Ultraviolet Camera Measurements of Passive and Explosive (Strombolian) Sulphur Dioxide Emissions at Yasur Volcano, Vanuatu

Here, we present the first ultraviolet (UV) camera measurements of sulphur dioxide (SO2) flux from Yasur volcano, Vanuatu, for the period 6–9 July 2018. These data yield the first direct gas-measurement-derived calculations of explosion gas masses at Yasur. Yasur typically exhibits persistent passive gas release interspersed with frequent Strombolian explosions. We used compact forms of the “PiCam” Raspberry Pi UV camera system powered through solar panels to collect images. Our daily median SO2 fluxes ranged from 4 to 5.1 kg s−1, with a measurement uncertainty of −12.2% to +14.7%, including errors from the gas cell calibration drift, uncertainties in plume direction and distance, and errors from the plume velocity. This work highlights the use of particle image velocimetry (PIV) for plume velocity determination, which was preferred over the typically used cross-correlation and optical flow methods because of the ability to function over a variety of plume conditions. We calculated SO2 masses for Strombolian explosions ranging 8–81 kg (mean of 32 kg), which to our knowledge is the first budget of explosive gas masses from this target. Through the use of a simple statistical measure using the moving minimum, we estimated that passive degassing is the dominant mode of gas emission at Yasur, supplying an average of ~69% of the total gas released. Our work further highlights the utility of UV camera measurements in volcanology, and particularly the benefit of the multiple camera approach in error characterisation. This work also adds to our inventory of gas-based data, which can be used to characterise the spectrum of Strombolian activity across the globe.

Enhanced ablation of a vertical ice wall due to an external freshwater plume

We investigate the effect of an external freshwater plume on the dissolution of a vertical ice wall in salty water using laboratory experiments. We measure the plume velocity, the ablation velocity of the ice and the temperature at the ice wall. The freshwater volume flux, $Q_{s}$, is varied between experiments to determine where the resultant wall plume transitions from being dominated by the distributed buoyancy flux due to dissolution of the ice, to being dominated by the initial buoyancy flux, $B_{s}$. We find that when $B_{s}$ is significantly larger than the distributed buoyancy flux from dissolution, the plume velocity is uniform with height and is proportional to $B_{s}^{1/3}$, the interface temperature is independent of $B_{s}$, and the ablation velocity increases with $B_{s}$.

The effect of a salinity gradient on the dissolution of a vertical ice face

We investigate experimentally the effect of stratification on a vertical ice face dissolving into cold salty water. We measure the interface temperature, ablation velocity and turbulent plume velocity over a range of salinity gradients and compare our measurements with results of similar experiments without a salinity gradient (Kerr & McConnochie, J. Fluid Mech., vol. 765, 2015, pp. 211–228; McConnochie & Kerr, J. Fluid Mech., vol. 787, 2016, pp. 237–253). We observe that stratification acts to reduce the ablation velocity, interface temperature, plume velocity and plume acceleration. We define a stratification parameter, $S=N^{2}Q/{\it\Phi}_{o}$, that describes where stratification will be important, where $N$ is the Brunt–Väisälä frequency, $Q$ is the height-dependent plume volume flux and ${\it\Phi}_{o}$ is the buoyancy flux per unit area without stratification. The relevance of this stratification parameter is supported by our experiments, which deviate from the homogeneous theory at approximately $S=1$. Finally, we calculate values for the stratification parameter at a number of ice shelves and conclude that ocean stratification will have a significant effect on the dissolution of both the Antarctic and Greenland ice sheets.

The turbulent wall plume from a vertically distributed source of buoyancy

We experimentally investigate the turbulent wall plume that forms next to a uniformly distributed source of buoyancy. Our experimental results are compared with the theoretical model and experiments of Cooper & Hunt (J. Fluid Mech., vol. 646, 2010, pp. 39–58). Our experiments give a top-hat entrainment coefficient of $0.048\pm 0.006$. We measure a maximum vertical plume velocity that follows the scaling predicted by Cooper & Hunt but is significantly smaller. Our measurements allow us to construct a turbulent plume model that predicts all plume properties at any height. We use this plume model to calculate plume widths, velocities and Reynolds numbers for typical dissolving icebergs and ice fronts and for a typical room with a heated or cooled vertical surface.

Theoretical Analysis and Semianalytical Solutions for a Turbulent Buoyant Hydrogen-Air Jet

Semianalytical solutions are developed for turbulent hydrogen-air plume. We derived analytical expressions for plume centerline variables (radius, velocity, and density deficit) in terms of a single universal function, called plume function. By combining the obtained analytical expressions of centerline variables with empirical Gaussian expressions of the mean variables, we obtain semianalytical expressions for mean quantities of hydrogen-air plume (velocity, density deficit, and mass fraction).