Modeling the effect of plume-rise on the transport of carbon monoxide over Africa with NCAR CAM
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Abstract. We investigated the effects of fire-induced plume-rise on the simulation of carbon monoxide (CO) over Africa and its export during SAFARI 2000 using the NCAR Community Atmosphere Model (CAM) with a CO tracer and a plume-rise parameterization scheme. The plume-rise parameterization scheme simulates the consequences of strong buoyancy of hot gases emitted from biomass burning, including both dry and cloud-associated (pyro-cumulus) lofting. The current implementation of the plume-rise parameterization scheme into the global model provides an opportunity to examine the effect of plume-rise on long-range transport. The CAM simulation with the plume-rise parameterization scheme seems to show a substantial improvement of the agreements between the modeled and aircraft-measured vertical distribution of CO over Southern Africa biomass-burning area. The plume-rise mechanism plays a crucial role in lofting biomass-burning pollutants to the middle troposphere. In the presence of deep convection we found that the plume-rise mechanism results in a decrease of CO concentration in the upper troposphere. The plume-rise depletes the boundary layer, and thus leaves lower concentrations of CO to be lofted by the deep convection process. The effect of the plume-rise on free troposphere CO concentration is more important for the source area (short-distance transport) than for remote areas (long-distance transport). A budget analysis of CO burden over Southern Africa reveals the plume-rise process to have a similar impact as the chemical production of CO by OH and CH4. In addition, the plume-rise process has an minor impact on the regional export. These results further confirm and extend previous findings in a regional model study. Effective lofting of large concentration of CO by the plume-rise mechanism also has implications for local air quality forecasting in areas affected by other fire-related pollutants.Keywords:
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Abstract We describe a series of new laboratory experiments which examine the rise of a two‐dimensional buoyancy‐driven plume of freshwater through a porous layer initially saturated with aqueous saline solution. Measurements show that the plume head accounts for a constant fraction of about 0.7 of the buoyancy supplied at the source and that it grows as it rises through the porous layer. However, the morphology of the plume head becomes increasingly complex as the ratio of the injection speed to the buoyancy rise speed increases, with the fluid spreading laterally and developing localized buoyant fingers which intermingle with the ambient fluid. Behind the plume head, a tail of nearly constant width develops providing a pathway from the source to the plume head. These starting plume dynamics may be relevant for buoyancy‐driven contaminant dispersal and also for the convection which develops during CO 2 sequestration as CO 2 dissolves into aquifer water.
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Abstract The occurrence of buoyancy‐induced vertical flow (sinking) of a bromide (Br − ) tracer plume at small injection concentrations is investigated in transport experiments conducted in a large‐scale physical aquifer model containing a homogeneous and isotropic sand pack. Two‐well tracer tests are conducted using Br − at concentrations ranging from 50 to 1000 mg/1, corresponding to relative densities between 7.5 × 10 −5 and 1.5 × 10 −3 . Analysis of three‐dimensional solute concentration data indicates that the center of mass of the Br − plume was displaced downward as the denser tracer solution sank through the sand pack. Plume sinking occurred at all solute concentrations investigated; the magnitude of the vertical displacement increased with increasing Br − concentration of the injected tracer solution. The dynamic collapse of the Br − plume caused by buoyancy forces resulted in increased apparent transverse and longitudinal dispersivities. The results suggest that the possibility of buoyancy‐induced flow must be considered when interpreting tracer tests conducted with anion concentrations as low as 50 mg/1. The occurrence of buoyancy‐induced flows at such low relative densities also suggest that the phenomenon may be more widespread than is generally recognized.
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Abstract This paper considers the effects of turbulent plumes with time-varying buoyancy released from a point source in a confined region. Attention is concentrated on the case of cyclically varying buoyancy sources. After many cycles, an asymptotic regime is created in which the fluid away from the plume is stably stratified. During the statically unstable phase of the cycle, the plume extends farther and farther into the fluid, spreading out horizontally at a level determined predominantly by the density in the plume at that depth. For about one half of this phase the plume extends almost the full depth of the container and then retreats back toward the source.
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A steady state bubble‐plume model is evaluated using full‐scale temperature, salinity, and dissolved oxygen data collected in a Swiss lake. The data revealed a plume‐generated near‐field environment that differed significantly from the ambient far‐field water column properties. A near‐field torus of reduced stratification developed around the plume, the extent of which is on the same lateral scale as the horizontal dislocations generated by persistent first‐mode seiching. The plume fallback water was found to penetrate much deeper than expected, thereby maintaining reduced vertical gradients in the near‐field torus. The plume entrains a portion of the fallback water leading to short‐circuiting, which generates a complex plume‐lake interaction and reduces far‐field downwelling relative to the upward plume flow. As the integral plume model incorporates the entrainment hypothesis, it is highly sensitive to the near‐field environmental conditions. After identifying appropriate near‐field boundary conditions the plume model predictions agree well with the field observations.
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