Numerical model simulations of a mesoscale gravity wave event: Sensitivity tests and spectral analyses

This study presents numerical model experiments and spectral investigations involving a mesoscale gravity wave event. Its purposes are to determine the sensitivity of mesoscale gravity wave simulation to model configuration and physics and to evaluate spectrally both an observed and simulated wave episode. The case is the large-amplitude wave event of 15 December 1987 in the central United States, and the model employed is the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model 5 (MM5). The primary MM5 configuration features a nested domain with IO-km horizontal resolution, 41 sigma-level vertical resolution, nonhydrostatic physics, a radiative upper boundary condition. an explicit moist process scheme with ice physics, and the Grell cumulus parameterization. Experiments are performed to investigate the effects on wave simulation of upper boundary condition, hydrostatics, vertical and horizontal resolution, and moist physics. From the sensitivity tests it is found that wave development and maintenance are insensitive to the upper boundary condition, that wave simulation is insensitive to hydrostatic/nonhydrostatic differences at IO-km horizontal resolution, and that wave production and structure are insensitive to vertical resolution. With respect to horizontal resolution. an expanded wave scale spectrum and shorter minimum wavelengths appear as grid size is decreased. With respect to moist physics, latent heating is found robe necessary for model wave development, and model wave production and strength are, to an extent, sensitive to the moist process package employed. Elevated convection is the model wave forcing mechanism, and, ata given grid size, wave response varies with the degree to which such convection is explicit. Statistical analyses consisting of high-pass filtering and power spectrum analysis of observed and model surface pressure data are performed. The filtering analyses indicate more realistic simulated wave activity as horizontal resolution is increased. The spectral analyses uncover bimodal distributions in both the observed and model spectra and show that the model, in general, does not overproduce mesoscale gravity wave energy. With respect to the model alone, the spectral analyses reveal that as grid size is decreased the significant spectral frequencies shift upward while both the average power in the mesoscale wave band and spatial variability of the power in such band decrease.