Striving towards simulating all known atmospheres equally well: vertical coordinate and pressure-gradient force

This talk presents a review of two new algorithms in the Explicit Planetary Isentropic-Coordinate (EPIC) atmospheric model designed to accurately simulate both terrestrial and gas-giant atmospheres. The model now has a pure-sigma coordinate region underlying a hybrid sigma-theta region, which can be used as a planetary boundary layer for terrestrial planets or the deep atmosphere for gas-giant planets. In the sigma region, the potential temperature, theta, is treated as a prognostic variable and the pressure is a diagnostic variable of the vertical coordinate and the surface pressure, as is standard. The novelty of our approach is to do the complement in the hybrid region: we treat the pressure as a prognostic variable and the potential temperature as a diagnostic variable of the vertical coordinate and the pressure. The effect of heating enters exclusively through the vertical velocity, which is a diagnostic variable in this hydrostatic model, just as is the case in a pure-theta coordinate model. This contrasts with other hybrid-coordinate models for which potential temperature is treated as a prognostic variable, which we argue undermines the full potential of using hybrid isentropic coordinates.

Having added a sigma region to a model that once had a pure-theta coordinate, we now have all the problems that come with sigma, most notably the large errors from the horizontal pressure-gradient force (PGF) terms in steep topography. We have experienced these problems in Venus superrotation spinup simulations with topography. We argue that for hybrid-coordinate models, with or without topography, treating the PGF with a weak-formulation (finite volume) instead of a strong formulation (traditional finite difference) has advantages for both terrestrial and gas-giant atmospheres. We have developed a fully three-dimensional finite-volume version of the PGF that we fit to our existing C-grid, which is now the model's default, and explain how it works.

This research is supported by NASA's Planetary Atmospheres and Outer Planets Research Programs and NSF's Planetary Astronomy Program.