THE EXPLICIT PLANETARY ISENTROPIC-COORDINATE (EPIC) ATMOSPHERIC MODEL

We describe a new general circulation model (GCM) designed for planeta ry atmospheric studies called the EPIC model. This is a finite-differe nce model based on the isentropic-coordinate scheme of Hsu and Arakawa (1990. Mon. Wea. Rev. 118, 1933-1959). We report on previously undocu mented modifications, additions, and key practical issues that experie nce running the model has revealed to be important. The model integrat es the hydrostatic primitive equations, which are valid for large-scal e atmospheric dynamics and include gravity waves (buoyancy waves), pla netary waves (Rossby waves), and horizontally propagating sound waves (Lamb waves), but not vertically propagating sound waves because of th e 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 e xcept where there is heating, which improves accuracy and helps the mo del maintain conservation properties over long integrations. An isentr opic vertical coordinate is natural for the atmospheres of Jupiter, Sa turn, Uranus, and Neptune, which are believed to have essentially adia batic interiors that match up with the bottom of the model and is also excellent for middle-atmosphere studies on any planet. Radiative proc esses are parameterized by Newtonian cooling, and the latent heat of o rtho-para hydrogen conversion is included when appropriate, with a sui tably defined mean potential temperature. The model is written with ge neral map factors that make it easy to configure in oblate spherical, cylindrical, or Cartesian coordinates. The code includes optional Mess age Passing Interface (MPI) library calls and hence runs on any Unix-b ased 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 res olution or range. Applications to date include comet impact forecasts and hindcasts for Jupiter, meridional circulation studies of Uranus an d Neptune, and the accompanying paper on three-dimensional simulations of Neptune's Great Dark Spot (1998. Icarus 132, 239-265). (C) 1998 Ac ademic Press.