Physicists have long known that turbulence is a fundamental process in Earth’s atmosphere, where it facilitates mixing, contributes to energy and momentum transport and deposition, and has important implications for weather and climate prediction. However, the mechanisms driving the transitions from laminar airflow to 3D turbulence are complex and poorly understood. In new research, Hecht et al. describe the first atmospheric observations of a new mechanism of strong turbulence generation involving the interaction of atmospheric gravity waves (GWs) and Kelvin-Helmholtz instabilities (KHIs).
GWs are ubiquitous and have many sources at lower and higher altitudes, including airflow over topography, convection, and wind shears, and they often lead to turbulence as they grow in amplitude with increasing altitude. KHIs occur in fluids like air that experience a strong gradient interface in velocity as they flow. At this strongly sheared interface, small perturbations can lead to growing instabilities that can be seen in thin cloud layers with distinctive wave crest–like shapes. Such KHIs can lead to turbulence even in the absence of other influences.
In the new work, the researchers used airglow imaging and lidar observations to study a large-scale KHI event that occurred at about 85 to 90 kilometers altitude over Chile on 1 March 2016. The images revealed a series of KHI billows forming as GWs propagated through the region and appeared to perturb the KHI formation. This perturbation led to misalignments along the KHI billows, causing them to interact with one another as they grew in amplitude. The very high spatial resolution of the imaging revealed details of the KHIs evolving to turbulence, including the formation of “knots” and vortex “tubes” where the KHI billows interact.
The interactions observed between GWs and KHIs motivated a companion modeling study by Fritts et al. that considered conditions such as those that would accompany GW modulations along KHI billow cores. The simulated interactions bore a striking resemblance to the observations reported by Hecht et al.: Specifically, misaligned KHI billows induced vortex tubes linking adjacent KHI billow cores, and their subsequent evolution to knots then drove strong turbulence. The researchers note that the results of both studies are strikingly similar to prior laboratory work confirming the expectation of rapid and strong turbulence transitions accompanying such events.
These papers mark the first quantitative observations and modeling of KHI tubes and knots in Earth’s atmosphere and suggest that these processes are common and have significant effects in the upper atmosphere. They also reveal the benefits, the authors say, of combining theory, laboratory experiments, atmospheric observations, and numerical simulations in studying such dynamic processes as significant pathways to atmospheric turbulence. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1029/2020JD033414 and https://doi.org/10.1029/2020JD033412, 2020).
—Morgan Rehnberg, Science Writer