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1.
On equilibrium profiles of suspended particles 总被引：1，自引：1，他引：0
A power law is often used to represent the vertical profile of uniform suspended particles above a horizontally homogeneous surface. It serves as an analytical solution representing an equilibrium between vertical turbulent diffusion and gravitational settling, andcan be used to extract settling velocity information from observed particle number density profiles. In this note, we analyse this situation and use a numerical model to investigate the temporal change of particle number density and of the net vertical flux due to turbulent diffusion and gravitational settling. The results show that the net flux approaches zero very slowly for small particles (ws/ u* < 1), and show that the power law does not hold for small particles. If the power law is used to extract settling velocities from observed vertical distributions of particle number density in these cases, the estimated settling velocity may be unrealistically large. 相似文献
2.
The dispersion of heavy particles subjected to a turbulent forcing is often simulated with Lagrangian stochastic models. Although
these models have been employed successfully over land, the implementation of traditional LS models in the marine boundary
layer is significantly more challenging. We present an adaptation of traditional Lagrangian stochastic models to the atmospheric
marine boundary layer with a particular focus on the representation of the scalar turbulence for temperature and humidity.
In this new model, the atmosphere can be stratified and the bottom boundary is represented by a realistic wavy surface that
moves and deforms. Hence, the correlation function for the turbulent flow following a particle is extended to the inhomogenous,
anisotropic case. The results reproduce behaviour for scalar Lagrangian turbulence in a stratified airflow that departs only
slightly from the expected behaviour in isotropic turbulence. When solving for the surface temperature and the radius of evaporating
heavy water droplets in the airflow, the modelled turbulent forcing on the particle also behaves remarkably well. We anticipate
that this model will prove especially useful in the context of sea-spray dispersion and its associated sensible heat, latent
heat, and gas fluxes between spray droplets and the atmosphere. 相似文献
3.
We study the dynamics of sea-spray particles in the coastal region of La Reunion Island on the basis of numerical simulations using the transport aerosol model MACMod (Marine Aerosol Concentration Model) and a survey of the aerosol size distributions measured at four locations at two different heights in the north-west part of the island. This allows evaluation of the performance of our model in case of pure marine air masses with implementation of accurate boundary conditions. First of all, an estimate of the aerosol concentration at 10-m height at the upwind boundary of the calculation domain is obtained using a revisited version of the MEDEX (Mediterranean Extinction) model. Estimates of the vertical profile of aerosol concentrations are then provided using aerosol data obtained at two different heights at the upwind boundary of the calculation domain. A parametrization of the vertical profiles of aerosol concentrations for maritime environment is proposed. The results are then compared to the vertical profiles of 0.532 \(\upmu \)m aerosol particle extinction coefficient obtained from lidar data provided by the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) and also to the data provided by the Aerosol Robotic Network (AERONET). This allows validation of the complete vertical profiles in the mixed layer and shows the validity of satellite data for determination of the vertical profiles. Two kinds of simulation were made: one without a particle advection flux at the upwind boundary of the numerical domain, whereas the second simulation was made with a particle advection flux. In the first case, the influence of the distance to the shoreline on the local sea-spray dynamics is investigated. In the second set of simulation, the particles issued from the local production in the surf zone near the shoreline are mixed with aerosols advected from the remote ocean. A good agreement between the model calculations using our boundary conditions and the data was found. The present results then attest the ability of this kind of model, as a first approach to predicting the local transport of sea-spray particles in a pure marine environment. 相似文献
4.
A Lagrangian Stochastic Model for Heavy Particle Dispersion in the Atmospheric Marine Boundary Layer
The dispersion of heavy particles and pollutants is often simulated with Lagrangian stochastic (LS) models. Although these
models have been employed successfully over land, the free surface at the air-sea interface complicates the implementation
of traditional LS models. We present an adaptation of traditional LS models to the atmospheric marine boundary layer (MBL),
where the bottom boundary is represented by a realistic wavy surface that moves and deforms. In addition, the correlation
function for the turbulent flow following a particle is extended to the anisotropic, unsteady case. Our new model reproduces
behaviour for Lagrangian turbulence in a stratified air flow that departs only slightly from the expected behaviour in isotropic
turbulence. When solving for the trajectory of a heavy particle in the air flow, the modelled turbulent forcing on the particle
also behaves remarkably well. For example, the spectrum of the turbulence at the particle location follows that of a massless
particle for time scales approximately larger than the Stokes’ particle response time. We anticipate that this model will
prove especially useful in the context of sea-spray dispersion and its associated momentum, sensible and latent heat, and
gas fluxes between spray droplets and the atmosphere. 相似文献
5.
DAIZHOU ZHANG 《Tellus. Series B, Chemical and physical meteorology》2008,60(4):641-646
Dust particles frequently become mixtures of mineral dust and sea salt during their transport in the marine boundary layer, consequently growing in size, which causes changes in their settling velocities. In this study, the effect of sea salt on the gravitational settling of dust particles is investigated. Results show that the adhering of sea salt to dust particles can dramatically increase the gravitational settling of the particles, in particular if the particles become larger than 3–4 μm. Estimates with the observational data from six dust events in southwestern Japan revealed that, due to sea salt adhering, the gravitational settling flux of mineral dust increased approximately 14–17% in well-mixed events and 4–6% in less-mixed events, indicating a potential significant effect of sea salt on dust settling and the importance of considering this effect in the schemata of particle gravitational settling when mapping dust flux to the ocean. 相似文献
6.
Marcelo Chamecki René van Hout Charles Meneveau Marc B. Parlange 《Boundary-Layer Meteorology》2007,125(1):25-38
An expression for the vertical equilibrium concentration profile of heavy particles, including the effects of canopy on the
eddy diffusivity as well as corrections for atmospheric stability, is proposed. This expression is validated against measurements
of vertical concentration profiles of corn pollen above a corn field. The fitted theoretical profiles show that particle settling
is correctly accounted for. The sensitivity to variations in the turbulent Schmidt number, settling velocity and stability
corrections are explicitly characterized. The importance of independent measurements of the surface flux of pollen in future
experiments is noted. 相似文献
7.
Measurements of vertical profiles of pollen concentration from local and remote sources have been made during convective conditions with balloon-mounted rotating impaction samplers.The vertical profiles of local particles appear to be usually characterized by a decrease in the ground layer and by almost constant values aloft. A minimum often occurs at levels between 100 and 200 m.The concentration profiles of pollen from remote sources present a peak at heights of some hundred meters.By comparing the observed profiles with some published models of transport, it appears that: (i) because of differing eddy scales, turbulent transport affects the settling of pollen differently close to the ground than aloft; and (ii) turbulent anisotropies related to the dynamic and thermal structure of the atmosphere may affect the vertical distribution of particulate matter in the boundary layer. 相似文献
8.
Large-scale turbulent motions enhancing horizontal gas spread in an atmospheric boundary layer are simulated in a wind-tunnel
experiment. The large-scale turbulent motions can be generated using an active grid installed at the front of the test section
in the wind tunnel, when appropriate parameters for the angular deflection and the rotation speed are chosen. The power spectra
of vertical velocity fluctuations are unchanged with and without the active grid because they are strongly affected by the
surface. The power spectra of both streamwise and lateral velocity fluctuations with the active grid increase in the low frequency
region, and are closer to the empirical relations inferred from field observations. The large-scale turbulent motions do not
affect the Reynolds shear stress, but change the balance of the processes involved. The relative contributions of ejections
to sweeps are suppressed by large-scale turbulent motions, indicating that the motions behave as sweep events. The lateral
gas spread is enhanced by the lateral large-scale turbulent motions generated by the active grid. The large-scale motions,
however, do not affect the vertical velocity fluctuations near the surface, resulting in their having a minimal effect on
the vertical gas spread. The peak concentration normalized using the root-mean-squared value of concentration fluctuation
is remarkably constant over most regions of the plume irrespective of the operation of the active grid. 相似文献
9.
在夜间稳定边界层中, 湍流热通量往往具有间歇性特征。在一阵阵出现的强度较大的湍流热通量之间, 混杂着弱噪声或其他微弱的难以辨识的高频脉动信号。为了研究间歇性湍流热通量的特征, 必须将这些无关信 号剔除, 以提取出干净的湍流热通量。本文提出了一种新的提取间歇性湍流热通量的方法, 该方法通过分析 湍流热通量的概率密度函数, 并与稳定分布进行比较, 湍流热通量的概率密度函数开始偏离稳定分布的位置, 即是间歇性湍流热通量开始出现的阈值。本文通过夜间稳定边界层外场试验数据的验证, 发现利用稳定分布确定 的阈值可有效地提取出间歇性湍流热通量。在此基础之上, 本文对比了提取前后湍流热通量的功率谱, 发现 提取后低频信号的方差所占比重下降, 而高频信号略有上升。此外, 间歇性湍流热通量在高频区的功率谱满足“-7/6”律。 相似文献
10.
Peter P. Sullivan James C. McWilliams Chin-Hoh Moeng 《Boundary-Layer Meteorology》1996,80(1-2):167-202
A method for performing nested grid calculations with a large-eddy simulation code is described. A common numerical method is used for all meshes, and the grid architecture consists of a single outer or coarse grid, and nested or fine grids, which overlap in some common region. Inter-grid communication matches the velocity, pressure and potential temperature fields in the overlap region. Resolved and sub-grid scale (SGS) turbulent fluxes and kinetic energy on the fine grid are averaged to the coarse grid using a conservation rule equivalent to Germano's identity used to develop dynamic SGS models.Simulations of a slightly convective, strong shear planetary boundary layer were carried out with varying surface-layer resolutions. Grid refinements in the (x, y, z) directions of up to (5, 5, 2) times were employed. Two-way interaction solutions on the coarse and fine meshes are successfully matched in the overlap region on an instantaneous basis, and the turbulent motions on the fine grid blend smoothly into the coarse grid across the grid interface. With surface-layer grid nesting, significant increases in resolved eddy fluxes and variances are found. The energy-scale content of the vertical velocity, and hence vertical turbulent fluxes, appear to be most influenced by increased grid resolution. Vertical velocity spectra show that the dominant scale shifts towards higher wavenumbers (smaller scales) and the magnitude of the peak energy is increased by more than a factor of 3 with finer resolution. Outside of the nested region the average heat and momentum fluxes and spectra are slightly influenced by the fine resolution in the surface layer. From these results we conclude that fine resolution is required to resolve the details of the turbulent motions in the surface layer. At the same time, however, increased resolution in the surface layer does not appreciably alter the ensemble statistics of the resolved and SGS motions outside of the nested region. 相似文献
11.
Impact of Sea-Spray on the Atmospheric Surface Layer 总被引：1，自引：0，他引：1
The feedback effects of sea-spray on the heat and momentum fluxes under equilibrium conditions associated with winds of tropical
cyclones are investigated using a one-dimensional coupled sea-spray and atmospheric surface-layer (ASL) model. This model
is capable of simulating the microphysical aspects of the evaporation of saline water droplets of various sizes and their
dynamic and thermal interaction with the turbulence mixing that is simulated by the Mellor–Yamada 1.5-order closure scheme.
Sea-spray droplet generation is described by a state-of-the-art parametrization that predicts the size spectrum of sea-spray
droplets for a given surface forcing. The results from a series of simulations indicate the way in which evaporating droplets
of various sizes modify the turbulence mixing near the surface, which in turn affects further droplet evaporation. All these
results are direct consequences of the effects of sea-spray on the balance of turbulent kinetic energy in the spray-filled
surface layer. In particular, the overall impact of sea-spray droplets on the mean wind depends on the wind speed at the level
of sea-spray generation. When the wind speed is below 40 m s−1, the droplets are small in size and tend to evaporate substantially and thus cool the spray-filled layer, while for wind
speeds above 50 m s−1, the size of the droplets is so large that they do not have enough time to evaporate much before falling back into the sea.
The sensible heat carried by the droplets is released to the ambient air, increasing the buoyancy of the surface layer and
enhancing the turbulent mixing. The suspension of sea-spray droplets reduces the buoyancy and makes the surface layer more
stable, decreasing the friction velocity and the downward turbulent mixing of momentum. The results from the numerical experiments
also suggest that, in order not to violate the constant flux assumption critical to the Monin–Obukhov similarity theory, a
displacement equal to the mean wave height should be included in the logarithmic profiles of the wind and thermal fields. 相似文献
12.
Richard Bintanja 《Boundary-Layer Meteorology》2000,95(3):369-395
An atmospheric surface-layer model is used to investigate the interactionbetween suspended snow particles and the near-surface flow. Themodel incorporates the effects of upward diffusion, gravitational settling and sublimation of snow particles in 48 size classes, the effects of snowdrift sublimation on the heat and moisture budget of the surface layer, and the buoyancy destruction of turbulent kinetic energy (TKE) caused by the presence of suspended particles. A new term in the E- closure model representing the buoyancy destruction due to suspended particles is included in the prognostic equation for TKE. Generally, model results indicate that the presence of suspended particles causes significant decreases in TKE, the dissipation rate, turbulent length scales and eddy exchange coefficients (up to 40%). It is found that the reduction in the eddy exchangecoefficients is due mainly to reductions in turbulent length scales. Theassociated particle Richardson number peaks near the saltation-suspensioninterface, but at higher levels in the surface layer the particle-induced buoyancy can also significantly affect the flow. A detailed analysis of the various snowdrift quantities, the TKE budget and the particle buoyancy effects on the flow is presented. 相似文献
13.
Richard Bintanja 《Boundary-Layer Meteorology》2000,95(3):343-368
Snowdrift is one of the manymanifestations of two-phase flow, in which theinteraction between suspended particles and theambient fluid brings about some interesting features.Specifically, the drag required to keep particles insuspension against the downward gravitational pullrequires expenditure of turbulent kinetic energy(TKE). Other effects include the increased density of theair-snow mixture and the stable thermal stratificationcaused by the snowdrift sublimation-induced cooling.An atmospheric surface-layer model that includes snowdriftsuspension is described that includes the effects ofupward diffusion, gravitational settling andsublimation of snow particles in 48 size classes, theeffects of snowdrift sublimation on the heat andmoisture budget of the surface layer and the dampingof turbulence in the presence of suspended particles. Thewell-known E- closure model is applied toevaluate the eddy exchange coefficient, with a newterm representing buoyancy reduction induced by thestably stratified suspended particle profile includedin the prognostic equation for TKE. 相似文献
14.
Spectral Structure of Small-Scale Turbulent and Mesoscale Fluxes in the Atmospheric Boundary Layer over a Thermally Inhomogeneous Land Surface 总被引：1，自引：0，他引：1
Spectral analysis was performed on aircraft observations of a convective boundary layer (CBL) that developed over a thermally
inhomogeneous, well-marked mesoscale land surface. The observations, part of the GAME-Siberia experiment, were recorded between
April and June 2000 over the Lena River near Yakutsk City. A special integral parameter termed the ‘reduced depth of the CBL’
was used to scale the height of the mixed layer with variable depth. Analysis of wavelet cospectra and spectra facilitated
the separation of fluxes and other variables into small-scale turbulent fluctuations (with scales less than the reduced depth
of the CBL, approximately 2 km) and mesoscale fluctuations (up to 20 km). This separation approach allows for independent
exploration of the scales. Analyses showed that vertical distributions obeyed different laws for small-scale fluxes and mesoscale
fluxes (of sensible heat, water vapour, momentum and carbon dioxide) and for other variables (wind speed and air temperature
fluctuations, coherence and degree of anisotropy). Vertical profiles of small-scale turbulent fluxes showed a strong decay
that differed from generally accepted similarity models for the CBL. Vertical profiles of mesoscale fluxes and other variables
clearly showed sharp inflections at the same relative (with respect to the reduced depth of the CBL) height of approximately
0.55 in the CBL. Conventional similarity models for sensible heat fluxes describe both small-scale turbulent and mesoscale
flows. The present results suggest that mesoscale motions that reach up to the relative level of 0.55 could be initiated by
thermal surface heterogeneity. Entrainment between the upper part of the CBL and the free atmosphere may cause mesoscale motions
in that region of the CBL. 相似文献
15.
The main goals of this work are climatological analysis of characteristics of vertical wind in the stratosphere and estimation of potential opportunities of its influence on stratospheric aerosol particles. High-altitude, temporal, and latitude dependences of zonal mean vertical wind velocity for the period of 1992?C2006 from the UKMO atmospheric general circulation model are analyzed. It is shown that monthly averaged amplitudes of the vertical wind are approximately ±5?mm/s, while annual averaged ones are ±1?mm/s. The upward wind can provide the vertical lifting against gravity for sufficiently large (up to 3?C5???m) aerosol particles with a density up to 1.0?C1.5?g/cm3 at stratospheric and mesospheric altitudes. The vertical wind, probably, is a substantial factor for particle motion up to altitudes of 30?C40?km and can change essentially the sedimentation velocities and the residence times of stratospheric aerosols. The structure of the averaged fields of vertical wind supposes the opportunity of formation of dynamically stable aerosol layers in the middle stratosphere. With the problem regarding the action of a permanent source of monodisperse particles near the stratopause taken as an example, it is shown that if the action of the averaged vertical component is taken into account along with the gravitational sedimentation and turbulent diffusion, the standard vertical profiles of the relative concentration of particles change cardinally. Estimations for the levitation heights for particles of different densities and sizes in the stratosphere under action of gravity and vertical wind pressure are presented. 相似文献
16.
The Near-Calm Stable Boundary Layer 总被引：3，自引：3，他引：0
Larry Mahrt 《Boundary-Layer Meteorology》2011,140(3):343-360
For the near-calm stable boundary layer, nominally 2-m mean wind speed <0.5 ms−1, the time-average turbulent flux is dominated by infrequent mixing events. These events are related to accelerations associated
with wave-like motions and other more complex small-scale motions. In this regime, the relationship between the fluxes and
the weak mean flow breaks down. Such near-calm conditions are common at some sites. For very weak winds and strong stratification,
the characteristics of the fluctuating quantities change slowly with increasing scale and the separation between the turbulence
and non-turbulent motions can become ambiguous. Therefore, a new analysis strategy is developed based on the scale dependence
of selected flow characteristics, such as the ratio of the fluctuating potential energy to the kinetic energy. In contrast
to more developed turbulence, correlations between fluctuating quantities are small, and a significant heat flux is sometimes
carried by very weak vertical motions with large temperature fluctuations. The relation of the flux events to small-scale
increases of wind speed is examined. Large remaining uncertainties are noted. 相似文献
17.
During the SABLES2006 (Stable Atmospheric Boundary Layer Experiment in Spain 2006) field campaign, a gravity-wave episode
was observed on the night of July 11 by the microbarometers deployed at the surface and on the 100-m tower. The high-amplitude,
low-frequency periodic pressure fluctuations were very well correlated with the wind speed and direction. Data from neighbouring
automatic stations showed that the gravity wave was not local, but long-lived and mesoscale. The propagation of the wave over
the experimental site had significant effects on the structure of the weakly-stratified nocturnal boundary layer that developed
that night: the stability increased, turbulent vertical motions were suppressed, the nocturnal low-level jet was disrupted,
and periodic temperature fluctuations of amplitude up to 3–4 K were observed. In this work we analyse the different available
data sources (tower data, RASS-SODAR, microbarometric, satellite imagery, automatic stations) to describe the phenomena in
depth and to find a suitable explanation for the generation and propagation of the wave. The linear wave theory explains remarkably
well most of the observations, and the wave parameters could be estimated by applying a wavelet-based technique to surface
microbarometric measurements. We also analyse the vertical structure of the wave and find wave ducting conditions above the
surface. Finally, by means of the multi-resolution flux decomposition, we analyse in detail the changes in vertical turbulent
fluxes and the spectra of turbulent motions produced by the interaction between the gravity wave and the local flow. 相似文献
18.
Coherent Structures and the Dissimilarity of Turbulent Transport of Momentum and Scalars in the Unstable Atmospheric Surface Layer 总被引：7，自引：7，他引：0
Atmospheric stability effects on the dissimilarity between the turbulent transport of momentum and scalars (water vapour and
temperature) are investigated in the neutral and unstable atmospheric surface layers over a lake and a vineyard. A decorrelation
of the momentum and scalar fluxes is observed with increasing instability. Moreover, different measures of transport efficiency
(correlation coefficients, efficiencies based on quadrant analysis and bulk transfer coefficients) indicate that, under close
to neutral conditions, momentum and scalars are transported similarly whereas, as the instability of the atmosphere increases,
scalars are transported increasingly more efficiently than momentum. This dissimilarity between the turbulent transport of
momentum and scalars under unstable conditions concurs with, and is likely caused by, a change in the topology of turbulent
coherent structures. Previous laboratory and field studies report that under neutral conditions hairpin vortices and hairpin
packets are present and dominate the vertical fluxes, while under free-convection conditions thermal plumes are expected.
Our results (cross-stream vorticity variation, quadrant analysis and time series analysis) are in very good agreement with
this picture and confirm a change in the structure of the coherent turbulent motions under increasing instability, although
the exact structure of these motions and how they are modified by stability requires further investigation based on three-dimensional
flow data. 相似文献
19.
Three aircraft-based studies of boundary-layer fronts (BLFs) werecarried out during the experiment KABEG in April 1997 near thesea-ice edge over the Davis Strait. The boundary-layer flow wasparallel to the ice edge and hence two independent turbulent regimescould develop in an identical synoptic framework, separated by thefrontal zone along the ice edge. The zone of strongest crosswindhorizontal gradients was typically 20 km wide, while the downstreamscale of the BLF was observed to be several hundreds of kilometres.For two of the three cases the investigation of turbulence structureswas possible and the results are given herein.Horizontal and vertical profiles of turbulent fluxes and other turbulentquantities are presented. A spectral analysis reveals the coexistence ofsmall-scale turbulence with roll motions. These roll motions can behidden or can be visible as cloud streets. The associated transportmechanisms are highly relevant for the choice of suitable averagingintervals for turbulent flux calculations and model validation.Parameterizations for the vertical velocity variance, countergradienttransport, sea surface roughness and eddy diffusivity are evaluatedand compared for this baroclinic strong-wind convective boundary-layerenvironment. Analogously, drag coefficients and bulk transfer coefficientsare derived from measurements. 相似文献
20.
Stefan Zacharias Mark Reyers Joaquim G. Pinto Jan H. Schween Susanne Crewell Michael Kerschgens 《Meteorology and Atmospheric Physics》2012,117(1-2):47-61
High-resolution simulations with a mesoscale model are performed to estimate heat and moisture budgets of a well-mixed boundary layer. The model budgets are validated against energy budgets obtained from airborne measurements over heterogeneous terrain in Western Germany. Time rate of change, vertical divergence, and horizontal advection for an atmospheric column of air are estimated. Results show that the time trend of specific humidity exhibits some deficiencies, while the potential temperature trend is matched accurately. Furthermore, the simulated turbulent surface fluxes of sensible and latent heat are comparable to the measured fluxes, leading to similar values of the vertical divergence. The analysis of different horizontal model resolutions exhibits improved surface fluxes with increased resolution, a fact attributed to a reduced aggregation effect. Scale-interaction effects could be identified: while time trends and advection are strongly influenced by mesoscale forcing, the turbulent surface fluxes are mainly controlled by microscale processes. 相似文献