Numerical Simulation of the Large-Amplitude Mesoscale Gravity-Wave Event of 15 December 1987 in the Central United States

Abstract

An observational study, employing spectral methods, is first made to establish a background for a modeling effort of the mesoscale gravity-wave event of 15 December 1987. The waves are found to have wavelengths of 100?160 km, phase speeds of approximately 30 m s?1, and lifetimes of over 6 h. Conditions for their maintenance are evaluated, indicating the presence of a wave duct and a supportive role for wave-CISK. Convection, shearing instability, and geostrophic adjustment are all implicated as possible source mechanisms for the observed waves. The case is then simulated with the Pennsylvania State University?National Center for Atmospheric Research MM4 mesoscale forecast model, with the following primary objectives: (i) to test the model's ability to simulate a mesoscale gravity-wave event, (ii) to examine in detail the environments of mesoscale gravity-wave development, and (iii) to investigate the mechanisms of mesoscale gravity-wave generation and maintenance. The full-physics control experiment employed a 30-km grid, the Hsie et al. scheme for explicit moist processes, and a modified Arakawa?Schubert cumulus parameterization. From this experiment it is found that the model can successfully simulate mesoscale gravity waves and can capture many aspects of an observed wave event. For this case the model mesoscale gravity waves arose, matured, and decayed in the same regions as those observed and had similar timing and amplitudes. Model wave speeds, however, were 1?1.8 times those observed. The model output showed that although a good wave duct covered the wave activity area, the model waves were maintained and amplified by wave-CISK processes. These waves appeared to be generated by convection of mesoscale extent above a stable duct. This convection moved with the waves and was associated with steering levels. Model sensitivity experiments showed that (i) the model mesoscale gravity waves do not stern from initial data imbalances, (ii) model mesoscale gravity-wave development does not occur when latent heating is removed, (iii) model mesoscale gravity-wave production is not necessarily limited to the early hours of a simulation, and (iv) model mesoscale gravity waves can be produced using grid sizes up to 45 km. As applied to the actual case, it is concluded from the simulations that both ducting and wave-CISK contributed to the maintenance of the observed waves. Convection is indicated as the primary wave source, although evidence of shearing instability is also found. The model results, however, do not support the idea of generation by geostrophic adjustment.