Studies of Velocity Fluctuations in the Lower Atmosphere Using the MU Radar. Part I: Azimuthal Anisotropy

Abstract

We present in this paper a study of the azimuthal anisotropy of the motion field observed during a six-day campaign in March 1986 using the MU radar in Shigaraki, Japan. The radial wind velocity was observed at 20° zenith angle, at every 30° of azimuth during four days, and at every 45° during two days. A jet stream was present during the entire six days. The average radial velocity variance from 10.4 to 19.2 km was calculated every four minutes and then averaged over 20 minutes or one hour. The average variance was found to be a strong function of both azimuth and time. The azimuthal variations were analyzed in terms of the mean and the first and second harmonics. The mean is proportional to the kinetic energy per unit mass of the radial wind fluctuations, and the first harmonic is proportional to the vertical flux of horizontal momentum per unit mass. The strong azimuthal variation was usually dominated by the second harmonic; i.e., with two peaks, but was occasionally dominated by the first harmonic, with one peak. The phase of the first harmonic was usually westward, but the phase of the second harmonic was quite variable. It was shown by a development of gravity wave theory that all of the observed azimuthal variations could probably be caused by a gravity wave field whose parameters vary with time.