In Situ Observations of Neutral Shear Instability in the Statically Stable High‐Latitude Mesosphere and Lower Thermosphere During Quiet Geomagnetic Conditions

Abstract

Though the Kelvin‐Helmholtz instability (KHI) has been extensively observed in the mesosphere, where breaking gravity waves produce the conditions required for instability, little has been done to describe quantitatively this phenomenon in detail in the mesopause and lower thermosphere, which are associated with the long‐lived shears at the base of this statically stable region. Using trimethylaluminum (TMA) released from two sounding rockets launched on 26 January 2018, from Poker Flat Research Range in Alaska, the KHI was observed in great detail above 100 km. Two sets of rocket measurements, made 30 min apart, show strong winds (predominantly meridional and up to 150 ms−1) and large total shears (90 ms−1 km−1). The geomagnetic activity was low in the hours before the launches, confirming that the enhanced shears that triggered the KHI are not a result of the E‐region auroral jets. The four‐dimensional (three‐dimensional plus time) estimation of KHI billow features resulted in a wavelength, eddy diameter, and vertical length scale of 9.8, 5.2, and 3.8 km, respectively, centered at 102‐km altitude. The vertical and horizontal root‐mean‐square velocities measured 29.2 and 42.5 ms−1, respectively. Although the wind structure persisted, the KHI structure changed significantly with time over the interval separating the two launches, being present only in the first launch. The rapid dispersal of the TMA cloud in the instability region was evidence of enhanced turbulent mixing. The analysis of the Reynolds and Froude numbers (Re = 7.2 × 103 and Fr = 0.29, respectively) illustrates the presence of turbulence and weak stratification of the flow.

Kelvin‐Helmholtz Billow Interactions and Instabilities in the Mesosphere Over the Andes Lidar Observatory: 1. Observations

“…They measured the vertical wind and horizontal and vertical dimensions of the feature, and then they inferred a horizontal root mean square velocity. Mesquita et al (2020) did follow a KHI crest and their measured root mean square velocity was comparable to what was found for the inferred horizontal wind. In our case, we only have horizontal measurements.…”

“…The sequence of this section is as follows: (1) We first infer the background turbulent viscosity following our previous work (Fritts et al, 2014; Hecht et al, 2014, 2018). (2) Then we use the images to estimate the energy dissipation associated with the decay of the KHI billows following Chau et al (2020) and Mesquita et al (2020). (3) Following that, a comparison is made to Thorpe's laboratory data that were the first to reveal KHI tube and knot development.…”

“…The size of the feature L e was 1.85 km and thus, following Chau et al (2020) and Mesquita et al (2020), an energy dissipation rate, ϵ , was calculated from where C is taken to be 1. Using our data, ϵ is 0.97 W/kg.…”

“…(2) Then we use the images to estimate the energy dissipation associated with the decay of the KHI billows following Chau et al (2020) and Mesquita et al (2020). 3Following that, a comparison is made to Thorpe's laboratory data that were the first to reveal KHI tube and knot development.…”

“…This is because large AGW The full potential of imaging is only realized, though, when those data are combined with other techniques that can provide details on the vertical structure of the instability dynamics. Examples of these techniques that have been used to study KHI dynamics include the innovative use of radars (Chau et al, 2020;Hysell et al, 2012;Lehmacher et al, 2007), chemical releases from sounding rockets (Larsen et al, 2005;Mesquita et al, 2020), and lidars used in several of the studies cited above.…”