J. Klemp, MMM/NCAR

For applications extending into the thermosphere, realistic simulations may require an upper boundary that permits vertical expansion/contraction of the atmosphere in response to strong internal heating/cooling rates. In adapting MPAS for geospace applications, a modification of the height-based vertical coordinate is presented that relaxes the rigid-lid constraint and permits the coordinate surfaces at upper levels to transition toward a constant pressure surface at the model’s upper boundary. This modification is conceptually similar to a terrain-following coordinate at low levels, but now modifies the coordinate surfaces at upper levels to conform to a constant pressure surface at the model top. Since this surface is evolving in time, the height of the upper boundary is adaptively adjusted to follow a designated constant pressure upper surface. This alteration in the original height-based vertical coordinate employed in MPAS requires only minor modifications to the numerics and little additional computational expense. The viability of this modified vertical coordinate formulation is verified in a 2-D prototype of MPAS for an idealized case of upper-level diurnal heating.