The EPIC atmospheric model with an isentropic/terrain-following hybrid vertical coordinate

The explicit planetary isentropic coordinate (EPIC) atmospheric model has been upgraded to use a hybrid vertical coordinate, ζ, that transitions continuously from potential temperature, θ, aloft to a function of a pressure coordinate, σ, that is terrain following near topography. The result is a model that simulates terrestrial and gas-giant atmospheres equally well. Considering that surface pressure varies by orders of magnitude from one planet to the next whereas topography has a roughly similar geometric scale everywhere, we define σ in terms of log p rather than the traditional p. We include a pure-sigma region at the bottom that allows for accurate modeling of the planetary boundary layer (PBL) for terrestrial applications and the deep atmosphere for gas-giant applications. We describe the functional form for ζ ( θ, σ ), the method used to calculate θ, and the method used to calculate the hybrid vertical velocity, over(ζ, ̇), all of which are new. Potential temperature is only predicted in the pure-sigma region while in the hybrid region it is found diagnostically; in a complementary manner, pressure is predicted in the hybrid region and at the surface but found diagnostically in the pure-sigma region. The hybrid vertical velocity, over(ζ, ̇), is calculated directly near the beginning of each timestep rather than iteratively at the end. A brief description of the model's new turbulence scheme is included. To compare with previous models and to illustrate the flexibility of the hybrid coordinate, we run the Held-Suarez benchmark for Earth and a published Great Dark Spot simulation for Neptune.