Interaction between upper-ocean submesoscale currents and convective turbulence

The interaction between upper-ocean submesoscale fronts evolving with coherent features, such as vortex filaments and eddies, and finescale convective turbulence generated by surface cooling of varying magnitude is investigated. While convection is energized by gravitational instability, predominantly at the finescale (FS), which feeds off the potential energy that is input through cooling, the submesoscale (SMS) is energized at larger scales by the release of available potential energy stored in the front. Here, we decompose the flow into FS and SMS fields explicitly to investigate the energy pathways and the strong interaction between them. Overall, the SMS is energized due to surface cooling. The frontogenetic tendency at the submesoscale increases, which counters the enhanced horizontal diffusion by convection-induced turbulence. Downwelling/upwelling strengthens, and the peak SMS vertical buoyancy flux increases as surface cooling is increased. Furthermore, the production of FS energy by SMS velocity gradients is significant, up to half of the production by convection. Examination of potential vorticity reveals that surface cooling promotes higher levels of secondary symmetric instability, which coexists with the persistent baroclinic instability.

Persistence analysis in convective turbulence

<p>Persistence is defined as the probability that the local value of a fluctuating field remains at a particular state for a certain amount of time, before being switched to another state. The concept of persistence has been found to have many diverse practical applications, ranging from non-equilibrium statistical mechanics to financial dynamics to distribution of time scales in turbulent flows and many more. In this study, we carry out a detailed analysis of the statistical characteristics of the persistence probability density functions (PDFs) of velocity and temperature fluctuations in the surface layer of a convective boundary layer, using a field-experimental dataset. Our results demonstrate that for the time scales smaller than the integral scales, the persistence PDFs of turbulent velocity and temperature fluctuations display a clear power-law behavior, associated with a self-similar eddy cascading mechanism. Apart from that, we show that the effects of non-Gaussian temperature fluctuations act only at those scales which are larger than the integral scales, where the persistence PDFs deviate from the power-law and drop exponentially.</p><p>To advance our knowledge, we also investigate how the turbulent structures associated with velocity and temperature fluctuations interact to produce the emergent flux signatures, a vexing problem but of paramount importance for a plethora of applications, encompassing both engineering and Earth sciences. We discover that the persistence patterns for heat and momentum fluxes are widely different. Moreover, we uncover the power-law scaling and length scales of turbulent motions that cause this behavior. Furthermore, by separating the phases and amplitudes of flux events, we explain the origin and differences between heat and momentum transfer efficiencies in convective turbulence. In summary, our findings provide a new understanding of the connection between flow organization and flux generation mechanisms, two cornerstones of turbulence research.</p>

The Zilitinkevich Scale in Life and Science

<div>A prominent scientist, a leader in environmental turbulence and planetary boundary layer research Sergej Zilitinkevich passed away on February 15, 2021.  His scientific  contribution is of a remarkably large scale, and so was his life. In fact, one of his most commonly referred research achievements is now widely known as The Zilitinkevich scale - a length scale of a rotation-stratification turbulent mixing in stably stratified planetary boundary layers. In fact, one of his most significant life achievements can be characterized as organizing large international consortiums to advance the boundary layer research into new territories of non-local turbulence, organized structures, and extreme stratifications. Many of his discoveries are now in textbooks, often cited as classical common knowledge without references. Sergej Zilitinkevich was passionate for truly large-scale international science that combined the best of multidisciplinary approaches and multicultural research schools. In the last decade, he has been leading international efforts to develop an energetically consistent turbulence theory that could be applied to strongly stratified, very-high-Reynolds-number environmental flows. He and collaborators obtained ground-breaking results in the studies of key mechanisms controlling both the stably stratified and convective turbulence. Until present, turbulence in strongly stratified flows such as those found in the free atmosphere and hydrosphere has been parameterized only heuristically; the theoretical and experimental basis behind the computing schemes was rather fragmented. Convective turbulence is still parameterized only heuristically. These and other theoretical advances open opportunities for a novel class of parametrizations for modelling and forecasting of climate, weather, wind-energy potential, and air and water quality. His last project - the Pan-Eurasian experiment - connects more than 100 research groups from 20 countries. It aims to solve interlinked challenges of global warming, atmospheric pollution, biodiversity loss, energy production, and fresh water recognizing the increasing role of the cold climate areas in the context of global change. This presentation commemorates the life and work of our collegue and friend.</div>

On the Potential Optical Signature of Convective Turbulence over the West Florida Shelf

Atmospheric cold front propagation across the northern Gulf of Mexico is characterized by elevated surface wind velocities and a ~10–15 °C drop in surface air temperatures. These meteorological conditions result in significant heat energy losses from the surface ocean to the overlying atmosphere. These seasonally recurring cold-air outbreak events may penetrate the southern portion of the West Florida continental shelf and initiate turbulent and convective overturn of the water column. Examination of true color images derived from ocean-viewing, satellite-based radiometer data reveals coincident and substantial surface water discolorations that are optically similar to smaller-scale “whiting events,” despite the regional-scale extent of the observed phenomenon (>25,000 km2). Coupled air–sea numerical simulations suggest the surface water discoloration occurs and is sustained where the entire water column is dynamically unstable. The simulation results indicate significant density (σt) inversions between the surface and bottom waters. Thus, the combined numerical model and remote sensing analysis suggest that convective turbulence may be contributing to the sustained ventilation of bottom waters containing a high concentration of suspended particulates. High-temporal resolution true color images rendered from the GOES-R Advanced Baseline Imager (ABI) data appear to support the surface water discoloration’s turbulent-driven nature.

Small-scale convective turbulence constrains microbial patchiness

Microbes play a primary role in aquatic ecosystems and biogeochemical cycles. Patchiness is a critical component of these activities, influencing biological productivity, nutrient cycling and dynamics across trophic levels. Incorporating spatial dynamics into microbial models is a long-standing challenge, particularly where small-scale turbulence is involved. Here, we combine a realistic simulation of turbulence with an individual-based microbial model to test the key hypothesis that the coupling of motility and turbulence drives intense microscale patchiness. We find that such patchiness is depth-structured and requires high motility: Near the fluid surface, strong convective turbulence overpowers motility, homogenising motile and non-motile microbes equally. In deeper, thermocline-like conditions, highly motile microbes are up to 1.6-fold more patch-concentrated than non-motile microbes. Our results demonstrate that the delicate balance of turbulence and motility that triggers micro-scale patchiness is not a ubiquitous consequence of motility, and that the intensity of such patchiness in real-world conditions is modest.