Fig. 4

a Time–height cross section of vertical velocity W \(({\text{ms}}^{ - 1} )\) measured by the vertically pointing beam of the MU radar. The horizontal dashed line shows the altitude of the critical level of the KH instability (defined as the altitude where phase shift of W disturbances was observed). b The corresponding time–height cross section of horizontal wind speed \(({\text{ms}}^{ - 1} )\). The black contours show an arbitrary level of radar echo power for delineating the KH billows. The missing data are due to contaminations in the Doppler spectra by a UAV flying near the MU radar