Observations and modeling of Secondary Instabilities and Billows in Kelvin-Helmholtz Instabilities

Abstract:

PMC-Turbo is a balloon-borne experiment that imaged Polar Mesospheric Clouds (PMCs) in the upper mesosphere above the Arctic from 8 July to 14 July 2018. PMC-Turbo was designed to further our understanding of gravity waves (GWs) and GW breaking and shear instabilities accounting for turbulence, mixing, and gravity wave energy and momentum deposition via high-resolution imaging of the dynamics over the life cycles of multiple events. The PMC Turbo payload includes seven optical cameras and a Rayleigh backscatter lidar. PMC Turbo imaging captured large scale dynamics with scales of 10-100 km, instability dynamics at scales from about 1-10 km, and fine structure down to scales of approximately 100 m.

On July 12th around 1300 UTC, we captured images of PMCs tracing KHI containing numerous complex dynamics. The formation and subsequent turbulent dissipation of KHI is an important method of energy and momentum deposition by GWs. We observed complicated interactions between KHI billows which break into turbulence. The data was recorded with high temporal resolution of 2 seconds and spatial resolution resolving fine structure, while the full field of view spans hundreds of kilometers.

Modeling Kelvin-Helmholtz instabilities exhibiting mis-aligned billow cores enable dynamics known from earlier laboratory studies, but only confirmed to occur in the mesosphere by recent ground-based and balloon-borne observations. Furthermore, modeling of these dynamics reveals the tubes and knots to drive turbulence intensities ~3-10 times stronger than in their absence. Modeling efforts applied to our PMC Turbo KHI observations capture multiple features of the multi-scale KHI observed by PMC Turbo. These include secondary convective instabilities within the primary billow cores aligned in the plane of large-scale shear, secondary KHI in the stratified braids between and around the primary billows and initially aligned along their axes, vortex tubes that arise due stretching of the vortex sheets between mis-aligned billow "ends", and knots that arise where vortex tubes interact strongly in close proximity. All of these features, and their consequences, can be seen in the PMC Turbo imaging of this KHI event. We will discuss the implications of these dynamics to the atmospheric conditions allowing for their formation.