The form of metre-scale turbulence at mesopause heights in summer

Observations with a 46.5 MHz radar system at Aberystwyth (52.4°N, 4.1°W) have been used to examine the characteristics of echoes observed at heights above 80 km in daytime during the summer months of 1990–1992. Particular attention has been paid to the motion field observed during the two types of echo identified previously—spectrally broad, isotropic echoes observed between mid-May and early September, and spectrally narrow, anisotropic echoes observed on certain days in June and July. It has been found that both types of echo are associated with short-period gravity waves, those related to the spectrally narrow echoes being of smaller amplitude. The echoes are identified with isotropic and anisotropic turbulence, respectively, and data for days showing both types of echo have been used to examine the characteristic intervals of height and time over which a transition occurs.