Observations of QP radar echo structure consistent with neutral wind shearcontrol of the initiation mechanism

The semidiurnal variations in quasi-periodic echo structure parameters asso ciated with midlatitude sporadic E layers observed with the Chung-Li VHF ra dar 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 coverage of the ech oes increases. These characteristics are shown to be consistent with the va riations to be expected if the shears in the neutral wind and the variation s in the background mean flow are the dominant mechanisms controlling the i nitiation of the echo structures.