Gravity Waves in Idealized MM5 Simulations
Abstract
Propagation and dissipation of gravity waves in the NCAR/Penn State Mesoscale Model (MM5) are studied using numerical experiment, numerical analysis, and established theory. The waves, which are due to a small, fixed-frequency disturbance in an idealized atmosphere, form beams in an X shaped pattern known as St. Andrew's Cross. This choice of problem is accompanied by a theory, supported by physical experiment, and it allows the MM5 wave propagation by the model's dynamical core to be distinguished from other model functions, such as boundary conditions or physics parameterizations. Gravity-wave theory predicts the amount of amplitude loss along the beams in a viscous, Boussinesq fluid. This is adapted for compressibility and used as a gauge for the dissipation due to the dynamical core of the MM5, in which dissipation is strictly numerical. Eight numerical experiment cases with various grid and wave parameters are simulated and measured in terms of amplitude loss. Results show amplitude losses among the eight cases can be greater or less than theory predicts for a viscous fluid, depending on parameter values. As expected, numerical dissipation decreases or increases with respectively finer or coarser resolution of gravity waves. This is confirmed by the numerical analysis, which also predicts sensitivity of MM5 dissipation for grid and wave parameters not specifically considered by the numerical experiments. Numerical dissipation is more sensitive to changes in horizontal parameters than vertical ones, reflecting the impact of artificial dissipation used in MM5's horizontal advection scheme. Comparison of experimental and theoretical results shows the experimental numerical dissipation matches that of molecular viscosity when approximately eleven horizontal grid cells represent one wave. These results are relevant to selection of MM5 grid spacing when wave behavior is expected and interpretation of MM5 forecast dependence on grid spacings.
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 15, 2006
- Accession Number
- ADA452176
Entities
People
- Susan A. Triantafillou
Organizations
- Atmospheric and Environmental Research, Inc