Adapting the MPAS-Atmosphere Dynamical Core for Geospace Applications

In modeling the behavior of the whole earth atmosphere, there is increasing need for a consolidated nonhydrostatic atmospheric model that can accurately represent the influences of tropospheric and stratospheric dynamical variability on upper atmospheric circulations, and vice versa. The Model for Prediction Across Scales (MPAS) is a global nonhydrostatic model designed for the shallow atmosphere that has proven to be robust and efficient in simulations for a wide range of weather applications. As part of model unification efforts at NCAR , we are investigating the viability of extending the MPAS equations and numerics to effectively simulate the dynamics of the atmosphere from the surface to the upper regions of the thermosphere (∼500 km). For this purpose, a number of modifications to the MPAS dynamical equations are required to render the model potentially suitable for geospace applications. These include using the actual geocentric distance in the governing equations and grid mesh configuration, allowing variable atmospheric composition and its coupling to the dynamics and thermodynamics, and accommodating (potentially large) kinematic viscosity and thermal diffusivity. Using a 2-D MPAS prototype model, we have incorporated these extensions and have confirmed that the basic split-explicit numerical integration of the model equations remains viable for deep atmosphere configurations. Modifications to the dynamical solver are not extensive, partly because the influence of atmospheric composition on dynamics occurs only through variations in mean the gas constant R ̅ and heat capacity c ̅p. Initial testing for idealized mountain wave simulations demonstrate excellent performance over a wide range of horizontal scales, and emphasize the dramatic influences of molecular viscosity and thermal conductivity in the upper atmosphere.

Temperature perturbations for a mountain wave simulation with a 500 km domain height for a the representative temperature profile and corresponding variations in R and cp (left panel)
Figure: Temperature perturbations for a mountain wave simulation with a 500 km domain height for a the representative temperature profile and corresponding variations in R and cp (left panel). Nonhydrostatic mountain waves are produced for a 50 m/s wind flowing over a 100 m bell-shaped mountain having a 25 km half width. Eddy viscosity terms are included to constrain the exponential growth in wave amplitude with height that would occur in the absence of molecular viscosity.