OPERATIONAL FORECASTING AND DETECTION OF MESOSCALE GRAVITY-WAVES

Mesoscale gravity waves display periods of 1-4 h, have wavelengths of 50-500 km, and can have important effects upon the sensible weather. R eal-time prediction, detection, and nowcasting of these mesoscale phen omena is shown to be feasible, due to recent major advances in operati onal observing and modelling systems. The ability to predict the likel ihood of a gravity wave event rests upon recognizing the synoptic flow pattern in which such waves are consistently found to occur The delin eation of the most likely region for wave activity can be further refi ned 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 careful ly monitored for evidence of gravity wave activity. An automated gravi ty wave detection system is developed. It is shown that application of a time-to-space conversion adaptation of the Barnes objective analysi s scheme to bandpass-filtered 5-min surface observations enables the d etection of gravity waves with scales as small as 150 km and their sep aration from smaller-scale convective phenomena. This scheme requires accurate knowledge of the wave propagation velocity. A method is prese nted and successfully tested for this purpose, which is based on an ad aptation of wave-ducting theory to the mesoscale model forecast fields . The proposed procedure is demonstrated with a gravity wave event tha t occurred during STORM-FEST. A solitary wave of depression formed as an upper-level jet streak approached an inflection axis in the difflue nt height field downstream of the Rocky Mountains. This wave generatio n region was diagnosed from mesoscale model forecasts as being unbalan ced. 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 t he observed waves. The analysed pressure and wind perturbation fields successfully portray the evolution of the gravity wave into a wave tra in as strong thunderstorms developed with the wave. The mesoscale mode l produced a gravity wave similar in most respects to that analysed pr ior to the development of convection. These results suggest that mesos cale gravity waves can be predicted and analysed with operationally av ailable data and numerical model guidance.