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Observation of Kelvin‐Helmholtz billows and their mesoscale environment by radar, instrumented aircraft, and a dense radiosonde network

On 19 February 1970, a T-33 jet aircraft, equipped with fast-response meteorological instruments, obtained measurements of a wave structure and clear air turbulence. The clear air structure was simultaneously detected with a sensitive 10·7 cm wavelength radar at Wallops Island, Virginia. From a special network of radiosonde stations with separations of about 100 km, the detailed mesoscale synoptic conditions associated with the clear air structures were determined. The turbulence and wave structure occurred between heights of 2 and 3 km within an upper level frontal zone which had advanced well ahead of the surface cold front. The wave pattern consisted of two-dimensional Kelvin-Helmholtz billows. The billow velocity was close to the mean wind velocity of the sheared layer, and the Richardson number for the layer was about 0·15 which is consistent with the criterion for instability based on theoretical and wind tunnel results. The most intense turbulence occurred where the billows were rolling up or breaking.