A NEW ADIABATIC KERNEL FOR THE MC2 MODEL

Traditional semi-implicit formulations of nonhydrostatic compressible models may not be stable in the presence of steep terrain when pressur e gradient terms are split and lagged in time. If all pressure gradien t terms and the divergence are treated implicitly, the resulting wave equation for the pressure contains off-diagonal cross-derivative terms leading to a highly nonsymmetric linear system of equations. In this paper we present a more implicit formulation of the Mesoscale Compress ible Community (MC2) model employing a Generalized Minimal Residual (G MRES) Krylov iterative solver and a more efficient semi-Lagrangian adv ection scheme. Open boundaries now permit exact upwind interpolation a nd the ability to reproduce simulations to machine precision is illust rated for one-way nesting at equivalent resolution. Numerical simulati ons of hydrostatic and nonhydrostatic mountain waves demonstrate the s tability and accuracy of the new adiabatic kernel. The computational e fficiency of the model is reported for ID Jacobi and 3D Alternating Di rection Implicit (ADI) line relaxation preconditioners implemented wit h a parallel data transposition strategy.