The Explicit Planetary Isentropic-Coordinate (EPIC) Atmospheric Model

Abstract We describe a new general circulation model (GCM) designed for planetary atmospheric studies called the EPIC model. This is a finite-difference model based on the isentropic-coordinate scheme of Hsu and Arakawa (1990. Mon. Wea. Rev. 118, 1933–1959). We report on previously undocumented modifications, additions, and key practical issues that experience running the model has revealed to be important. The model integrates the hydrostatic primitive equations, which are valid for large-scale atmospheric dynamics and include gravity waves (buoyancy waves), planetary waves (Rossby waves), and horizontally propagating sound waves (Lamb waves), but not vertically propagating sound waves because of the hydrostatic approximation. The vertical coordinate is entropy in the form of potential temperature, which coincides with material surfaces for adiabatic motion. This means that there is no vertical velocity except where there is heating, which improves accuracy and helps the model maintain conservation properties over long integrations. An isentropic vertical coordinate is natural for the atmospheres of Jupiter, Saturn, Uranus, and Neptune, which are believed to have essentially adiabatic interiors that match up with the bottom of the model and is also excellent for middle-atmosphere studies on any planet. Radiative processes are parameterized by Newtonian cooling, and the latent heat of ortho–para hydrogen conversion is included when appropriate, with a suitably defined mean potential temperature. The model is written with general map factors that make it easy to configure in oblate spherical, cylindrical, or Cartesian coordinates. The code includes optional Message Passing Interface (MPI) library calls and hence runs on any Unix-based parallel computer or network cluster. An optional graphical user interface to commercial visualization software facilitates control of the model and analysis of output. Memory is allocated dynamically such that the user does not recompile to change horizontal or vertical resolution or range. Applications to date include comet impact forecasts and hindcasts for Jupiter, meridional circulation studies of Uranus and Neptune, and the accompanying paper on three-dimensional simulations of Neptune's Great Dark Spot (1998. Icarus 132, 239–265).