An observational study of primary and secondary billows in the free atmosphere

A case study is presented showing the kinematic structure of a train of large‐amplitude billows associated with a strongly sheared layer of high static stability in the upper troposphere. the observations were obtained using a high‐power microwave Doppler radar with a range resolution of 30 m. A beamwidth resolution almost as good as this was achieved at the ranges of interest using a 25 m paraboloid aerial. the resulting detailed measurements revealed the kinematic structure of secondary billows of wavelength 350m growing on the back of primary billows of wavelength 4.2km. the secondary billows were regions of moderate turbulence, the outer scale of which was small compared with the radar resoluton. Streamers of cirrus ice particles were present in the region of interest and these served as tracers showing how the billows perturbed the flow over a considerable depth above and below the primary billows.