Boundary Layer Transport Research Articles

Uncertainty in Estimating Boundary-Layer Transport during Highly Convective Conditions

Abstract Routine and supplemental rawinsonde observations collected during the Preliminary Regional Experiment for Storm-Central (PRE-STORM) were analyzed to assess the uncertainty in boundary-layer trajectory calculations due to imprecise interpolation of the horizontal wind field. This study was designed to complement our earlier analysis of rawinsonde data collected during the Cross Appalachian Tracer Experiment (CAPTEX; Kahl and Samson 1986). The present study is representative of widespread convective conditions, while our previous study was representative of fairly persistent, undisturbed flow. Spatial autocorrelation analysis revealed significant wind field variability on scales less than 100 km. Evaluation of several spatial and temporal interpolation techniques yielded mean absolute errors in estimation of u and v wind components ranging from 3.3–6.5 m −1. Spatial interpolation accuracy improved only slightly when supplemental measurements were included in the interpolation procedure. Estimates o...

A two‐stream model of the vertical transport of trace species in the convective boundary layer

A conceptually simple two‐stream model (TSM) of the convective planetary boundary layer (PBL) provides a surprisingly realistic description of basic boundary layer transport features. The model is empirically fitted to a variety of important passive tracer problems. This model, unlike previous eddy diffusion models of vertical transport, is constructed to have a basic similarity to the underlying physics of transport. One parameterization can describe sources at the top or the bottom boundary or at all points within the domain and thereby may be used in simulations describing chemical or radiative forcings throughout the PBL. The TSM also allows estimates of concentration variations due to sources, chemical processes, and temporal variation in the boundary conditions. However, it is intended for use only in problems in which the relatively exact description of the PBL provided by a large‐eddy model is not practical. The model is likely to be most useful in parameterizing vertical turbulent transfer in the convective boundary layer when the chemical time scale is comparable to or greater than the convective time scale zi/w*, around 15 min. We present one model set of parameters which appears to perform well in a variety of situations. A methodology is available which may be applied to fit the model to a variety of PBL transport situations as accurate tracer measurements or three‐dimensional model data become available.

Impact of natural hydrocarbons on hydroxyl and peroxy radicals at a remote site

The impact of non‐methane hydrocarbons of natural and anthropogenic origin on the photochemistry of the nonurban lower atmosphere has been studied using a photochemical model with detailed planetary boundary layer transport. The research was undertaken to evaluate the increase in peroxy radical concentrations due to hydrocarbon oxidation and their contribution to observed imbalances in the NOx, O3 photostationary state (Parrish et al., 1986a). The results show that these hydrocarbons, at concentrations observed in the atmosphere, reduce the hydroxyl radical concentration in clean air and increase the peroxy radical concentration to levels that are needed to explain the observed imbalance in the photostationary state. For the conditions simulated here, natural hydrocarbons are the principal contributors to the increase in the peroxy radical concentration at low NOx mixing ratios. At higher NOx levels the anthropogenic hydrocarbons that scale with NOx gain in importance. Model simulations that include the hydrocarbons reproduce the observed dependence of the concentration of ozone on various NOx concentrations.

Polymerization of olefins through heterogeneous catalysis. VI. Effect of particle heat and mass transfer on polymerization behavior and polymer properties

The multigrain model for polymerization of olefins over solid catalysts is used to predict kinetic behavior, molecular weights, and polydispersities. The effects of intraparticle and external boundary layer transport resistance on the kinetic behavior and polymer properties are explored. Means for the experimental detection of intraparticle diffusion resistance are suggested. The importance of catalyst physical properties, such as the porosity, and the catalyst loading is illustrated through simulation. Finally, the hypotheses of diffusion resistance and site heterogeneity as explanations for the broad molecular weight distributions of olefin polymers are critically evaluated, and molecular weight distribution control in industrial catalysts is discussed.

Interaction between soil hydrology and boundary-layer development

A two-layer model of soil hydrology and thermodynamics is combined with a one-dimensional model of the planetary boundary layer to study various interactions between evolution of the boundary layer and soil moisture transport. Boundary-layer moistening through surface evaporation reduces the potential and actual surface evaporation as well as the boundary-layer growth. With more advanced stages of soil drying, the restricted surface evaporation allows greater sensible heat flux which enhances boundary-layer growth and entrainment drying.

Thermal catastrophe in the plasma sheet boundary layer

This Letter presents a first step towards a substorm model including particle heating and transport in the plasma sheet boundary layer (PSBL). The heating mechanism discussed is resonant absorption of Alfven waves. For some assumed MHD perturbation incident from the tail lobes onto the plasma sheet, the local heating rate in the PSBL has the form of a resonance function of the one‐fluid plasma temperature. Balancing the local heating by convective transport of the heated plasma toward the central plasma sheet, an "equation of state" is found for the steady‐state PSBL whose solution has the form of a mathematical catastrophe: at a critical value of a parameter containing the incident power flux, the local density, and the convection velocity, the equilibrium temperature jumps discontinuously. Associating this temperature increase with the abrupt onset of the substorm expansion phase, the catastrophe model indicates at least three ways in which the onset may be triggered. Several other consequences related to substorm dynamics are suggested by the simple catastrophe model

Adjustment of a thermally-stratified fluid in a container with vertical through-flow

The adjustment process of a thermally-stratified fluid in a container in response to an imposed vertically upward through-flow, M 0, is investigated. Finite-difference numerical solutions to the timedependent Navier-Stokes equations for a cylindrical container are obtained to verify the theoretical model of Rahm ( Int. J. Heat Mass Transfer 29, 1479–1485 (1986)). The numerical results are in close agreement with the theoretical predictions for the interior temperatures. During the transient phase, the vertical sidewall boundary layer transport grows, and the vertical extent of the boundary layer increases. In the steady state, the boundary layer transport carries most of the imposed through-flow M 0. The interior is virtually stagnant with near-linear stratification. Because of the dominance of the advective activities, the time scale for the overall adjustment process is substantially reduced below the diffusive time. It is demonstrated that, with a proper choice of M 0, this method can be utilized as an effective tool to produce a stably stratified fluid in the geophysical laboratory.

Damage components of powdery mildew in winter wheat

The photosynthesis and transpiration rates of winter wheat leaves were measured at different developmental stages and at various levels of mildew infection. Even at low levels of infection, circa 4% leaf area being covered, both the assimilation and transpiration rates at light satiation were considerably reduced. Light use efficiency and dark respiration were not significantly affected. The physiological background of these effects was analysed by photosynthesis measurements at different external CO2 concentrations. It was shown that the carboxylation resistance was increased due to mildew infection and that stomatal resistance, boundary layer resistance and transport resistance in the mesophyll were only indirectly affected. A simulation model was used to compute the consequences of these effects on daily growth rate of a winter wheat canopy at different development stages and with different levels of mildew severity. These computations show that a low infection level may result in a considerable reduction of the crop growth rate. This effect was more pronounced when the sky was clear than overcast. Bepalingen van fotosynthese en verdamping werden verricht bij verschillende ontwikkelingsstadia van wintertarwe, die in verschillende mate was aangetast door meeldauw. Zelfs bij vrij lage infectiepercentages (circa 4%) was er al een aanzienlijke reductie van assimilatie en verdampingssnelheid bij lichtverzadiging. Lichtbenuttingsefficiëntie en donkerademhaling werden niet significant beïnvloed. De fysiologische achtergrond van deze effecten werd onderzocht door fotosynthesemetingen bij verschillende externe CO2 concentraties. Er werd aangetoond dat de carboxylatieweerstand werd verhoogd door meeldauwinfectie en dat de huidmondjesweerstand, grenslaagweerstand en transportweerstand in het mesofyl indirect werden beïnvloed. De gevolgen van deze effecten op de dagelijkse groeisnelheid van een wintertarwegewas in verschillende ontwikkelingsfasen en met verschillende LAI werd nagegaan met een simulatiemodel. Deze berekeningen tonen aan dat een geringe meeldauwaantasting resulteert in een aanzienlijke reductie van de groeisnelheid van het gewas. Dit effect was duidelijker bij heldere dan bij bewolkte hemel.

Transport in boundary layer of divertor tokamaks

Study of transport in the boundary layer of tokamak plasma in presence of magnetic divertors is extended to the second order in ion collision frequency. Numerical results for ion and energy losses to the collector plates are presented for toroidal and poloidal divertors. For the toroidal case, the Wiener-Hopf solution for the second order distribution function is obtained. An error occurring in an earlier first order calculation is pointed out and corrected first order fluxes are also given. For the poloidal divertor, asymmetry observed in ion and energy transport is found to persist in the second order result.

Tropospheric ozone: The role of transport

The Geophysical Fluid Dynamics Laboratory general circulation/transport model, with photochemistry in the top level (middle stratosphere) only, is used to simulate global tropospheric ozone distributions for upper and lower limits of surface removal rates. We compared these simulations with available observations and find that large‐scale atmospheric transport plays a major role in the behavior of tropospheric ozone. Furthermore, we identify potential roles for tropospheric chemistry, discover defects in the model's simulations of transport, and gain a more global picture of tropospheric ozone. The transport model's mean cross‐tropopause flux is in the range of previous estimates, and the shapes of the simulated vertical profiles of mixing ratio and percent standard deviation are in good agreement with observations. South of 40°N, the simulated and observed latitude gradients are the same, the upper and lower limit calculations bracket measured values, and the seasonal cycles are well reproduced. While the transport model simulates a wide range of tropospheric ozone climatology, there are a significant number of disagreements. The need for additional ozone destruction in the maritime boundary layer suggests a role for chemical destruction, while in the continental boundary layer, it appears that chemical production, a seasonal cycle in surface deposition, and improved boundary layer transport are all required. The two major defects in the simulated “free troposphere” are (1) excess ozone at high latitudes of the northern hemisphere (NH) and (2) spring rather than summer maxima and fall rather than winter minima at NH mid‐ and high latitudes. While defect 1 has a number of possible causes, deficiencies in model transport play a major role. Although similar transport defects have not been ruled out for defect 2, tropospheric chemistry appears to be needed. Separate calculations of the net chemical production and loss demonstrate that this is a complex problem. The most likely solution involves the transport control of NOx which controls the ozone chemistry.

Metallurgical reaction and deposit fusion boundary microstructure under a stationary plasma arc

Alloy element loss, decarburization in melted region and microstructural change of a deposit fusion boundary under a stationary plasma arc have been investigated by a first-order kinetic and interfacial microstructure. The cylindrical specimens of iron-based alloys, Fe-C, Ni-C, and Co-C alloys were locally melted by a plasma arc using argon plasma gas and Ar+H2 shielding gas, and the rates of alloy element loss and decarburization in the melted region were measured. Moreover, the fusion boundaries experienced when nickel, iron, Ni-Fe filler metals were deposited on iron, Fe-C, nickel and cobalt base metals, were evaluated metallographically. The initial rate of alloy element loss decreases as follows: Fe-Al>Fe-Mn>Fe-Cr>SGI-Mg>Fe-Ni. The loss reaction mechanism is metallic evaporation and the rate seems to be limited by transport in the gas boundary layer. The magnitude of decarburization is as follows: Ni-C>Fe-C> Co-C. The decarburization rate in Fe-C alloy is assumed to be governed by a process involving mass transfer in the gas phase and the molten metal. However, at low carbon concentrations, the rate appears to be limited by transfer in liquid metal. Fusion boundary deposited with nickel filler metal on iron is regular, but with carbon added to iron, an infiltration of deposit metal into adjacent base metal occurs. The fusion boundary with iron deposited on nickel is irregular where thin Ni-Fe solid solution is formed. In a deposited fusion boundary of cobalt with iron, nickel, and Ni-Fe filler metals, FeCo compound formation occurs, with cobalt dissolving into the nickel deposit metal resulting in a tongue-like structure produced by nickel penetration and a fine columnar substructure formation produced by Ni-Fe diffusion.

Heat Transport in the Atmospheric Boundary Layer

Abstract The structure of turbulence and transport of heat is examined from data obtained from 11 aircraft soundings executed in heated boundary layers during the Air Mass Transformation Experiment. Various influences on the turbulent transport are revealed by analyzing properties of the joint frequency distribution in polar coordinate. Such an analysis allows determination of a correlation coefficient and fluctuation amplitude as a function of properties of different types of motion comprising the total flow. In particular, the contribution of various events to the total flux is revealed without a priori specification of sampling criteria. In the present case, shear generation of turbulence and broken stratocumulus at the boundary layer top modify the profiles of virtual and sensible heat flux. The upward virtual heat flux shows the usual linear decrease with height but vanishes and becomes downward at the relatively low level of 60% of the boundary layer top. The vertical structure of the heat flux is e...

THE INFLUENCE OF INJECTION OR WITHDRAWAL OF FLUID ON FREE CONVECTION ABOUT A VERTICAL CYLINDER IN A POROUS MEDIUM

10. Free convection about a vertical cylinder in a porous medium subject to constant wall temperature or constant wall heat flux is considered. The influence of uniform lateral mass flux on the boundary-layer transport is studied. Numerical results for heat transfer along the boundary and temperature profiles are presented for various values of the parameter cj which is a measure of the effect of injection or withdrawal of fluid as compared to the combined transverse curvature and lateral mass flux effects.

Mass transport in a turbulent boundary layer under a progressive water wave

The bottom boundary layer under a progressive water wave is studied using Saffman's turbulence model. Saffman's equations are analysed asymptotically for the case Re [Gt ] 1, where Re is a Reynolds number based on a characteristic magnitude of the orbital velocity and a characteristic orbital displacement. Approximate solutions for the mass-transport velocity at the edge of the boundary layer and for the bottom stress are obtained, and Taylor's formula for the rate of energy dissipation is verified. The theoretical results are found to agree well with observations for sufficiently large Reynolds numbers.

Coupled boundary layer transport involving particles

AbstractParticle deposition by diffusiophoresis and thermophoresis is described using the linearized Fickian multicomponent diffusion equations. For these cases, the diagonal component of the diffusivity matrix for the particle is expected to be much smaller than the other diagonal components. Intuitive arguments, dimensional analyses and the standard interfacial transport theories in multicomponent form suggest that the larger boundary layer due to the larger diffusivity component describes the transition between bulk and surface values for both components.

Toward Convective Boundary Layer Parameterization: A Scalar Transport Module

Abstract Recent results from large-eddy simulations indicate that the eddy diffusivity for scalar diffusion through the top of a convective boundary layer is different from that for diffusion through the bottom. From this, one can show that the eddy diffusivity is not well behaved in general and can have singularities. As an alternative to the high-resolution approach using eddy-diffusivity closure, we propose an improved integral method for calculating scalar transport in the convective boundary layer. Based on an analytical parameterization for the mean scalar profile, it is computationally as simple as the familiar “mixed-layer” integral method but is physically more realistic, allowing a mean scalar gradient within the mixed layer. We present sample results for the evolution of humility, CO and O3 profiles during the day.

A Lagrangian study of the boundary layer transport of pollutants in the northeastern United States

This paper describes the Lagrangian aircraft sampling of a unique segment of the Baltimore/Washington urban plume as it was transported along the Northeast Corridor on 14 August 1980. Plume cross-sectional analyses of NO x and O 3 at four downwind distances are presented and discussed relative to physical processes and chemical interactions/transformations. The analyses indicated (1) longrange transport of O 3, at least to 400 km, along the Northeast Corridor, (2) significant O 3, scavenging when the NO 2 concentrations exceed 20 ppb and (3) O 3 concentrations within the urban plume during the daylight hours increased as much as 100 ppb, while the background concentrations increased only 30 ppb. In addition, the time of the highest 1-h average O 3 concentrations at surface monitoring sites beneath the urban plume increased with increasing distances from Baltimore to New York City. Significant surface O 3 peaks were observed under cloudy skies downwind of New York City after midnight and are believed to be the result of transport from the corridor region SW of New York City.

One-dimensional vertical model for ozone and other gases in the atmospheric boundary layer

A vertical eddy diffusion coefficient profile for mass transport in the atmospheric boundary layer is established on the basis of a calculation where the equations of motion and the thermodynamic energy equation are solved. A one-dimensional vertical model for the chemical turnover is then developed. It is assumed that the turbulent mass transport is equal to the product of the vertical mean concentration gradient and the vertical eddy diffusion coefficient for mass transport. About 40 chemical species are calculated in the model, and about 90 chemical reactions are involved. A log-linear grid is introduced, with close spacing near the ground and coarser towards the free troposphere. The diurnal cycle of ozone in an atmospheric boundary layer with strong convective mixing during the day and a shallow layer under the influence of mechanical stress at night, is computed and compares well with observations. It is found that in polluted air, depletion of ozone close to the ground through reaction with NO prevents ozone from reaching the ground and be removed by deposition. The concentration of hydroxyl has a marked gradient with height, with maximum in the layers where nitrogen oxides, hydrocarbons and sulphur dioxide are emitted. Nitrogen dioxide, sulphur dioxide and the hydrocarbons (ethane, n- butane , ethylene, propylene and m- xylene ) accumulate throughout the night in the shallow nocturnal layer.

On the second approximation to mass transport in the bottom boundary layer

The second approximation is obtained for the mass transport velocity within the oscillatory bottom boundary layer beneath sinusoidal progressive and standing waves of finite amplitude. This approximation includes a simple new term, which essentially ensures continuity of the vertical gradient of mass transport at the edge of the layer and is of third-order in the perturbation (or wave-slope) parameter. For long progressive waves in conditions of zero net mass flow, the term represents a moderate reduction in mass transport at the edge of the layer, compared with the first approximation of Longuet-Higgins. For standing waves of arbitrary length, the mass transport is reduced (increased) far from (near) the bottom, except near nodal locations where an increase (a reduction) is predicted. The proposed correction to the first approximation yields clearly improved results when compared with appropriate experimental evidence. Deficiencies in the higher-order theories of Sleath and Isaacson for propagating waves are disclosed.

Acoustic profiling of suspended sediments in the marine bottom boundary layer

An acoustic (3 MHz) backscattering profilometer has been developed for study of sediment transport in the bottom boundary layer. Laboratory experiments indicate reasonable correlations between backscattering intensity and mass concentrations for well‐sorted fine to coarse quartz sands typical of those found in marine sediments. Departure of the calibration curves from their expected values is thought to be due to the non‐spherical shape of the quartz grains. Field experiments on the inner shelf using the profilometer combined with velocity profiles show individual profiles (taken at 1 Hz) during erosion events are characterized by highly variable concentrations. Surface wave groups rather than individual waves appear to be correlated with groups of profiles with high concentrations extending 25‐100 cm above bottom. Averaging produces smooth profiles which fit well log‐normal distributions predicted by steady flow suspended load theories.