Four dimensional quantification of Kelvin-Helmholtz instabilities in the polar summer mesosphere using volumetric radar imaging

Abstract

We present and characterize in time and three spatial dimensions a Kelvin‐Helmholtz Instability (KHI) event from polar mesospheric summer echoes (PMSE) observed with the Middle Atmosphere Alomar Radar System. We use a newly developed radar imaging mode, which observed PMSE intensity and line of sight velocity with high temporal and angular resolution. The identified KHI event occurs in a narrow layer of 2.4 km thickness centered at 85 km altitude, is elongated along North‐South direction, presents separation between billows of ~8 km in the East‐West direction, and its billow width is~3 km. The accompanying vertical gradients of the horizontal wind are between 35 and 45 m/s/km and vertical velocities inside the billows are +/‐ 12 m/s. Based on the estimated Richardson (< 0.25), horizontal Froude (~0.8), and buoyancy Reynolds (~2.5 ×10⁴) numbers, the observed event is a KHI that occurs under weak stratification and generates strong turbulence.