The frequency of occurrence of quasi-monochromatic inertia-gravity waves in the lower stratosphere from MST radar observations

Quasi-monochromatic oscillations of the winds in the lower stratosphere are often detected in time series of radar wind observations, in vertical profiles of winds from balloons, and recently in the trajectories of stratospheric constant-density balloons. The purpose of this study is to use the long series of wind observations from the MST radars at White Sands Missile Range, New Mexico, and Vandenberg Air Force Base, California, to characterize the frequency of occurrence of these oscillations. Wave cases were defined as those where a single sinusoidal wave-form explained more than 25 percent of the variance of both the zonal and meridional winds over 72 hours in time at 17 to 19 km altitude. At Vandenberg about 30 percent of all cases, and about 60 percent of the cases during June-September, satisfied this condition. At White Sands the frequency of occurrence was less, about 17 percent of all cases and 30 percent during June-September. The meridional wind lags the zonal wind by about 90 degrees in most cases, as expected for classical inertia-gravity waves. The preferred periods of the detected waves span about 18 to 23 hours during all seasons. In one case during July 1995 at White Sands a quasi-monochromatic wave with period near 22 hours persisted for about 3 weeks. Vertical structure and other wave properties of this case will be discussed.