Showing papers on "Convective mixing published in 2012"

Scaling of convective mixing in porous media.

Abstract: Convective mixing in porous media is triggered by a Rayleigh-B\'enard-type hydrodynamic instability as a result of an unstable density stratification of fluids. While convective mixing has been studied extensively, the fundamental behavior of the dissolution flux and its dependence on the system parameters are not yet well understood. Here, we show that the dissolution flux and the rate of fluid mixing are determined by the mean scalar dissipation rate. We use this theoretical result to provide computational evidence that the classical model of convective mixing in porous media exhibits, in the regime of high Rayleigh number, a dissolution flux that is constant and independent of the Rayleigh number. Our findings support the universal character of convective mixing and point to the need for alternative explanations for nonlinear scalings of the dissolution flux with the Rayleigh number, recently observed experimentally.

Convective mixing in the central Irminger Sea: 2002-2010

Abstract: A near-continuous time series of 8 years of daily hydrographic profiles, recorded between fall 2002 and summer 2010 by moorings located in the central Irminger Sea, is presented. This record shows that convective mixing down to 400 m depth occurs in most winters. Under favorable conditions, convective mixing is seen to reach much deeper. During the cold winter of 2007–2008 mixed layers reached depths of 1 km. In the subsequent, more moderate winter of 2008–2009, a stronger preconditioning of the Irminger Gyre led to mixed layers down to 800 m depth. The convectively formed waters in the Irminger Sea are more saline and warmer than those formed in the Labrador Sea, but potential vorticity is reduced to 0.7 10

Horizontal Density Structure and Restratification of the Arctic Ocean Surface Layer

Abstract: Ice-tethered profiler (ITP) measurements from the Arctic Ocean’s Canada Basin indicate an ocean surface layer beneath sea ice with significant horizontal density structure on scales of hundreds of kilometers to the order 1 km submesoscale. The observed horizontal gradients in density are dynamically important in that they are associated with restratification of the surface ocean when dense water flows under light water. Such restratification is prevalent in wintertime and competes with convective mixing upon buoyancy forcing (e.g., ice growth and brine rejection) and shear-driven mixing when the ice moves relative to the ocean. Frontal structure and estimates of the balanced Richardson number point to the likelihood of dynamical restratification by isopycnal tilt and submesoscale baroclinic instability. Based on the evidence here, it is likely that submesoscale processes play an important role in setting surface-layer properties and lateral density variability in the Arctic Ocean.

Numerical Simulation of Natural Convection in Heterogeneous Porous media for CO2 Geological Storage

Abstract: We report a modeling and numerical simulation study of density-driven natural convection during geological CO2 storage in heterogeneous formations. We consider an aquifer or depleted oilfield overlain by gaseous CO2, where the water density increases due to CO2 dissolution. The heterogeneity of the aquifer is represented by spatial variations of the permeability, generated using Sequential Gaussian Simulation method. The convective motion of the liquid with dissolved CO2 is investigated. Special attention is paid to instability characteristics of theCO2 concentration profiles, variation ofmixing length, and averageCO2 mass flux as a function of the heterogeneity characterized by the standard deviation and the correlation length of the log-normal permeability fields. The CO2 concentration profiles show different flow patterns of convective mixing such as gravity fingering, channeling, and dispersive based on the heterogeneity medium of the aquifer. The variation of mixing length with dimensionless time shows three separate regimes such as diffusion, convection, and second diffusion. The average CO2 mass flux at the top boundary decreases quickly at early times then it increases, reaching a constant value at later times for various heterogeneity parameters.

Carbon and oxygen isotopic ratios in Arcturus and Aldebaran: Constraining the parameters for non convective mixing on the RGB

Abstract: We re-analysed the carbon and oxygen isotopic ratios in the atmospheres of the two bright K giants Arcturus and Aldebaran. Previous determinations of their 16O/18O ratios showed a rough agreement with FDU expectations; however, the estimated 16O/17O and 12C/13C ratios were lower than in the canonical predictions. These anomalies are interpreted as signs of the occurrence of non-convective mixing episodes. We re-investigated this issue in order to verify whether the observed data can be reproduced in this hypothesis and if the well determined properties of the two stars can help us in fixing the uncertain parameters characterizing non-convective mixing and its physical nature. We used high-resolution infrared spectra to derive the 12C/13C and 16O/17O/18O ratios from CO molecular lines near 5 mu. We also reconsidered the determination of the stellar parameters to build the proper atmospheric and evolutionary models. We found that both the C and the O isotopic ratios for the two stars considered actually disagree with pure FDU predictions. This reinforces the idea that non-convective transport episodes occurred in them. By reproducing the observed elemental and isotopic abundances with the help of parametric models of nucleosynthesis and mass circulation, we derived constraints on the properties of non convective mixing. We find that very slow mixing is incapable of explaining the observed data, which require a fast transport. Circulation mechanisms with speeds intermediate between those typical of diffusive and of convective mixing should be at play. We however conclude with a word of caution on the conclusions possible at this stage, as the parameters for the mass transport are rather sensitive to the stellar mass and initial composition.

Turbulent convection model in the overshooting region. i. effects of the convective mixing in the solar overshooting region

Abstract: The overshooting in the framework of the turbulent convection model is investigated in the solar overshooting region. The overshooting mixing is treated as a diffusive process. It is found that the sound speed profile can be improved to be in good agreement with helioseismic inversions. The bump in the sound speed differences between solar models and the helioseismical inversions below the base of the solar convective envelope is almost eliminated by the overshooting mixing. The overshooting mixing leads to a significant depletion of Li in the main-sequence stage. Li abundance in the solar surface can be reduced to about 1% of its initial abundance in the solar models with the overshooting mixing. The solar model with the overshooting shows a smooth profile of the temperature gradient, which is also favored by the helioseismology.

Modeling and Simulation of Nanoparticle Transport in Multiphase Flows in Porous Media: CO2 Sequestration

Abstract: Geological storage of anthropogenic CO2 emissions in deep saline aquifers has recently received tremendous attention in the scientific literature. Injected CO2 plume buoyantly accumulates at the top part of the deep aquifer under a sealing cap rock, and some concern that the high-pressure CO2 could breach the seal rock. However, CO2 will diffuse into the brine underneath and generate a slightly denser fluid that may induce instability and convective mixing. Onset times of instability and convective mixing performance depend on the physical properties of the rock and fluids, such as permeability and density contrast. The novel idea is to adding nanoparticles to the injected CO2 to increase density contrast between the CO2-rich brine and the underlying resident brine and, consequently, decrease onset time of instability and increase convective mixing. As far as it goes, only few works address the issues related to mathematical and numerical modeling aspects of the nanoparticles transport phenomena in CO2 storages. In the current work, we will present mathematical models to describe the nanoparticles transport carried by injected CO2 in porous media. Buoyancy and capillary forces as well as Brownian diffusion are important to be considered in the model. IMplicit Pressure Explicit Saturation-Concentration (IMPESC) scheme is used and a numerical simulator is developed to simulate the nanoparticles transport in CO2 storages.

Estimating effective rates of convective mixing from commercial-scale injection

Abstract: In this work, the ongoing CO2 injection at the Utsira formation is considered as a field-scale study for CO2 storage. We employ an upscaled model for CO2 migration that in addition to the standard two-phase flow physics includes dissolution, effective convective mixing and capillary trapping. The aim of this work is to get the first field-scale estimates of the effective upscaled convective mixing rates. To account for the uncertainties in the description of the storage formation, sensitivity studies are conducted relative to some of the most uncertain parameters. This paper thus presents the first field-scale estimates of upscaled convective mixing in the context of CO2 storage. Our result gives upscaled convective mixing rates in the order of 15 kg/m2/year. These numbers are comparable, but somewhat higher than previous analysis using high-resolution numerical simulations would indicate. As such, our work validates the use of numerical simulations to obtain upscaled convective mixing rates, while at the same time yielding validation of convective mixing as an important and quantifiable storage mechanism in the Utsira formation.

Scaling of Convective Mixing in Porous Media

Abstract: Convective mixing in porous media is triggered by a Rayleigh–Benard-type hydrodynamic instability as a result of an unstable density stratification of fluids. While convective mixing has been studied extensively, the fundamental behavior of the dissolution flux and its dependence on the system parameters are not yet well understood. Here, we show that the dissolution flux and the rate of fluid mixing are determined by the mean 10

Interpreting seasonal convective mixing in Devils Hole, Death Valley National Park, from temperature profiles observed by fiber-optic distributed temperature sensing

Abstract: [1] Devils Hole, a groundwater-filled fracture in the carbonate aquifer of the southern Nevada Mojave Desert, represents a unique ecohydrological setting, as home to the only extant population of Cyprinodon diabolis, the endangered Devils Hole pupfish. Using water column temperatures collected with a fiber-optic distributed temperature sensor (DTS) during four field campaigns in 2009, evidence of deep circulation and nutrient export are, for the first time, documented. The DTS was deployed to measure vertical temperature profiles in the system, and the raw data returned were postprocessed to refine the calibration beyond the precision of the instrument's native calibration routines. Calibrated temperature data serve as a tracer for water movement and reveal a seasonal pattern of convective mixing that is supported by numerical simulations of the system. The periodic presence of divers in the water is considered, and their impacts on the temperature profiles are examined and found to be minimal. The seasonal mixing cycle may deplete the pupfish's food supplies when nutrients are at their scarcest. The spatial and temporal scales of the DTS observations make it possible to observe temperature gradients on the order of 0.001°C m−1, revealing phenomena that would have been lost in instrument noise and uncertainty.

Internal waves downstream of Norfolk Ridge, western Pacific, and their biophysical implications

Abstract: An 11-d quasi-Lagrangian surface layer experiment to the east of Norfolk Island tracked a 140-m-deep drifting vertical array (DVA) of instruments, including conductivity sensors, thermistors, and current meters. These are the first in situ data from the area to measure large-amplitude internal waves. The observations show isotherm peak-to-peak excursions reaching 50 m and were dominated by the semidiurnal forcing frequency. The DVA exhibited horizontal loops at the semidiurnal tidal frequency as it tracked water at depths of 80–140 m to within 10% of total horizontal displacement. The large vertical isotherm excursions generated substantial vertical shear. Temperature microstructure estimates of the vertical diffusivity of scalar properties at 70-m depth, around the center of the thermocline, were on the order of 1024 m2 s21 and around an order of magnitude larger at shallower depths. When combined with nitrate data, this implies vertical fluxes of nitrate in the pycnocline of , 1 mmol m22 d21. The internal waves also potentially cause an interaction between the variability in tidal forcing and the diurnal radiation cycle that influences chlorophyll in the deep chlorophyll maximum. The average effect of the internal wave was to increase the light level for a particular isotherm over the static case. The internal waveinduced velocities were strong enough to dominate phytoplankton rise speeds and so potentially played a role in the formation and persistence of an observed Trichodesmium bloom. The majority of the world’s oceans are oligotrophic and so a good deal of effort has focused on describing the carbon cycle and resulting primary production in such waters in order to identify limits to production (Jenkins and Doney 2003). A key component of this description is a reliable nitrogen inventory and its associated transport fluxes. Such transport is typically dominated by turbulent mixing. While wind shear and convective mixing clearly dominate turbulence at the water surface, deeper in the water column internal waves become an important driver of vertical mixing (Naveira Garabato et al. 2004). The present work looks at the role of internal wave–driven mixing and its influence on carbon cycle processes in a region characterized as being oligotrophic (Law et al.

The interannual variability of potential temperature in the central Labrador Sea

Abstract: [1] The interannual variability of potential temperature in the central Labrador Sea is studied with a suite of numerical simulations with an eddy-resolving regional ocean model and compared with available observations. The model successfully reproduces the observed variations in potential temperature at depths comprised between 150 and 2000 m over the period 1980–2009, capturing also the warming trend of the last decade and the deep water formation event in 2008. The suite of experiments allows for quantifying the contribution from the physical forcings responsible for the interannual variability of potential temperature in the region. The local atmospheric forcing drives the interannual signal by driving convection, while the incoming current system along the east coast of Greenland is responsible for about half of the warming trend (∼0.3–0.4°C) during the last decade through restratification process. The lateral transport of Irminger water in the convective region into the central Labrador Sea is further analyzed integrating a passive tracer. It is found that the overall amount of Irminger water transported in the convective region of the Labrador Sea is directly correlated with the amount of vertical convective mixing. In the last decade, following the decrease in convective activity, the model reveals a substantial decrease in concentration of Irminger Current water below 500 m in the Labrador Sea interior: by 2010 the overall amount is less than half than in the previous 20 years.

Salinity intrusion and convective mixing in the Atlantic Equatorial Undercurrent

Abstract: This study investigates the advection of positive-salinity anomalies by the Equatorial Undercurrent (EUC) and their potential importance in inducing vertical convective mixing For this purpose we use hydrographic and velocity observations taken in April 2010 along the western Atlantic equatorial ocean (32 to 43°W) The high-salinity EUC core is a few tens of metres thick and occupies the base of the surface mixed layer and the upper portion of the surface thermocline It leads to high positive values of the vertical salinity gradient, which in many instances cause statically unstable conditions in otherwise well-stratified regions The unstable regions result in vertical convection, hence favouring the occurrence of step-like features We propose that this combination of horizontal advection and vertical-instability leads to a sequence of downward-convective events As a result the EUC salinity is diffused down to a potential density of 2643, or about 200 m deep This mechanism is responsible for water-mass and salt downwelling in the equatorial Atlantic Ocean, with a potentially large influence on the tropical and subtropical cells

A simple and self-consistent geostrophic-force-balance model of the thermohaline circulation with boundary mixing

Abstract: . A simple model of the thermohaline circulation (THC) is formulated, with the objective to represent explicitly the geostrophic force balance of the basinwide THC. The model comprises advective-diffusive density balances in two meridional-vertical planes located at the eastern and the western walls of a hemispheric sector basin. Boundary mixing constrains vertical motion to lateral boundary layers along these walls. Interior, along-boundary, and zonally integrated meridional flows are in thermal-wind balance. Rossby waves and the absence of interior mixing render isopycnals zonally flat except near the western boundary, constraining meridional flow to the western boundary layer. The model is forced by a prescribed meridional surface density profile. This two-plane model reproduces both steady-state density and steady-state THC structures of a primitive-equation model. The solution shows narrow deep sinking at the eastern high latitudes, distributed upwelling at both boundaries, and a western boundary current with poleward surface and equatorward deep flow. The overturning strength has a 2/3-power-law dependence on vertical diffusivity and a 1/3-power-law dependence on the imposed meridional surface density difference. Convective mixing plays an essential role in the two-plane model, ensuring that deep sinking is located at high latitudes. This role of convective mixing is consistent with that in three-dimensional models and marks a sharp contrast with previous two-dimensional models. Overall, the two-plane model reproduces crucial features of the THC as simulated in simple-geometry three-dimensional models. At the same time, the model self-consistently makes quantitative a conceptual picture of the three-dimensional THC that hitherto has been expressed either purely qualitatively or not self-consistently.

The HDO/H2O relationship in tropospheric water vapor in an idealized "last-saturation" model

Abstract: Previous model studies have shown that the isotopic composition of tropospheric water vapor is sensitive to atmospheric water transport processes, but compositional information is difficult to interpret due to the complexity of the models. Here an attempt is made to clarify the sensitivity by computing the relationship between tropospheric HDO (via dD) and H2O (via specific humidity q) in an idealized model atmosphere based on a "last-saturation" framework that includes convection coupled to a steady large-scale circulation with prescribed horizontal mixing. Multiple physical representations of convection and mixing allow key structural as well as parametric uncertainties to be explored. This model has previously been shown to reproduce the essential aspects of the humidity distribution. Variations of dD or q individually are dominated by local dynamics, but their relationship is preserved advectively, thus revealing conditions in regions of convection. The model qualitatively agrees with satellite observations, and reproduces some parametric sensitivities seen in previous GCM experiments. Sensitivity to model assumptions is greatest in the upper troposphere, apparently because in-situ evaporation and condensation processes in convective regions are more dominant in the budget there. In general, vapor recycling analogous to that in continental interiors emerges as the crucial element in explaining why dD exceeds that predicted by a simple Rayleigh process; such recycling involves coexistent condensation sinks and convective moisture sources, induced respectively by (for example) waves and small-scale convective mixing. The relative humidity distribution is much less sensitive to such recycling. © 2012 American Geophysical Union. All Rights Reserved.

A simple and self-consistent geostrophic-force-balance model of the thermohaline circulation with boundary mixing

Abstract: . A simple model of the thermohaline circulation (THC) is formulated, with the objective to represent explicitly the geostrophic force balance of the basinwide THC. The model comprises advective-diffusive density balances in two meridional-vertical planes located at the eastern and the western walls of a hemispheric sector basin. Boundary mixing constrains vertical motion to lateral boundary layers along these walls. Interior, along-boundary, and zonally integrated meridional flows are in thermal-wind balance. Rossby waves and the absence of interior mixing render isopycnals zonally flat except near the western boundary, constraining meridional flow to the western boundary layer. The model is forced by a prescribed meridional surface density profile. This two-plane model reproduces both steady-state density and steady-state THC structures of a primitive-equation model. The solution shows narrow deep sinking at the eastern high latitudes, distributed upwelling at both boundaries, and a western boundary current with poleward surface and equatorward deep flow. The overturning strength has a 2/3-power-law dependence on vertical diffusivity and a 1/3-power-law dependence on the imposed meridional surface density difference. Convective mixing plays an essential role in the two-plane model, ensuring that deep sinking is located at high latitudes. This role of convective mixing is consistent with that in three-dimensional models and marks a sharp contrast with previous two-dimensional models. Overall, the two-plane model reproduces crucial features of the THC as simulated in simple-geometry three-dimensional models. At the same time, the model self-consistently makes quantitative a conceptual picture of the three-dimensional THC that hitherto has been expressed either purely qualitatively or not self-consistently.

Efficiency of Dissolution Trapping in Geological Carbon Storage

Abstract: During geological storage of carbon dioxide (CO2), several mechanisms contribute to safe storage by immobilizing the CO2 in the injection formation. It has been shown that dissolution into resident brine can be one of the major contributors. The injected supercritical CO2 is buoyant, but dissolved CO2 increases brine density and therefore reduces the tendency for upward CO2 migration. The density increase with dissolved CO2 leads to convective mixing of the brine, thereby enabling more CO2 to dissolve. It is important to quantify the efficiency of CO2 dissolution, and therefore the efficiency of convective mixing. In previous work, we have shown that convective mixing can be considerably enhanced when taking into acccount the interaction between the two-phase region (supercritical CO2 and brine), and the single-phase brine region. Bounds on this impact were obtained for onset times, wavelengths of unstable fi ngers, and dissolution rates. The maximum increase in the dissolution rate was found to be large, when interaction with the plume was considered. In this paper, we use stability analysis to further study the dissolution in more detail. We make technical contributions to the field of stability analysis and in obtaining more reliable estimates of the efficiency of dissolution trapping.

Variability in Labrador Sea Water formation

Abstract: The Atlantic Meridional Overturning Circulation (AMOC) transports of a large amount of heat towards the North Atlantic region. Since this circulation is considered to have shown pronounced variability in the past, and a weakening is projected for the 21st century, it is very important to understand and monitor the mechanisms that determine its variability. Deep water formation is one of the most important of these mechanisms as it plays an important role in setting the shape and strength of the AMOC. It only takes place in a few locations in the ocean, one of which is the Labrador Sea. In this thesis two processes that play an important role in determining the variability of Labrador Sea Water formation are studied as well as the possibility to monitor this variability using satellite altimetry measurements. The first process study focused on the restratification period after a deep convection event. The dense water in the area affected by deep convection is then (partly) replaced by more buoyant water originating from the boundary currents that encircle the interior. Using a numerical model in an idealized configuration, the roles of three eddy types that are known to play a role in the restratification process were studied. It was found that the presence of Irminger Rings is essential for a realistic amount of restratification in the Labrador Sea. The second process study focused on the effects of a very fresh surface layer, which makes the surface layer lighter and can, if light enough, inhibit convective mixing. The well-known case of the Great Salinity Anomaly (1969-1971), which was fortuitously well documented by the measurements taken at ocean weather station “Bravo” in the central Labrador Sea, has been analyzed. In contrast to what is commonly assumed, only a combination of the fresh surface layer and the very mild winter conditions in 1969 could have started the convective shutdown, and only a combination of the extremely harsh winter and a salinification of the upper water column could have caused its resumption in 1972. Moreover, two so far undiscovered positive surface feedbacks were found (both acting through the low wintertime sea surface temperature) that limit the buoyancy flux to the atmosphere and thereby actively reinforce the shutdown state. Apart from understanding the variability in Labrador Sea Water formation, it is also important to monitor this variability. The network of satellite altimeters does not suffer from limitations as harsh winter conditions and poor coverage and can therefore give valuable additional information to in situ measurements. Altimeters can detect the sea surface lowering that accompanies the densification of the water column during deep water formation. Although this signal is small compared to variability in sea surface height induced by other processes, still the approximate depth of deep convection (less than 1000 m, between 1000 and 1500 m or more than 1500 m depth) and the location of the convection area at a larger scale can be determined