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**Semi-implicit formulations of the Navier-Stokes equations: application to nonhydrostatic atmospheric modeling.**
*(English)*
Zbl 1237.76153

The authors present semi-implicit formulations of five different forms of the compressible Navier-Stokes equations (NSE) used in nonhydrostatic atmospheric modeling. The compressible NSE in nonhydrostatic atmospheric modeling include buoyancy terms that require special handling if one wishes to extract the Schur complement form of the linear implicit problem. They present results for five different forms of the compressible NSE and describe in detail how to formulate the semi-implicit time-integration method for these equations. Finally, they compare all five equations and compare the semi-implicit formulations of these equations both using the Schur and No Schur forms against an explicit Runge-Kutta method. Their simulations show that, if efficiency is the main criterion, it matters which form of the governing equations you choose. Furthermore, the semi-implicit formulations are faster than the explicit Runge-Kutta method for all the tests studied, especially if the Schur form is used. While they have used the spectral element method for discretizing the spatial operators, the semi-implicit formulation that they derive are directly applicable to all other numerical methods. They show results for our five semi-implicit models for a variety of problems of interest in nonhydrostatic atmospheric modeling, including inertiagravity waves, density current (i.e., Kelvin-Helmholtz instabilities), and mountain test cases; the later test case requires the implementation of nonreflecting boundary conditions. Therefore, they show results for all five semi-implicit models using the appropriate boundary conditions required in nonhydrostatic atmospheric modeling: no-flux(reflecting) and nonreflecting boundary conditions (NRBCs). It is shown that the NRBCs exert a strong impact on the accuracy and efficiency of the models.

Reviewer: Qin Mengzhao (Beijing)

### MSC:

76N15 | Gas dynamics (general theory) |

65M60 | Finite element, Rayleigh-Ritz and Galerkin methods for initial value and initial-boundary value problems involving PDEs |

65M70 | Spectral, collocation and related methods for initial value and initial-boundary value problems involving PDEs |

86A10 | Meteorology and atmospheric physics |

35Q30 | Navier-Stokes equations |