Numerical Model Investigations of Mesoscale Gravity Waves.

To investigate mesoscale gravity waves and their modelling, experiments involving the simulation of three observed wave events are performed using the Penn State/NCAR Mesoscale Model 5 (MM5). The purposes of the work are: (i) to explore model performance in simulations of mesoscale gravity wave events, (ii) to determine the sensitivity of simulated mesoscale gravity waves to model configuration and physics, and (iii) to investigate the mesoscale gravity waves issues of wave event characteristics, wave generation, wave maintenance, and wave scale. The primary MM5 configuration consists of a nested domain with up to 10-km horizontal resolution, 41 sigma-level vertical resolution, nonhydrostatic physics, the Hsie et al. and Grell schemes for grid-resolved and sub-grid moist processes, ice physics, and a radiative upper boundary condition. With respect to the consistency of model performance in different events, it is found that the MM5 can reproduce a variety of cases. With respect to model configuration in mesoscale gravity wave simulation, it is found that: (i) wave development and maintenance are insensitive to the upper boundary condition, (ii) wave structure and development are insensitive to vertical resolution, with the waves themselves not due to in vertical/horizontal resolution inconsistency, but (iii) model wave scale is sensitive to horizontal resolution, with shorter minimum wavelengths, yet an expanded scale spectrum, seen as grid size is decreased. With respect to model physics it is found that latent heating is mandatory for model wave development, and model wave production and strength are sensitive to the moist process schemes employed. Grid-resolved convection is the model wave forcing mechanism. Shear has a secondary, but positive, role in model wave energizing. Both wave -CISK and ducting contribute to model wave maintenance. An analysis of the settings of wave events reveals that the typical synoptic environment naturally yields good conditions for ducting as well as generation by convection and shear. Both the observational and experimental results support convection as the source mechanism of the actual waves in the cases simulated. Model analyses, however, also reveal conditions favorable for generation by shearing instability in these cases. Furthermore, the finding of effective wave ducts supports conclusions of their help in maintaining the actual disturbances. The model results discount the idea of geostrophic adjustment as a cause of mesoscale gravity waves. Spectral analyses of both model and observed barograms support the realism of the simulations. In general, the model tends to underproduce mesoscale gravity wave energy, with total power and power variance decreasing as grid size is decreased. It is found that mesoscale gravity wave events are characterized by variability in wave attributes, such as wavelength and amplitude. Wave interference is hypothesized as one influence on wave scale.