During the period of 7--14 September 1998 a multiple instrument campaign was conducted at the Jicamarca Radio Observatory (JRO) near Lima, Peru, using a 50-MHz VHF radar and radiosondes. In this paper we discuss the radar and in situ observations and interpret them with insight gained from high-resolution numerical simulations of the Kelvin-Helmholtz instability (KHI). Evidence is presented that a large-scale shear in the upper troposphere on 8 September 1998 becomes locally unstable due to ambient gravity wave activity. Isolated KHI events result and subsequently grow and merge to form a large 3-km mixing layer. A 3-km-deep potential-temperature step is observed at the same altitude at least a full day after the initial observations. Analysis indicates that neither turbulent mixing nor radiative cooling is capable of destroying the fully developed temperature step in the 21 hours available, which lends credence to the hypothesis that the structure persisted for this period of time. Alternately, the structure could be continuously generated by a standing wave associated with mountain wave forcing.