The life cycle of instability features measured from the Andes Lidar Observatory over Cerro Pachon on 24 March 2012

Abstract

The Aerospace Corporation's Nightglow Imager (ANI) observes nighttime OH emission (near 1.6 μm) every 2 s over an approximate 73° field of view. ANI had previously been used to study instability features seen over Maui. Here we describe observations of instabilities seen from 5 to 8 UT on 24 March 2012 over Cerro Pachon, Chile, and compare them with previous results from Maui, with theory, and with Direct Numerical Simulations (DNS). The atmosphere had reduced stability because of the large negative temperature gradients measured by a Na lidar. Thus, regions of dynamical and convective instabilities are expected to form, depending on the value of the Richardson number. Bright primary instabilities are formed with a horizontal wavelength near 9 km and showed the subsequent formation of secondary instabilities, rarely seen over Maui, consistent with the primaries being dynamical instabilities. The ratio of the primary to secondary horizontal wavelength was greater over Chile than over Maui. After dissipation of the instabilities, smaller‐scale features appeared with sizes in the buoyancy subrange between 1.5 and 6 km. Their size spectra were consistent with the model of Weinstock (1978) if the turbulence is considered to be increasing. The DNS results produce secondary instabilities with sizes comparable to what is seen in the images although their spectra are somewhat steeper than is observed. However, the DNS results also show that after the complete decay of the primary features, scale sizes considerably smaller than 1 km are produced and these cannot be seen by the ANI instrument.

A fast SWIR imager for observations of transient features in OH airglow

“…Fritts et al (2014) show structures based on model calculations of the OH airglow response to KHIs looking very similar to these. Yamada et al (2001) and Hecht et al (2014) present similar phenomena in their measurements. The latter provide a detailed analysis combining the measurements of different instruments.…”

“…Where the superposition of both waves takes place, one can see even smaller structures in the order of about 2 km, which we assume to be Kelvin-Helmholtz instability billows (compare Fritts et al (2014); Hecht et al, 2014).…”

PMC Turbo: Studying Gravity Wave and Instability Dynamics in the Summer Mesosphere Using Polar Mesospheric Cloud Imaging and Profiling From a Stratospheric Balloon

“…Applications of DNS to KHI for various Reynolds and Richardson numbers, Re and Ri , respectively, for idealized shear flows and MSD arising from superposed higher‐ and lower‐frequency motions have yielded other comparisons that provide further evidence of the validity of DNS descriptions of such flows. Specifically, comparisons of PMC and OH airglow imaging and modeling have revealed tendencies for enhanced KHI accompanying significant GW amplitudes (Baumgarten & Fritts, ; Fritts, Baumgarten, et al, ; Fritts, Wan, et al, ; Hecht et al, , ). These features are consistent with regions of preferred KHI capping local GW breaking in MSD (Fritts et al, ) and apparent in radar and lidar profiling noted above.…”

“…These features are consistent with regions of preferred KHI capping local GW breaking in MSD (Fritts et al, ) and apparent in radar and lidar profiling noted above. PMC and airglow imaging have also revealed features aligned along the plane of Kelvin‐Helmholtz (KH) billow rotation that intensify, interact, and ultimately break down to smaller‐scale turbulence (Baumgarten & Fritts, ; Hecht et al, ). These secondary instability features comprise counterrotating vortices, with spanwise (normal to the evolution plane) wave numbers, that arise in the outer (inner) portions of the KH billows for smaller (larger) Ri that are relatively more (less) unstable (Fritts, Baumgarten, et al, ; Fritts, Wan, et al, ).…”

Numerical Simulations of High‐Frequency Gravity Wave Propagation Through Fine Structures in the Mesosphere

“…The Reynolds number ( Re ) is calculated from the GW length scale as where ν = μ / ρ is the kinematic viscosity. We apply a turbulent kinematic viscosity of ν =3 ν 0 based on estimates of an elevated effective viscosity due to preexisting turbulence (Baumgarten & Fritts, ; Fritts, Baumgarten, et al, ; Fritts, Wan, et al, ; Hecht et al, , ) in the manner of Fritts, Laughman, et al (), where ν 0 is the true kinematic viscosity ∼1.5×10 −5 m 2 s −1 at ground level and ν ∼2.8 m 2 s −1 is the kinematic viscosity specified in the model at 80 km. For GW with λ z =10 km, this results in Re ≈10 5 where FSs arise accompanying flow instabilities.…”