Simple finite elements and multigrid for efficient mass-consistent wind downscaling in a coupled fire-atmosphere model

We present a simple finite element formulation of mass-consistent approximation, and a fast multigrid iterative method with adaptive semicoarsening, which maintains the convergence of the iterations over a range of grids and penalty coefficients. The method is designed to run in each time step of WRF-SFIRE and replace the interpolation from the atmospheric mesh to the fire mesh by a diagnostic model of the air flow over a fine-scale terrain.