Development of the Nonhydrostatic Unified Model of the Atmosphere (NUMA): Limited-Area Mode

This paper describes a Nonhydrostatic Unified Model of the Atmosphere (NUMA) based on a spectral element, or high-order continuous Galerkin (CG) spatial discretization utilizing 3D hexahedral elements. The nonhydro-static dynamical core, based on the compressible Euler equations, is appropri-ate for both limited-area and global atmospheric simulations. In this paper, we restrict our attention to 3D limited-area phenomena; global atmospheric simulations will be presented in a follow-up paper. A suite of explicit and semi-implicit time-integrators is presented. Domain decomposition and com-munication algorithms utilized by our distributed memory implementation is presented, allowing efficient evaluation of the the direct stiffness summation (DSS) operator. Numerical verification of the model is performed using four test cases: 1) 2D inertia-gravity waves, 2) flow past a 3D linear hydrostatic mountain, 3) flow past a 3D nonlinear mountain and 4) 3D buoyant convec-tion of a bubble in a neutral atmosphere; these tests indicate that NUMA can simulate the necessary physics of a dry numerical weather prediction dy-namical core. Scalability for the explicit dynamical core is demonstrated for 12288 cores on TACC’s Ranger cluster, while the semi-implicit core is shown to scale to 4096 cores on the same architecture