Kelvin-Helmholtz billows generated at a cirrus cloud base within a tropopause fold/upper-level frontal system

For the first time, trains of Kelvin‐Helmholtz billows of ∼100–1500 m in depth persisting for about 60 hours at the south extremity of a tropopause fold are reported. The structures were monitored by the VHF Middle and Upper atmosphere radar (Shigaraki, 34.85°N, 136.10°E) in November 2008 at the base of cirriform clouds simultaneously detected by a Ka‐band radar. ECWMF reanalysis data indicate that these clouds were advected and spread out by the sub‐tropical jet‐stream from a tropical storm area while dry and cold air penetrated beneath the upper‐level front. The KH billows were likely continuously generated over horizontal extent of several thousand kilometers for several days without evidence of breaking into three‐dimensional turbulence. Rather than radiative effects, our observations suggest that cooling by sublimation of ice particles precipitating in the frontal layer contributed to their generation. These results should motivate further studies on the interaction between clouds and shear instabilities. Key Points Kelvin‐Helmholtz billows can persist over several days in frontal zones The presence of cloudy air above frontal zones is conducive to KH instabilities Large‐amplitude KH billows do not break into 3D turbulence