Capability Development for Planetary General Circulation Model, 15-R9807

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Principal Investigators
Erika L. Barth
Timothy I. Michaels
Scot C.R. Rafkin

Inclusive Dates:  04/01/08 – 10/01/09

Background - A general circulation model (GCM), a global scale model that simulates the large scale behavior of an atmosphere, is the most complete tool available to comprehensively model planetary atmospheres. The purpose of this project was to develop a unified "next-generation" general circulation model for simulating the atmospheres of solid-body planets and moons. The new model has been developed such that it can be applied to simulate thick atmospheres (Venus, Titan), thin atmospheres (Mars), and tenuous atmospheres (Pluto, Triton).

Approach - SwRI's technical approach was to create a portable, versatile, and lasting software framework for building (and maintaining) a climate model applicable to a wide range of known bodies with atmospheres. The Flexible Modeling System (FMS) package developed at the Geophysical Fluid Dynamics Laboratory was used as the dynamical core. The core modules are primarily independent of the specific target body, though some changes to the vertical grid structure were necessary to allow for particularly tenuous atmospheres, and the time module was modified to accurately resolve non-terrestrial diurnal and annual periods. The physics packages include boundary layer processes, radiative transfer, microphysics, surface processes and moist convection. Each package was built and tested as a stand-alone column model before coupling with the dynamical core. Other models used, including the Titan Regional Atmospheric Modeling System, TRAMS, and the Mars Regional Atmospheric Modeling System, MRAMS, served as the basis for many GCM physics packages and as standards for model testing. The basic equations for each physics package apply to all target bodies, such that only a single set of subroutines need be developed. Where knowledge of a specific target body is necessary (e.g. setting up the optical constants for dust on Mars in the radiative transfer routine, or parameterizing the photochemical production of Angstrom-sized haze particles in Titan's atmosphere as a part of the microphysics package), the code is directed to subroutines from the specific target body module. Preprocessor conditional blocks are used to direct the code to this target body module; this is specified during compile time and thus does not slow down execution of the code.

Accomplishments - SwRI has successfully developed the framework to allow the dynamical core to model the atmospheres of various planetary bodies by testing it on Titan, Mars, and Pluto. A microphysics package that simulates the coagulation and sedimentation of involatile aerosols has been coupled to this framework, and simulations to create the main haze layer in Titan's atmosphere have been conducted. A radiative transfer package (including Rayleigh scattering, a mie code for determining optical parameters for aerosols, a correlated-k scheme for absorption by atmospheric gases, and two-stream computations) has been developed and tested for Mars and Titan. The surface/subsurface package from MRAMS was generalized to work on Titan and other worlds. Moist, convective parameterizations were not implemented, as they are not needed for present-day Mars, and it was determined that more research is required to accurately modify a terrestrial moist convective code for use with Titan's methane/ethane clouds.

A GCM simulation shows the formation of Titan's main haze layer. A production function simulates the photochemical creation of Angstrom-sized particles (multi-colored contours). The production function peaks near the 0.1 mbar pressure level (about 250 km above the surface). Larger particles are produced by coagulation. This is seen by the appearance of particles that have doubled in radius (green contours) about two hours into the simulation, followed by particles with radii increased by a factor of 10 appearing about nine hours into the simulation (white contours). The numbers on the contours indicate particle number density (per cubic centimeter).

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