Semi-Implicit Computation of Fast Modes in a Scheme Integrating Slow Modes by a Leapfrog Method based on a Selective Implicit Time Filter

A scheme for integration of atmospheric equations containing terms with differing time scales is developed. The method employs a filtered leapfrog scheme utilizing a fourth-order implicit time-filter with one function evaluation per time step to compute slow propagating phenomena such as advection and rotation. The terms involving fast-propagating modes are handled implicitly with an unconditionally stable method that permits application of larger time steps and faster computations compared to fully explicit treatment. Implementation using explicit and recurrent formulation is provided. Stability analysis demonstrates that the method is conditionally stable for any combination of frequencies involved in the slow and fast terms as they approach the origin. The implicit filter used in the method damps the computational modes without noticeably sacrificing the accuracy of the physical mode. The O[(Δt4)] accuracy for amplitude errors achieved by the implicitly filtered leapfrog is preserved in applications where terms responsible for fast propagation are integrated with a semi-implicit method. Detailed formulation of the method for soundproof non-hydrostatic anelastic equations is provided. Procedures for implementation in global spectral shallow water models are also given. Examples comparing numerical and analytical solutions for linear gravity waves demonstrate the accuracy of the scheme. The performance is also shown in more practical nonlinear applications, where numerical solutions accomplished by the method are evaluated against those computed from a scheme where the slow terms are handled by the third-order Runge-Kutta scheme. It demonstrates that the method is able to accurately resolve fine-scale dynamics of Kelvin-Helmholtz shear instabilities, the evolution of density current and nonlinear drifts of twin tropical cyclones.