Plume spreading test case for coastal ocean models

We present a test case of river plume spreading to evaluate numerical methods used in coastal ocean modeling. It includes an estuary–shelf system whose dynamics combine nonlinear flow regimes with sharp frontal boundaries and linear regimes with cross-shore geostrophic balance. This system is highly sensitive to physical or numerical dissipation and mixing. The main characteristics of the plume dynamics are predicted analytically but are difficult to reproduce numerically because of numerical mixing present in the models. Our test case reveals the level of numerical mixing as well as the ability of models to reproduce nonlinear processes and frontal zone dynamics. We document numerical solutions for the Thetis and FESOM-C models on an unstructured triangular mesh, as well as ones for the GETM and FESOM-C models on a quadrilateral mesh. We propose an analysis of simulated plume spreading which may be useful in more general studies of plume dynamics. The major result of our comparative study is that accuracy in reproducing the analytical solution depends less on the type of model discretization or computational grid than it does on the type of advection scheme.

On the dynamics of stratified particle-laden plumes

An experimental study on stratified particle-laden plumes is presented and five steady-state flow regimes have been identified. The steady-state behaviour of the plume is directly related to the magnitude of the convective velocity associated with particle-induced instabilities, $U_c$ , in relation to the terminal settling velocity of each individual particle, $u_{st}$ . When $u_{st}>U_c$ , the ratio of particle to fluid buoyancy flux at the source, $P$ , becomes important. For $P<0.2$ , the plume dynamics appears very similar to a single-phase plume as particle recycling has minimal impact on the steady-state plume height. When $P>0.2$ , the plume height decreases significantly, creating an anvil-shaped intrusion similar to those associated with explosive volcanic eruptions. Importantly, the measured steady-state heights of plumes within this settling regime validate the collapse model of Apsley & Lane-Serff (J. Fluid Mech., vol. 865, 2019, pp. 904–927). When $u_{st}\leqslant U_c$ , particle re-entrainment behaviour changes significantly and the plume dynamics becomes independent of $P$ . When $u_{st}\approx U_c$ , a trough of fluid becomes present in the sedimenting veil due to a significant flux of descending particles at the edge of the plume. Once $u_{st}< U_c$ , the particles spreading in the intrusion become confined to a defined radius around the plume due to the significant ambient convection occurring beneath the current. For $u_{st}\ll U_c$ , or in the case of these experiments, when $U_c\geqslant 1\ \text{cm s}^{-1}$ , ambient convection becomes so strong that intrusion fluid is pulled down to the plume source, creating a flow reminiscent of a stratified fountain with secondary intrusions developing between the original current and the tank floor. Through an extension of the work of Cardoso & Zarrebini (Chem. Engng Sci., vol. 56, issue 11, 2001a, pp. 3365–3375), an analytical expression is developed to determine the onset of convection in the environment beyond the edge of the plume, which for a known particle settling velocity, can be used to characterise a plume's expected settling regime. In all plume regimes, the intrusion fluid is observed to rise in the environment following the sedimentation of particles and a simple model for the change in intrusion fluid height has been developed using the steady-state particle concentration at the spreading level.

Asthenospheric zircon below Galápagos dates plume activity

Mantle plumes are active for long periods of time1,2, however dating the onset of their activity is difficult. The magmatic products of the Galápagos plume, for example, have been subducted and fragmentarily accreted to the Caribbean and South American plates3,4. Based on submarine and terrestrial exposures it is inferred that the plume has been operating for ~90 Myrs5 or perhaps even longer (e.g., ~139 Myrs6). Here we show that the activity of the plume dates back to ~170 Ma. Evidence for this comes from 0 to 168 Ma zircon with isotopic plume signature (Galápagos Plume Array; GPA) recovered from lavas and sediments from ten islands of the archipelago. Given lithospheric plate motion, this result implies that GPA zircon predating the Galápagos lithosphere (i.e., >14 Ma) formed at asthenospheric depths. Thermo-mechanical numerical experiments of plume-lithosphere interaction show that old zircon grains can be stored within local astenospheric stable domains to be later captured by subsequent rising plume magmas. These results open new avenues for research on mantle plume dynamics in similar tectonic settings.

PM2.5 dispersion modelling from La Ronde fireworks events in Montreal using AERMOD and ArcMap

Particulate matter from fireworks events are poorly understood sources of PM2.5 despite their potential to add significant quantities of PM2.5 to the atmosphere. PM2.5 has been found to aggravate various cardiovascular and respiratory illnesses and has been linked to premature death. Each year La Ronde amusement park on Sainte-Helene Island exhibits numerous firework events in what is considered one of the world's premiere pyrotechnic competitions. These individual events are the centre of study for this project. Each event was modelled using Lakes Environmental's version of AERMOD, which estimated PM2.5 concentration plumes which then underwent geospatial analysis using ArcMap. This project details the PM2.5 plume dynamics from La Ronde fireworks events from 1990-2004, and how these events impact a ten kilometer radius around the island of Sainte-Helene in Montreal, Canada.