The semidiumal variations in quasi-periodic echo structure parameters associated with midlatitude sporadic E layers observed with the Chung-Li VHF radar located in Taiwan (24.9 N, 121.2 E; 52 MHz) are presented. The results show that the periodicity decreases with increasing range rate and that the spread of Doppler velocities increases as the altitude coverge of the echoes increases. These characteristics are shown to be consistent with the variations to be expected if the shears in the neutral wind and the variations in the background mean flow are the dominant mechanisms controlling the initiation of the echo structures. ¿ 2000 American Geophysical Union