Observations of the Breakdown of Mountain Waves Over the Andes Lidar Observatory at Cerro Pachon on 8/9 July 2012

Abstract

Although mountain waves (MWs) are thought to be a ubiquitous feature of the wintertime southern Andes stratosphere, it was not known whether these waves propagated up to the mesopause region until Smith et al. (2009) confirmed their presence via airglow observations. The new Andes Lidar Observatory at Cerro Pachon in Chile provided the opportunity for a further study of these waves. Since MWs have near‐zero phase speed, and zero wind lines often occur in the winter upper mesosphere (80 to 100 km altitude) region due to the reversal of the zonal mean and tidal wind, MW breakdown may routinely occur at these altitudes. Here we report on very high spatial/temporal resolution observations of the initiation of MW breakdown in the mesopause region. Because the waves are nearly stationary, the breakdown process was observed over several hours; a much longer interval than has previously been observed for any gravity wave breakdown. During the breakdown process observations were made of initial horseshoe‐shaped vortices, leading to successive vortex rings, as is also commonly seen in Direct Numerical Simulations (DNS) of idealized and multiscale gravity wave breaking. Kelvin‐Helmholtz instability (KHI) structures were also observed to form. Comparing the structure of observed KHI with the results of existing DNS allowed an estimate of the turbulent kinematic viscosity. This viscosity was found to be around 25 m2/s, a value larger than the nominal viscosity that is used in models.

Large‐Amplitude Mountain Waves in the Mesosphere Accompanying Weak Cross‐Mountain Flow During DEEPWAVE Research Flight RF22

“…To date, multiple DEEPWAVE studies have addressed a diversity of MW dynamics extending to altitudes of ~90 km and of their effects extending to higher altitudes (e.g., Bossert et al, , ; Bramberger et al, ; Broutman et al, ; Eckermann et al, ; Fritts, Smith, et al, ; Kaifler et al, ; Pautet et al, ). Similar MW and more general GW studies are now being performed using ground‐based lidars, radars, and airglow imagers (e.g., Baumgarten et al, ; Hecht et al, ). To our knowledge, however, no other studies have approximated the capabilities of DEEPWAVE airborne measurements to quantify MW, and more general GW, horizontal and vertical scales, temporal variability, and their linear and nonlinear dynamics from ~0 to 100 km.…”

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.…”

“…Importantly, transitional instabilities that have been quantified in PMC and OH airglow imaging have enabled quantitative estimates of the underlying dynamics in the various DNS, including specific GW, KHI, and/or MSD character and scales. These enabled, in turn, quantitative estimates of ε and more qualitative estimates of the turbulent kinematic viscosity based on the corresponding DNS instability scales and turbulence intensities (Fritts et al, ; Fritts, Baumgarten, et al, ; Hecht et al, , ). In particular, ε inferred from EBEX and ground‐based PMC imaging by Fritts et al () for various idealized and MSD events were in the range of those estimated in multiple in situ rocket measurement programs and occasionally very large.…”

“…Yamada et al, 2001). More recent contributions by radar and lidar profiling (Franke & Collins, 2003;Lehmacher et al, 2007;Pfrommer et al, 2009) and especially in combination with ground-based or airborne imaging (Eckermann et al, 2016;Fritts, Vosper, et al, 2018;Hecht et al, 1997Hecht et al, , 2018Hecht et al, , 2005Hecht et al, , 2014Pautet et al, 2016) have revealed instability character, spatial and temporal scales, and evidence of the environments in which they arose.…”

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.…”