Abstract We report herein the direct digital remote sensing of schools of Pacific herring ( Clupea harengus pallasi), using a newly developed Compact Airborne Spectrographic Imager (CASI). We use the spectrometer capabilities of the instrument to obtain the spectral signature of the schools and their natural background. The imaging capabilities of the instrument are then used to collect spatial data for five different spectral bands. Simple image processing procedures are used to calculate school areas. Compared to aerial photography, digital remote sensing of fish schools offers several advantages including rapid turn around of results, digital image processing and archiving, as well as the possibility ofstudying the distribution of fish in relation to other parameters which can be remotely measured. Such as phyloplankton (via chlorophyll fluorescence or the blue/green colour), sediment concentration, oil slicks, and water mass boundaries.
The emplacement dynamics of lava flows on a slope is investigated using theoretical analyses and laboratory experiments for the case where a fixed volume of lava is rapidly released and propagates downhill as a two‐dimensional flow. When the lava has no internal yield strength, we identify four dynamical flow regimes that can arise: an inertial slumping regime, a horizontal viscous regime, a sloping viscous regime, and a crust yield strength regime that finally stops the flow. When the lava has an internal yield strength, it can also flow in a sloping viscoplastic regime which is accurately predicted by a simple box model that we derive. Our results are applied to predict the propagation downhill of various volumes of two typical lavas: a Hawaiian lava with no internal yield strength and a Mount Etna lava with an internal yield strength. In particular, we find that sloping flows of the Mount Etna lava are stopped by the surface crust strength rather than the internal yield strength.
The formation of channelized lava flows on a wide uniform slope is investigated both theoretically and experimentally. When a lava is released at a constant flow rate from a point source, we predict that it flows both down and across the slope at the same rate in a early time regime before undergoing a transition to a long‐time regime where down‐slope flow is faster than lateral flow. Eventually, the lateral flow is stopped by the strength of the growing surface crust, and the flow then travels down slope in a channel of constant width. Using scaling analysis, we derive expressions for the final channel width in both flow regimes, as a function of the flow rate, the slope, the density difference driving the flow, the lava viscosity, the thermal diffusivity, and the yield strength of the crust. We also find a dimensionless flow morphology parameter that controls whether the subsequent channel flow occurs in a “mobile crust” regime or in a “tube” regime. These theoretical predictions are in good agreement with laboratory experiments in which polyethylene glycol wax flows down a wide uniform slope under cold water. The theory is also applied to the understanding of the formation of a basaltic sheet flow lobe in Hawaii, which had an estimated crust yield strength of order 6 × 10 4 Pa.
We examine the dissolution of a sloping solid surface driven by turbulent compositional convection. The scaling analysis presented by Kerr & McConnochie ( J. Fluid Mech. , vol. 765, 2015, pp. 211–228) for the dissolution of a vertical wall is extended to the case of a sloping wall. The model has no free parameters and no dependence on height. It predicts that while the interfacial temperature and interfacial composition are independent of the slope, the dissolution velocity is proportional to $\cos ^{2/3}\unicode[STIX]{x1D703}$ , where $\unicode[STIX]{x1D703}$ is the angle of the sloping surface to the vertical. The analysis is tested by comparing it with laboratory measurements of the ablation of a sloping ice wall in contact with salty water. We apply the model to make predictions of the turbulent convective dissolution of a sloping ice shelf in the polar oceans.
An extensive series of laboratory experiments is used to quantify the circumstances under which fluids can be mixed by natural convection at high flux Rayleigh number. A compositionally buoyant fluid was injected at a fixed rate into an overlying layer of ambient fluid from a planar, horizontally uniform source. The nature of the resulting compositional convection was found to depend on two key dimensionless parameters: a Reynolds number Re and the ratio U of the ambient fluid viscosity to the input fluid viscosity. Increasing the Reynolds number corresponded to increasing the vigor of the convection, while the viscosity ratio was found to determine the spacing between plumes and whether buoyant fluid rose as sheets ( U < 1) or axisymmetric plumes ( U > 1). From measurements of the final density profile in the fluid after the experiments we quantified the extent to which buoyant liquid was mixed in terms of a thermodynamic mixing efficiency E . The mixing efficiency was found to be high ( E > 0.9) when either the Reynolds number was large ( Re > 100) or the viscosity ratio was small ( U < 0.2) and was found to be low ( E < 0.1) when both Re < 1 and U > 200. The amount of mixing was related to whether ascending plumes generated a large‐scale circulation in the ambient fluid. When our results are applied to the differentiation of the Earth's core, we suggest that the convection resulting from the release of buoyant residual liquid into the liquid outer core due to crystallization at the boundary between the inner and the outer core will probably lead to nearly complete mixing. In the dynamically very different context of the mantle, mantle plumes are predicted to ascend through the mantle and pond beneath the lithosphere, whereas convection driven by the subduction of oceanic lithosphere is expected to produce moderate to extensive mixing of the mantle. When the competing plate and plume modes of mantle convection are considered together, we find that owing to a larger driving buoyancy flux, the plate‐scale flow will destroy any stratification at the top of the mantle produced by mantle plumes. Applying our results to the “stagnant lid” style of thermal convection predicted to occur in the mantles of the Moon, Mercury, Mars, Venus, and pre‐Archean Earth, we expect the respective flows to produce minor thermal stratification at the respective core‐mantle boundaries. In part 2 of this study [ Jellinek and Kerr , this issue] we apply our results to the differentiation of magma chambers and komatiite lava flows.
The thermal erosion of cold felsic ground by the steady laminar flow of a hot basaltic lava is examined theoretically and experimentally. Initially, a chill layer is grown and then remelted at the base of the lava flow. A steady thermal erosion velocity is then established, which is limited by the buoyant instability of the melted ground or by the effective freezing temperature of the basaltic lava. When the theoretical analysis is applied to the longest lava tube system of the Cave Basalt on Mount St. Helens, it is found that about 100 days of flow is sufficient to produce the observed ground erosion in Little Red River, Ape and Lake Caves.
Abstract During the postcumulus stage of solidification in layered intrusions, fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks. One mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir. Convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma. Another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir. This process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile. Calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions, but could not form pure monomineralic rocks. Intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment. Dense magma, emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers. Reactions between melt and matrix may lead to changes in mineral compositions, mineral textures and whole rock isotope compositions. Another important mechanism for forming adcumulate rocks is compaction, in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled. For cumulate layers from 10 to 1000 metres in thickness, compaction can reduce porosities to very low values (< 1%) and form monomineralic rocks. The characteristic time-scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion. During compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix. Both compaction and compositional convection can be interrupted by solidification in the pore spaces. Compositional convection will only occur if the Rayleigh number is larger than 40, if the residual melt becomes lower in density, and the convective velocity exceeds the solidification velocity (measured by the rate of crystal accumulation in the chamber). Orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection.
The Dissolution of Polar Ice into a Stratified Ocean Craig D. McConnochie and Ross C. Kerr Research School of Earth Sciences, The Australian National University, Canberra craig.mcconnochie@anu.edu.au Abstract We use laboratory experiments to investigate the effect of stratification on the dissolution of polar ice. The temperature at the ice-ocean interface, the ablation rate, and the plume velocity are measured as a function of height and of stratification. We observe that stratification reduces the interface temperature, the ablation velocity and plume velocity. To compare our results with geophysical ice shelves we propose a stratification parameter that describes where stratification will be important. We use the stratification parameter to predict that ocean stratification will have an important effect on the dissolution of ice shelves in the polar regions. Finally, we compare our experimental results to a common numerical parameterization of ice-ocean interactions and note some significant differences. Introduction An important component of global climate is the increasingly rapid decrease in the mass of the Antarctic and Greenland Ice Sheets (Rignot et al., 2011). This mass loss is occurring on the underside and fronts of ice shelves formed where glaciers reach the polar oceans and from the icebergs that calve from them. The polar oceans provide a source of heat and salt that controls the dissolution of ice shelves and icebergs. The transport of heat and salt to the ice-ocean interface is assisted by a turbulent wall plume that forms next to the ice face. Recent observations made in the cavity of Pine Island Glacier ice shelf show that the ocean is unstably stratified in temperature and stably stratified in salinity (Jenkins et al., 2010). This stratification will affect the turbulent wall plume and hence the transport of heat and salt to the ice-ocean interface. As such, the ambient stratification could have an important effect on the mass loss from the Antarctic and Greenland Ice Sheets. Previously we have conducted experiments in a homogeneous ambient fluid over a range of far field conditions. It was observed that both the ablation rate and interface temper- ature are uniform with height (Kerr and McConnochie, 2015). In contrast, the maximum plume velocity increases like z 1/3 where z is the distance from the transition to turbulence (McConnochie and Kerr, 2016). This is inconsistent with the standard three-equation parameterization of ice shelf melting that typically predicts that the ablation rate and plume velocity are proportional to one another (Jenkins, 2011). It is important to assess whether this contradiction between laboratory observations and numerical parameteriza- tions remains once stratification is included in the laboratory experiments. Method We have conducted experiments that investigate the dissolution of a vertical ice face in cold salty water with a stable salinity gradient. The experiments were conducted in a 1.2 m high, 1.5 m wide, and 0.2 m long tank that was kept in a temperature controlled room. The far field temperature was kept constant at 3.5 o C and the mean far field salinity VIII th Int. Symp. on Stratified Flows, San Diego, USA, Aug. 29 - Sept. 1, 2016
Abstract Numerical models of ice‐ocean interactions typically rely upon a parameterization for the transport of heat and salt to the ice face that has not been satisfactorily validated by observational or experimental data. We compare laboratory experiments of ice‐saltwater interactions to a common numerical parameterization and find a significant disagreement in the dependence of the melt rate on the fluid velocity. We suggest a resolution to this disagreement based on a theoretical analysis of the boundary layer next to a vertical heated plate, which results in a threshold fluid velocity of approximately 4 cm/s at driving temperatures between 0.5 and C, above which the form of the parameterization should be valid.
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