TRAM: A New Nonhydrostatic Fully Compressible Numerical Model Suited for All Kinds of Regional Atmospheric Predictions

A new limited-area numerical model (TRAM, for Triangle-based Regional Atmospheric Model) has been built using a nonhydrostatic and fully compressible version of the Navier-Stokes equations. Advection terms are solved using a Reconstruct-Evolve-Average (REA) strategy over the computational cells. These cells consist of equilateral triangles in the horizontal. The classical z-coordinate is used in the vertical, allowing arbitrary stretching (e.g. higher resolution in the Planetary Boundary Layer, PBL). Proper treatment of terrain slopes in the bottom boundary conditions allows for representing accurately the orographic forcing. To gain computational efficiency, time-splitting is used to integrate separately fast and slow terms and acoustic modes in the vertical are solved implicitly. For real cases on the globe, the Lambert map projection is applied, and all Coriolis and curvature terms are retained. No explicit filters are needed.