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

Abstract

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.

Four‐Dimensional Quantification of Kelvin‐Helmholtz Instabilities in the Polar Summer Mesosphere Using Volumetric Radar Imaging

“…Mesosphere Stratosphere Troposhere radars are able to measure the neutral wind from Bragg scale ( ) atmospheric irregularities, where is the radar wavelength (e.g., Woodman & Guillén, ). Traditionally, they have provided atmospheric measurements with relative high temporal and vertical resolution over a particular region (e.g., Fukao et al, ; Lehmacher et al, ; Luce et al, ). Using phased arrays with electronic beam steering, modern atmospheric radars also provide horizontal information using a multibeam approach (e.g., Fukao et al, ; Latteck et al, ; Sato et al, ).…”

Satellite Observations of an Unusual Cloud Formation near the Tropopause

“…7) but, as we discuss later, the magnitude of shear is insufficient to explain the observed cloud structure. It is unlikely that conditions are met for the onset of Kelvin-Helmholtz instabilities and billows (Luce et al 2012). Last, there is no apparent source of convective overshooting sufficient to generate a 700-km-wide field of 4-km-deep gravity waves.…”

Convective Instability Underneath Midlevel Clouds: Comparisons between Numerical Simulations and VHF Radar Observations

“…The radar-derived wind speed and direction plots also suggest the absence of significant wind shear at the layer base. The characteristics of the radar echoes and vertical wind velocity fluctuations were not consistent with those produced by a KH instability (e.g., Fukao et al 2011;Luce et al 2012). Specifically, d KH billows are usually identified by slanted ''S shaped'' structures in time-height cross sections of radar echo power; here, the structures were rather suggestive of large vortices generated by convection; d even if the KH wave is an evanescent mode, KH billows produce vertical wind velocity perturbations far above and below the altitude of the billows; here, the largest vertical wind velocity fluctuations were limited to the nearly neutral or convectively unstable layer; and d at the mean altitude of the turbulent layer, there was no amplitude minimum and no phase shift of the vertical wind velocity that would suggest a critical level of KH instability.…”

“…The radar is mainly sensitive to air refractive index irregularities in both clear and cloudy air conditions through various backscattering mechanisms. Partial reflection from horizontally stratified temperature and humidity gradient sheets can cause strong enhancements of echo power at vertical incidence (e.g., Luce et al 1995). On the contrary, Bragg scatter from turbulence can produce a weak angular (azimuthal and zenithal) dependence of the radar echo power if the turbulent refractive index irregularities are statistically homogeneous in all directions at half the radar wavelength (e.g., Doviak and Zrni c 1984;Röttger and Larsen 1990).…”

“…The range resolution was 150 m for winds and spectral width and a few tens of meters for echo power owing to the range imaging technique using frequency diversity [e.g., see Luce et al (2006) for a detailed description of the technique]. Vertical profiles of pressure, temperature, humidity, and zonal and meridional wind velocities were collected simultaneously during the radar operation by RS92G Vaisala radiosondes.…”