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- Author or Editor: W. S. Lewellen x
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Abstract
A probabilistic framework for incorporating uncertainty in air quality models is described. The quantitative dependence of the uncertainty in calculated air quality concentrations on the uncertainty in the input meteorological data is illustrated using a simple Second-order Closure integrated Model Plume in combination with the EPRI Plume Model Validation and Development Data Set. Evaluation of the model results demonstrate that even though individual hourly samples cannot be deterministically predicted downwind of a powerplant stack, statistical representations of the observed cumulative distribution of the sample values are quite predictable. We discuss the data needed to improve the definition of the range of meteorological uncertainty within an ensemble of flows defined by given meteorological data, and thus provide for improvements in predictability models of the type illustrated. We argue that attempts to collect the data needed to define more precisely the variance within the ensemble of compatible flows will prove more productive than attempts to eliminate meteorological uncertainties in given datasets.
Abstract
A probabilistic framework for incorporating uncertainty in air quality models is described. The quantitative dependence of the uncertainty in calculated air quality concentrations on the uncertainty in the input meteorological data is illustrated using a simple Second-order Closure integrated Model Plume in combination with the EPRI Plume Model Validation and Development Data Set. Evaluation of the model results demonstrate that even though individual hourly samples cannot be deterministically predicted downwind of a powerplant stack, statistical representations of the observed cumulative distribution of the sample values are quite predictable. We discuss the data needed to improve the definition of the range of meteorological uncertainty within an ensemble of flows defined by given meteorological data, and thus provide for improvements in predictability models of the type illustrated. We argue that attempts to collect the data needed to define more precisely the variance within the ensemble of compatible flows will prove more productive than attempts to eliminate meteorological uncertainties in given datasets.
Abstract
An investigation of the influence of the ratio of surface sensible heat flux to latent heat flux, the Bowen ratio. on the structure of boundary-layer clouds is carried out utilizing numerical large eddy simulations (LES). The role of cloud-top radiational cooling, cloud-top temperature and moisture jump conditions, and wind shear are included in a secondary way. Although no detailed comparisons have been made, the LES results appear to be qualitatively consistent with the Atlantic Stratocumulus Transition Experiment, the recent field study on marine boundary-layer cloud structure. Some conclusions that follow from an examination of these LES results are the following: First, there is a highly bimodal character to the cloud ceiling frequency within a very low Bowen ratio boundary layer. The updrafts tend to produce a lower cloud ceiling than the surrounding environment with its weak downdrafts. Second, a very low Bowen ratio with the aid of some boundary-layer shear makes the development of persistent microcell cloud circulations possible within the boundary layer. Third, when the surface latent heat flux is the dominant factor in the dynamics of the boundary layer, the approach to a conditionally stable lapse rate results in the potential for subsequent decoupling. Last, the maximum partial cloud fraction is very well represented by the relation suggested by Sommeria and Deardorff for a Gaussian probability distribution for the range of conditions studied.
Abstract
An investigation of the influence of the ratio of surface sensible heat flux to latent heat flux, the Bowen ratio. on the structure of boundary-layer clouds is carried out utilizing numerical large eddy simulations (LES). The role of cloud-top radiational cooling, cloud-top temperature and moisture jump conditions, and wind shear are included in a secondary way. Although no detailed comparisons have been made, the LES results appear to be qualitatively consistent with the Atlantic Stratocumulus Transition Experiment, the recent field study on marine boundary-layer cloud structure. Some conclusions that follow from an examination of these LES results are the following: First, there is a highly bimodal character to the cloud ceiling frequency within a very low Bowen ratio boundary layer. The updrafts tend to produce a lower cloud ceiling than the surrounding environment with its weak downdrafts. Second, a very low Bowen ratio with the aid of some boundary-layer shear makes the development of persistent microcell cloud circulations possible within the boundary layer. Third, when the surface latent heat flux is the dominant factor in the dynamics of the boundary layer, the approach to a conditionally stable lapse rate results in the potential for subsequent decoupling. Last, the maximum partial cloud fraction is very well represented by the relation suggested by Sommeria and Deardorff for a Gaussian probability distribution for the range of conditions studied.
Abstract
Debris clouds provide an important visual signature of tornadoes and can potentially significantly affect the wind structure, damage potential, and Doppler radar measurements of tornado wind speeds. To study such issues, the dynamics of finescale debris have been added to an existing high-resolution large-eddy simulation model of tornado dynamics. A so-called “two-fluid” or “Eulerian–Eulerian” approach is employed, together with a surface layer model for lofting and depositing debris. In this paper the debris implementation is described, three critical dimensionless parameters governing tornado debris effects are identified, and sample results from a large set of simulations of tornadoes with idealized debris are presented. The results demonstrate that the accumulation of small-scale debris within the surface layer and corner flow can significantly alter the wind speeds and flow structure of the tornado vortex within a few hundred meters of the surface. They suggest that the total mass of the debris cloud can reach tens of thousands of tons. Near the surface, the debris mass loading can be well above 1, the peak mean velocities can be reduced by as much as half, and the total momentum (air plus debris) can either significantly increase or decrease. Local air and debris velocities can differ significantly and in a nontrivial fashion, thereby complicating the interpretation of Doppler radar measurements of tornado structure. Debris fluctuations, centrifuging, negative buoyancy, and angular momentum transport are all significant mechanisms for the debris effects. A negative physical feedback reduces the sensitivity of the results to changes in the parameterization of the surface debris fluxes. The realistic simulation of tornado debris clouds and surface damage tracks should prove useful in identifying the dynamics governing their observed counterparts.
Abstract
Debris clouds provide an important visual signature of tornadoes and can potentially significantly affect the wind structure, damage potential, and Doppler radar measurements of tornado wind speeds. To study such issues, the dynamics of finescale debris have been added to an existing high-resolution large-eddy simulation model of tornado dynamics. A so-called “two-fluid” or “Eulerian–Eulerian” approach is employed, together with a surface layer model for lofting and depositing debris. In this paper the debris implementation is described, three critical dimensionless parameters governing tornado debris effects are identified, and sample results from a large set of simulations of tornadoes with idealized debris are presented. The results demonstrate that the accumulation of small-scale debris within the surface layer and corner flow can significantly alter the wind speeds and flow structure of the tornado vortex within a few hundred meters of the surface. They suggest that the total mass of the debris cloud can reach tens of thousands of tons. Near the surface, the debris mass loading can be well above 1, the peak mean velocities can be reduced by as much as half, and the total momentum (air plus debris) can either significantly increase or decrease. Local air and debris velocities can differ significantly and in a nontrivial fashion, thereby complicating the interpretation of Doppler radar measurements of tornado structure. Debris fluctuations, centrifuging, negative buoyancy, and angular momentum transport are all significant mechanisms for the debris effects. A negative physical feedback reduces the sensitivity of the results to changes in the parameterization of the surface debris fluxes. The realistic simulation of tornado debris clouds and surface damage tracks should prove useful in identifying the dynamics governing their observed counterparts.
Abstract
Since maximum Mach numbers in an F5 tornado are expected to, at least, reach ∼0.4, a study was undertaken to determine the major potential compressibility effects in such flows. Results from compressible large-eddy simulations of tornado corner flow dynamics are summarized. Comparison with previous incompressible simulations indicates that Mach number effects tend to be modest and may be estimated by an isentropic approximation. As the average maximum Mach number M within the tornado increases above one-half, the largest changes occur for “low-swirl” corner flows, those exhibiting a central vertical jet off the surface capped by a vortex breakdown, with the vortex breakdown found to occur at significantly greater heights as M increases. It also appears that the highest Mach numbers are most likely to occur during rapid transients in near-surface intensification that can sometimes occur during a tornado's evolution.
Abstract
Since maximum Mach numbers in an F5 tornado are expected to, at least, reach ∼0.4, a study was undertaken to determine the major potential compressibility effects in such flows. Results from compressible large-eddy simulations of tornado corner flow dynamics are summarized. Comparison with previous incompressible simulations indicates that Mach number effects tend to be modest and may be estimated by an isentropic approximation. As the average maximum Mach number M within the tornado increases above one-half, the largest changes occur for “low-swirl” corner flows, those exhibiting a central vertical jet off the surface capped by a vortex breakdown, with the vortex breakdown found to occur at significantly greater heights as M increases. It also appears that the highest Mach numbers are most likely to occur during rapid transients in near-surface intensification that can sometimes occur during a tornado's evolution.
Abstract
The results of high-resolution, fully three-dimensional, unsteady simulations of the interaction of a tornado with the surface are presented. The goal is to explore some of the range of structures that should be expected to occur in nature within the tornadic “corner flow”—that region where the central vortex meets the surface. The most important physical variables considered are the tornado-scale circulation and horizontal convergence, the effective surface roughness, the tornado translation speed, the low-level inflow structure, and the upper-core structure. A key ingredient of the corner flow dynamics is the radial influx of fluid in the surface layer with low angular momentum relative to that of the fluid in the main vortex above it. This low swirl fluid arises initially from outside or below the larger-scale vortex or through frictional loss of angular momentum to the surface and forms much of the vortex core flow after it exits the corner flow region. Changes in the surface layer inflow or upper-core structure can dramatically affect the level of intensification and turbulent structure in the corner flow even when the swirl ratio of the tornado vortex as a whole is unchanged. The authors define a local corner flow swirl ratio, S c , based on the total flux of low angular momentum fluid through the corner flow and show that it parameterizes the leading effects on the corner flow of changes to the flow conditions immediately outside of the corner flow. As S c decreases, the low-level vortex intensity rises to a maximal level where mean swirl velocities near the surface reach 2.5 times the maximum mean swirl velocity aloft; further decreases force a transition to a much weaker low-level tornado vortex. This sensitivity suggests that differences in the near-surface inflow layer may be a critical factor in determining whether an existing supercell low-level mesocyclone spawns a tornado or not.
Abstract
The results of high-resolution, fully three-dimensional, unsteady simulations of the interaction of a tornado with the surface are presented. The goal is to explore some of the range of structures that should be expected to occur in nature within the tornadic “corner flow”—that region where the central vortex meets the surface. The most important physical variables considered are the tornado-scale circulation and horizontal convergence, the effective surface roughness, the tornado translation speed, the low-level inflow structure, and the upper-core structure. A key ingredient of the corner flow dynamics is the radial influx of fluid in the surface layer with low angular momentum relative to that of the fluid in the main vortex above it. This low swirl fluid arises initially from outside or below the larger-scale vortex or through frictional loss of angular momentum to the surface and forms much of the vortex core flow after it exits the corner flow region. Changes in the surface layer inflow or upper-core structure can dramatically affect the level of intensification and turbulent structure in the corner flow even when the swirl ratio of the tornado vortex as a whole is unchanged. The authors define a local corner flow swirl ratio, S c , based on the total flux of low angular momentum fluid through the corner flow and show that it parameterizes the leading effects on the corner flow of changes to the flow conditions immediately outside of the corner flow. As S c decreases, the low-level vortex intensity rises to a maximal level where mean swirl velocities near the surface reach 2.5 times the maximum mean swirl velocity aloft; further decreases force a transition to a much weaker low-level tornado vortex. This sensitivity suggests that differences in the near-surface inflow layer may be a critical factor in determining whether an existing supercell low-level mesocyclone spawns a tornado or not.
Abstract
High-resolution, fully three-dimensional, unsteady simulations of the interaction of a tornado vortex with the surface were performed in an attempt to answer questions about the character of turbulent transport in this unique flow. The authors demonstrate that sufficient resolution was achieved for the particular physical conditions of their example that the time-averaged velocity and pressure distributions showed little sensitivity in the region of maximum velocities to either finer resolution or modified subgrid turbulent model. The time-averaged velocity distributions show the maximum velocity values occurring within 50 m of the surface. The instantaneous velocity distributions show the turbulence dominated by a relatively small number of strong secondary vortices spiralling around the main vortex with the maximum instantaneous velocities typically one-third larger than the maximum time-averaged velocity. These eddies are centered a little inside of the cone of maximum mean swirl velocity and spiral around the mean vortex at velocities less than the average maximum velocity. Statistical analysis of the velocity fluctuations induced by the secondary vortices shows that the turbulent transport of angular momentum is predominantly inward at low levels, allowing the inner recirculating flow to acquire values of angular momentum of up to 30% of that provided by the inflow boundary conditions, thus enhancing the surface intensification of the velocities.
Abstract
High-resolution, fully three-dimensional, unsteady simulations of the interaction of a tornado vortex with the surface were performed in an attempt to answer questions about the character of turbulent transport in this unique flow. The authors demonstrate that sufficient resolution was achieved for the particular physical conditions of their example that the time-averaged velocity and pressure distributions showed little sensitivity in the region of maximum velocities to either finer resolution or modified subgrid turbulent model. The time-averaged velocity distributions show the maximum velocity values occurring within 50 m of the surface. The instantaneous velocity distributions show the turbulence dominated by a relatively small number of strong secondary vortices spiralling around the main vortex with the maximum instantaneous velocities typically one-third larger than the maximum time-averaged velocity. These eddies are centered a little inside of the cone of maximum mean swirl velocity and spiral around the mean vortex at velocities less than the average maximum velocity. Statistical analysis of the velocity fluctuations induced by the secondary vortices shows that the turbulent transport of angular momentum is predominantly inward at low levels, allowing the inner recirculating flow to acquire values of angular momentum of up to 30% of that provided by the inflow boundary conditions, thus enhancing the surface intensification of the velocities.
Abstract
The boundary-layer eddy structure under conditions similar to the cold-air outbreak of GALE IOP 2 is studied using numerical simulations. The simulations are run in two basic modes: a quasi-two-dimensional version that takes advantage of the observed “cloud-street” character of the flow, and a fully three-dimensional, unsteady simulation on a limited domain where periodic conditions are assumed to prevail. The two-dimensional simulation exhibits a cloud structure similar to that observed when the surface fluxes agree with the aircraft measurements. This requires very different values of effective surface roughness for temperature and humidity, which is unlikely to have been assumed in the absence of data. The three-dimensional simulation reveals that even when the eddy structure on this severely limited domain does not exhibit a dominant two-dimensional roll structure, the average turbulent statistics are quite consistent with those from the two-dimensional simulation. It is argued that a larger domain than can be readily used is needed to see a distinct cloud street pattern.
Abstract
The boundary-layer eddy structure under conditions similar to the cold-air outbreak of GALE IOP 2 is studied using numerical simulations. The simulations are run in two basic modes: a quasi-two-dimensional version that takes advantage of the observed “cloud-street” character of the flow, and a fully three-dimensional, unsteady simulation on a limited domain where periodic conditions are assumed to prevail. The two-dimensional simulation exhibits a cloud structure similar to that observed when the surface fluxes agree with the aircraft measurements. This requires very different values of effective surface roughness for temperature and humidity, which is unlikely to have been assumed in the absence of data. The three-dimensional simulation reveals that even when the eddy structure on this severely limited domain does not exhibit a dominant two-dimensional roll structure, the average turbulent statistics are quite consistent with those from the two-dimensional simulation. It is argued that a larger domain than can be readily used is needed to see a distinct cloud street pattern.
Abstract
A practical model of atmospheric dispersion of a passive tracer based on systematic reduction of the second-order closure transport equations using Gaussian shape assumptions is presented. The model is comparable with conventional Gaussian plume models in complexity, but still maintains the capability to also predict concentration fluctuation variance and to utilize direct measurements of turbulent velocity variances in a consistent manner. Comparison with laboratory data demonstrates the model's ability to produce reasonable predictions for the concentration field.
Abstract
A practical model of atmospheric dispersion of a passive tracer based on systematic reduction of the second-order closure transport equations using Gaussian shape assumptions is presented. The model is comparable with conventional Gaussian plume models in complexity, but still maintains the capability to also predict concentration fluctuation variance and to utilize direct measurements of turbulent velocity variances in a consistent manner. Comparison with laboratory data demonstrates the model's ability to produce reasonable predictions for the concentration field.
Abstract
Two-dimensional numerical computations of the developing boundary layer with a positive surface heat flux are presented. The model incorporates moisture phase-change effects; we are particularly interested in the cloud-street formation. The results show that cloud streets with large aspect ratios of about 10 can develop under certain conditions, and cloud top entrainment is identified as a crucial feature. Runs without phase change indicate roll merging to a certain extent, but do not product aspect ratios greater than 4.
Abstract
Two-dimensional numerical computations of the developing boundary layer with a positive surface heat flux are presented. The model incorporates moisture phase-change effects; we are particularly interested in the cloud-street formation. The results show that cloud streets with large aspect ratios of about 10 can develop under certain conditions, and cloud top entrainment is identified as a crucial feature. Runs without phase change indicate roll merging to a certain extent, but do not product aspect ratios greater than 4.
Abstract
No abstract available.
Abstract
No abstract available.
