First observations of Kelvin‐Helmholtz billows in an upper level jet stream using VHF frequency domain interferometry

Abstract

In this paper we report the first high‐resolution observations of upper tropospheric Kelvin‐Helmholtz billows using VHF frequency domain interferometry (FDI). The measurements were made using the sounding system VHF radar located in the German Harz Mountains operating at the frequencies of 53.25 and 53.75 MHz. Through an application of the FDI technique it has been possible to track the altitude of thin scattering layers with an accuracy of a few tens of meters and a temporal resolution of 13 s. Taking advantage of the FDI technique, we found several examples of Kelvin‐Helmholtz instability (KHI) in regions coinciding with large values of wind shear. One particularly good example is presented in detail. The KHI occurred just below the axis of the jet at an altitude of 9.1 km. It produced oscillations in the vertical velocity field in the altitude range of 8–10 km. The oscillations had a local period of 90 s and persisted over a time interval of 10 min. On the basis of the magnitude of the horizontal wind at the time and height of the KHI, the billow train had a horizontal extent of about 27 km, while the horizontal length of the individual billow amounted to about 4.0 km. The 90‐s oscillations were also observed in the radar signal power, the FDI‐derived layer height, and the coherence between the received radar signals for the two frequencies. Using the FDI data, we were able to identify the individual billows located within the train and observe their maximum crest‐to‐trough amplitudes to be between 220 and 230 m. This was less than the conventional radar resolution of 300 m used during the experiment.