A study of gravity waves generated by convective systems in Eta model forecasts

Spectral filtering and visualization reveal a multitude of gravity waves in the fine mesh (29 km) Eta model output, extending into the "large scale," masking the vertical motions associated with Rossby-modes. The 6h forecast of the Era model from 00 UTC 22 December 1994 initialization contains gravi ty waves recognizable over a broad spectrum of wavelengths as concentric ci rcles radiating outward from centers of model convection. Contamination of the signal is therefore identifiable in any spectral range, as rings. Postp rocessing techniques are presented to eliminate gravity wave contamination and to reveal the meteorologically significant signal. Low-pass filtering i s found to be an effective means of extracting the Rossby signal, but with loss in Rossby signal that renders the results obtained with a high resolut ion model no better than what could have been obtained from a coarser resol ution model. Time filtering, is suggested as a more effective method for el iminating the effects of fast modes while maintaining mesoscale details of the slow modes.