Operational Forecasting and Detection of Mesoscale Gravity Waves

Abstract

Mesoscale gravity waves display periods of 1?4 h, have wavelengths of 50?500 km, and can have important effects upon the sensible weather. Real-time prediction, detection, and nowcasting of these mesoscale phenomena is shown to be feasible, due to recent major advances in operational observing and modeling systems. The ability to predict the likelihood of a gravity wave event rests upon recognizing the synoptic flow pattern in which such waves are consistently found to occur. The delineation of the most likely region for wave activity can be further refined by computing simple indicators of unbalanced flow and conducting a cursory search for a suitable wave ?duct? with meso-Eta Model data. Particular emphasis should be placed on propagating unbalanced fields. Whenever and wherever a suitable gravity wave environment is found, the Automated Surface Observing System pressure data should be carefully monitored for evidence of gravity wave activity. An automated gravity wave detection system is developed. It is shown that application of a time-to-space conversion adaptation of the Barnes objective analysis scheme to bandpass-filtered 5-min surface observations enables the detection of gravity waves with scales as small as 150 km and their separation from smaller-scale convective phenomena. This scheme requires accurate knowledge of the wave propagation velocity. A method is presented and successfully tested for this purpose, which is based on an adaptation of wave-ducting theory to the mesoscale model forecast fields. The proposed procedure is demonstrated with a gravity wave event that occurred during STORM-FEST. A solitary wave of depression formed as an upper-level jet streak approached an inflection axis in the diffluent height field downstream of the Rocky Mountains. This wave generation region was diagnosed from mesoscale model forecasts as being unbalanced. A wave duct was diagnosed north of a warm front in both the model forecasts and the STORM-FEST soundings over the region traversed by the observed waves. The analyzed pressure and wind perturbation fields successfully portray the evolution of the gravity wave into a wave train as strong thunderstorms developed with the wave. The mesoscale model produced a gravity wave similar in most respects to that analyzed prior to the development of convection. These results suggest that mesoscale gravity waves can be predicted and analyzed with operationally available data and numerical model guidance.