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  • Author or Editor: J. W. Lavelle x
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J. W. Lavelle and D. C. Smith IV

Abstract

Effects of rotation on finite-length line plumes are studied with a three-dimensional nonhydrostatic numerical model. Geophysical convection with this source geometry occurs, for example, as the result of fissure releases of hot hydrothermal fluids at the seafloor from terrestrial release of hot gases and ash during volcanic activity along fissures and in the descent from the sea surface of brines formed during freezing of ice leads at high latitudes. Here the model treats the case of a starting plume of dense fluid descending into a rotating environment. Results are compared with laboratory experiments so that the validity of the model, particularly the nonlinear subgrid-scale mixing formulation, might first be established. Differences in plumes caused by varying rotation rate, &ohm, and buoyancy flux, B 0, are the primary focus, with experiments in fluid of depth h spanning a convective Rossby number [B 0 1/3/(2Ωh)] of 0.01−1.0. Rotation initiates spiraling of the descending plumes but it has little effect on the speed of plume descent; the latter depends on the strength of turbulent mixing. Low rotation rates allow the descending plume cap to be broad and the stem to be narrow. Higher rotation rates retard the lateral spread of the plume cap and widen the plume stem. Updraft at the stem edge is very much larger at higher rotation rates, and that appears to be instrumental in determining stem and cap width. Values of turbulent mixing coefficients within the plume are dependent on B 0 but not on Ω. Thus rotational effects on turbulence are not needed to account for differences in plume structure arising solely from Ω variation. Agreement between model and laboratory results did not occur without a nonlinear time- and space-dependent subgrid-scale mixing parameterization, suggesting that model applications to convective geophysical problems identified above require the same.

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H. O. Mofjeld and J. W. Lavelle

Abstract

Frequently the mixing length l in second-order closure models is assumed to have a constant value l 0 = γL at large distances from the bottom with a magnitude proportional to the first moment L of turbulent intensity. Although it is often stated that turbulence closure model results are relatively insensitive to the value of mixing length parameter γ, we show that this is not the case for a second-order Level II model of the steady bottom boundary layer in an unstratified fluid. In particular, the eddy viscosity and diffusivity depend strongly on γ. Available oceanic data on geostrophic drag ratio lead to a value of γ of approximately 0.2–0.3. Atmospheric data for steady flow suggest a smaller value of 0.05–0.1 although the atmospheric observations are ambiguous about the choice of γ, possibly because it is difficult to find truly neutrally stratified and steady-state conditions in the bottom boundary layer. A value of γ between 0.18 and 0.20 is required for the model to match a similarity theory having a linear–exponential form for viscosity. Fitting an M2 tidal current profile at a station in Admiralty Inlet, Washington, with an oscillatory analog of the model yields γ = 0.20 ± 0.04.

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J. W. Lavelle, M. A. Wetzler, E. T. Baker, and R. W. Embley

Abstract

Tidal and inertial currents and profuse hydrothermal discharge at recently erupted Axial Volcano, Juan de Fuca Ridge, cause relatively large and rapid temperature (T) changes in the near-bottom water column. Measurements show short-term T variations of as much as 0.13°C at 30 m and 0.18°C at 150 m above bottom and currents that have strong tidal components and means of 3–5 cm s−1. Locations and magnitudes of the hydrothermal sources leading to the observed T variations have been inferred via an inverse calculation. Results imply noncongruent source regions around the mooring site for plumes from low- and high-buoyancy flux sources. Water column and seafloor observations in the volcano’s caldera region generally support the distribution of source types and sites inferred. A high-buoyancy flux, ephemeral venting site, unexpected on the eastern shoulder of the volcano, is also indicated by the inverse calculation and supported by water-column survey data. Over the O(10 km2) calculation region, heat flux from low-buoyancy hydrothermal sources is apparently less than heat flux from high-buoyancy hydrothermal sources, a result that is in disagreement with previous reports on the balance of heat flux between vent source types.

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