During the passage of a polar front jet stream, one wind component and the radar echo power were recorded by the SOUSY VHF radar located in the Harz Mountains in Germany. Th measurements were performed in the troposphere with height and time resolutions of 150 m and 10 s, respectively. In the lower shear region of the jet stream a Kelvin-Helmholtz instability (KHI) occurred with a period near 4 min and velocity amplitudes of the order of 1 m s-1. During the growth stage of the KHI the echo power increased significantly and remained strong even after the KHI had disappeared. It is concluded that a 3-m turbulence structure which produced the enhanced echo power is part of a broad spectrum of instabilities which started with the generation of the 4-min KHI. The properties of the observed KHI are compared with those of the fastest growing instability computed from a linear operational model taking into account vertical profiles of the mean wind and temperature which were obtained from the radar and a radiosonde ascent. The computed frequency, growth rate, velocity profile show excellent qualitative and, partly, even quantitative agreement with the observational results.