We present an analysis of the vertical wave number and frequency spectra of atmospheric motions in the height ranges between 5 and 25 km observed using the Shigaraki, Japan, MU radar during a 4-day period in January 1988. The vertical wave number spectrum of the horizontal velocity fluctuation is found to saturate at large wave numbers satisfying power law ~ N2 2K3z while departing from this -3 power law at small wave numbers. Frequency spectra of the oblique radial velocity fluctuations can be fitted by a Garrett-Munk gravity wave model spectrum. However, the vertical velocity fluctuation cannot be fitted simultaneously. The observed spectra are too steep and their energy levels are too low compared with the results from model prediction. Also, the vertical profiles of the energy densities of the horizontal velocity fluctuations are found to be positively correlated to the background wind velocity profile. These characteristics of the observed spectra are satisfactorily explained by dynamic instability and wave-wave interactions in the regions below the critical layer through nonlinear numerical simulations. The correlation between the background wind and the horizontal velocity fluctuations is shown to result from wave-shear interaction.