Four-Dimensional Quantification of Kelvin-Helmholtz Instabilities in the Polar Summer Mesosphere Using Volumetric Radar Imaging

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 similar to 8 km in the east-west direction, and its billow width is similar to 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 (similar to 0.8), and buoyancy Reynolds (similar to 2.5 x 10(4)) numbers, the observed event is a KHI that occurs under weak stratification and generates strong turbulence.