Doppler lidar vertical stare observation of a Kelvin-Helmholtz instability in the nocturnal boundary-layer

Kelvin–Helmholtz (K-H) or shear instability may occur in continuously stratified flows and along interfaces between fluids associated with strong vertical shear. This process is suspected to be a major source of clear air turbulence and a contributor to vertical mixing of momentum and scalars in the atmospheric boundary layer. It is believed that it plays a substantial role in intermittent turbulence generation and vertical mixing in a stable boundary layer. Occasionally, a K-H instability becomes visible as K-H billow clouds.  

Here we present an observation of a K-H instability on the early morning of July 25, 2022 at the Cabauw atmospheric research station in The Netherlands. The K-H instability is observed in Doppler lidar vertical stare measurements (with 1s temporal resolution) as a prominent wavelike pattern in the vertical velocity centered at altitude of 1000m, in which the upper and lower halves are shifted in phase. These K-H waves have a period a few minutes, with vertical velocity amplitudes up to 2m/s, and last for about 15 minutes. Maximum eddy dissipation rate is found to be 2x10^-3m²s^-3. The residual layer that night was up to 2000m, providing sufficient aerosol backscatter signal for Doppler lidar measurements to observe these K-H waves.

The Doppler lidar wind profile measured just before the appearance of the K-H instability showed a strong wind shear (0.04s^-1) with a shear layer depth of about 300m. The Richardson number (Ri) profiles, derived from combining the vector wind shear (Doppler lidar) and Brunt-Vaisala frequency profiles (microwave radiometer), shows Ri<0.25 just before to the appearance of the K-H waves, confirming the K-H instability scenario.

To our knowledge this is the first Doppler lidar vertical stare observation of a K-H instability in the nocturnal boundary layer with its signature in the vertical velocity. Within our four years of Doppler lidar operation at Cabauw this K-H wave observation is unique, while less prominent elevated intermittent turbulence patches in the nocturnal boundary layer are more often observed. The K-H waves are also distinct from the more often observed gravity waves [1].  Our observations demonstrate that Doppler lidar vertical stare measurements, preferably combined with wind and temperature profile measurements, can contribute to the understanding of waves and enhanced turbulence in the nocturnal boundary layer.

[1] Knoop, S., Assink, J., Tijm, S., and Leijnse, H.: High-resolution observations of a gravity wave event over the Netherlands, EMS 2024, https://doi.org/10.5194/ems2024-445