary condition of the stream function at 2 km is interactive with the model-derived wave forcings. The temperature in the troposphere is explicitly calculated from thermodynamics consideration, through the specification of tropospheric wave momentum flux and latent heating according to climatology. This lessens the constraint of the lower stratospheric circulation to the lower boundary condition, and allows some degree of interaction between the troposphere and the stratosphere.

The most significant improvement made in the chemical module of the model is in treating explicitly the diurnal variation affecting chemical species. For this purpose, the timestep in the chemical equation has been shortened (8 timesteps per day). This eliminates the need to diurnally average the photolysis rates for the chemical time integration. In addition, the chemical family technique used in the previous version of the model has been replaced by a formulation in which each chemical compound is treated seperately. Another improvement made is in the representation of tropospheric chemistry, in particular hydrocarbon chemistry. Chemical species and reactions important in the troposphere which may have non-negligible consequences in the stratosphere were added, including the chemistry of C2H6, C3H6, CH2O, PAN etc. In addition, a simple parameterization of vertical tropospheric tracer transport in convective and frontal regions is included in the model.

The logistical structure of SOCRATES (shown in Figure 1 ) is slightly modified from the previous version of the model in that solar heating rate is estimated along with the photolysis calculation instead of being separately calculated. The model starts off with using initial temperature and concentrations of chemical species to calculate the zonal wind (from geostrophic approximation), solar heating and infrared cooling rates. The planetary and gravity wave forcing, along with the eddy diffusivity, is then estimated according to the zonal wind profile. With this wave forcing and heating information, the circulation is derived. Subsequently, the model calculates the temperature for the next time step, according to thermodynamic principles from the circulation and heating rates. From here on, it calculates the photolysis rates, solar heating rate, and the concentration of the chemical species, and then continues on to the next time loop.