Other
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusmsa51b..01d&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #SA51B-01
Other
3346 Planetary Meteorology (5445, 5739)
Scientific paper
Creating a single general circulation model (GCM) that achieves both accuracy and impartiality for gas-giant and terrestrial atmospheres, and is implemented for all known atmospheres, is the primary development goal for the Explicit Planetary Isentropic-Coordinate (EPIC) atmospheric model. The idea is to make it easy to practice comparative planetology on the now more than one dozen atmospheres observed inside and outside the solar system, and---given that many parameterizations are needed to make GCMs work---to make it hard to fall into the trap of artificial tuning. Towards the goal of accuracy, we are using an isentropic vertical coordinate, which minimizes vertical truncation errors and increases the accuracy of long-range transport of chemical species and of moisture; interestingly, the latter yields better predictions than non-isentropic coordinate models even for non-isentropic storm activity, because the "fuel" is delivered more precisely. However, the isentropic-coordinate approach is not accurate at the bottom of either terrestrial or a gas-giant atmospheres, each for a different reason. For terrestrial atmospheres, isentropes tend to intersect topography at steep angles, causing technical headaches, and for gas giants, convection in their interiors renders entropy nearly constant and therefore not a viable coordinate. Enter the idea of a hybrid-isentropic vertical coordinate, which smoothly transitions into a pressure coordinate towards the bottom of the model. The hybrid idea provides both accuracy and impartiality because topography can be handled with a terrain-following pressure coordinate, traditionally called sigma, and a sigma coordinate works equally well for the deep atmospheres of gas giants, where the model's bottom layer may be chosen conveniently to be a constant-pressure surface. We describe the implementation of this idea in the EPIC model, including our definition of the hybrid coordinate, the calculation of the hybrid vertical velocity, and the calculation of potential temperature (the preferred isentropic variable in meteorology), all of which are new. In addition to core features like its coordinate system, the choices one makes in developing GCM components affect how well they can handle a variety of atmospheres. On the one hand, it is not obvious at this point that we know enough to develop a planetary boundary layer (PBL) model that is equally accurate for Venus, Mars, Earth, and Titan, not to mention Io, Triton, and Pluto. On the other hand, we arguably do know enough to achieve this goal for other subgrid-scale processes, most notably turbulence and cloud microphysics. As an example of the tenor of algorithm we seek, a new turbulence model for EPIC is under development based on the Detached Eddy Simulation (DES) approach, which, unlike the traditional Reynolds Averaged Navier-Stokes (RANS) or Large-Eddy Simulation (LES) approaches, is equally accurate for modeling both wall-bounded and interior flows.
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