Three-Dimensional Air Motions over the Baiu Front Observed by a VHF-Band Doppler Radar: A Case Study

Abstract

Upper-tmpospheric three dimensional air motions have been observed for the first time during the Baiu period in 1984 by using a 46.5 MHz Doppler radar in Japan. This radar, called the MU radar, operates with an antenna aperture of 8330 m2 and peak and average radiation powers of 1000 and 50 kW, respectively. It can steer the antenna beam up to 30° from the zenith in each interpulse period. With the aid of this fast beam steerability the MU radar can measure the three dimensional air motion. Resolutions in time and altitude of, the present observations are 100 s and 150 m, respectively. Referring to the routine rawinsande observations the following results are obtained on the air motion over the Baiu front: 1) the observed mean meridional motion is upward and northward as expected but deviates upward from the frontal surface and pseudo-isentropes, 2) the upper-tropospheric mesoscale wind variations are not strongly correlated with the lower-tropospheric frontal activity such as precipitation and 3) intense updrafts of 0.5?1 m?1 appear at an interval of approximately 22 h. This interval suggests that the updrafts are caused by neutral symmetric motion.