Adapting the MPAS dynamical core for applications extending into the thermosphere

To extend the nonhydrostatic global Model for Prediction Across Scales (MPAS) for deep-atmosphere (geospace) applications, we have modified the model equations and numerics to include variable atmospheric composition and (potentially large) molecular viscosity and thermal conductivity. The split-explicit numerical integration techniques in MPAS remain stable in idealized test cases for atmospheric domains extending into the upper thermosphere and continue to provide an efficient numerical framework for nonhydrostatic simulations. Variations in the atmospheric constituents influence the dynamical equations by altering the heat capacity and ideal gas constants. These feedbacks require little alteration of the dynamical equations although our testing reveals that the amplitude of disturbances may be sensitive to even small variations in the thermodynamic coefficients. Although the potential temperature is no longer formally conserved for adiabatic flow, it remains effective as a prognostic thermodynamic variable in the model equations. Molecular viscosity and thermal conductivity are dominant influences in the upper thermosphere and are represented implicitly in the model numerics. Because of the large magnitude of these terms, their treatment, though stable, may significantly under represent the true magnitude of their damping effects. Further consideration of these deep-atmosphere extensions to MPAS will be explored in more realistic simulations of thermospheric dynamics.