Observations with the SOUSV VHF Radar in the mesolpause region during summer 1989 show lay- ers of strong echo ictensity and wind speeds increasing with height. Occasionally, quasi-manochromatic internal gravity waves occur in the bottom side of the echo layer and vanish in the height intervaI around the echo intensity maximum which tends to increase withh increasing wave amplitude The data anaiysis can be performed using a Boussieesq and a WKB approximation, It shows that the horizontal phase trace velocity of the observed gravity waves is equal to the background wind velocity at the height of the echo intensity maximum. The observations thus reprsent nonlinear gravity wave-critical level encounters pruducing strong turbulence in the neutral gas. But the, estimated euergy disspation rates are too small to produce neutral gas turbulence at scales equal to haft the radar wave!ength yieiding an additional indication for yet unidentified mechanism generating small-scale structure in the electron gas.