PRE-SUNRISE MESOSPHERIC ECHOES AND TURBULENT WIND STRUCTURE OBSERVED WITH THE MU-RADAR

Summer mesospheric observations using the MU radar at Shigaraki, Japan show that radar echoes could be detected before sunrise under some ci rcumstances. The pre-sunrise radar echoes observed at heights above 80 km are weak but clearly occur in a layer. Strong radar echoes observe d after sunrise at similar heights show an unusual broadening of the s pectral width in the echo power. This indicates the presence of strong atmospheric turbulence. In addition, frequency spectra for the vertic al wind fluctuations show an enhancement of the spectral power density in a wide range of periods shorter than the local Brunt-Vaisala one. This is consistent with the occurrence of strong turbulence layer in t he upper mesosphere though not immediately identified with the turbule nce activity. Thus the presence of strong turbulence layer might yield us a possibility of detecting radar echoes before sunrise.