Global MPAS-A Simulations with a Constant Pressure Upper Boundary

Abstract. The height based vertical coordinate in the Model for Prediction Across Scales – Atmosphere (MPAS-A) was recently modified to optionally allow the upper boundary of the model domain to follow a constant pressure surface, which enables the vertical expansion and contraction of the atmosphere in response to diabatic heating and cooling for deep atmosphere applications extending into the mesosphere and thermosphere. The current study extends the testing of this generalized upper boundary formulation to assess its viability in global environments and for its use in real data applications. For a domain extending well into the thermosphere, an idealized 2-D diurnal heating test case was adapted to a 3-D global configuration and used to simulate a nearly steady periodic diurnal response that is qualitatively similar to the behavior seen in a full physics simulation with the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) at the spring equinox. For the real data test cases, the top boundary height was lowered to a nominal height of 70 km in order to initialize MPAS-A with data interpolated within the data range of IFS ERA5 analyses. Simulations on a 120 km global mesh conducted in each of the four seasons confirmed the stability of the numerical integrations and were in good qualitative agreement with the ERA5 analyses in comparisons at 10 days. The suitability of this constant pressure upper boundary formulation for higher horizontal resolution applications was also documented in simulations on a 15 km global mesh and a 60-3 km variable resolution mesh.