Atmospheric Kelvin\u2013Helmholtz billows captured by the MU radar, lidars and a fish-eye camera

Masanori Yabuki,Lakshmi Kantha,Hubert Luce,Hiroyuki Hashiguchi

doi:10.1186/s40623-018-0935-0

Abstract

On June 11, 2015, a train of large-amplitude Kelvin–Helmholtz (KH) billows was monitored by the Middle and Upper Atmosphere (MU) radar (Shigaraki MU Observatory, Japan) at the altitude of ~ 6.5 km. Four to five KH billows in formation and decay stages were observed for about 20 min at the height of a strong speed shear (> ~ 30 m s−1km−1), just a few hundred meters above a mid-level cloud base. The turbulent billows had a spacing of about 3.5–4.0 km (3.71 km in average) and an aspect ratio (depth/spacing) of ~ 0.3. The turbulence kinetic energy dissipation rate estimated was of the order of 10–50 {text{mWkg}}^{ - 1}, corresponding to moderate turbulence according to ICAO (2010) classification. By chance, an upward-looking fish-eye camera producing pictures once every minute detected smooth protuberances at the cloud base caused by the KH billows so that comparisons of their characteristics could be made for the first time between the radar observations and the pictures. The main characteristics of the KH wave (horizontal wavelength, phase front direction and phase speed) obtained from the analysis of the pictures were fully consistent with those found from radar data. The pictures indicated that the billows were advected by the wind observed at the height of the critical level. They also revealed a very small transverse extent (about twice the horizontal spacing) suggesting that the large-amplitude KH billows were generated by a very localized source. Micro-pulse lidar and Raman–Rayleigh–Mie lidar data also collected during the event permitted us to confirm some of the characteristics of the billows.

Highlights

Shear flow instabilities leading to Kelvin–Helmholtz (KH) billows are quite common in the atmosphere
For the first time, characteristics of KH billows simultaneously captured by the Middle and Upper Atmosphere (MU) radar and two lidars are described with observations made by a vertically pointing fisheye camera
Concluding remarks The MU radar observed a train of deep KH billows inside a mid-level cloud, but close to the cloud base in an upper level front

Summary

Introduction

Shear flow instabilities leading to Kelvin–Helmholtz (KH) billows are quite common in the atmosphere. Fish-eye camera photographs are commonly used for estimating cloud fraction, classifying cloud type and measuring the height of the cloud base (e.g., Long et al 2006 and references therein) They have been used in combination with remote sensing data such as lidar data for helping interpret the remote sensing observations of clouds (e.g., Sassen et al 2003; Schultz et al 2006). The Vaisala Radiosonde A Vaisala RS92G Radiosonde was released from the radar site at 0706 LT on June 11, 2015, and provided vertical profiles of pressure, temperature, relative humidity and zonal and meridional wind components, u and v, at a frequency rate of 1 Hz (corresponding to a vertical sampling of ~ 5 m) From these data, it is possible to retrieve vertical profiles of potential temperatures and atmospheric stability parameters

Analyses of the observation results

Lqs RdT