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Vertical Velocity (vertical + velocity)

Distribution by Scientific Domains

Earth and Environmental Science	72%
Chemistry	9%

Selected Abstracts

Three-dimensional flow modelling and sediment transport in the River Klarälven

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2004
Bijan Dargahi
Abstract A three-dimensional ,ow model that uses the RNG k - , turbulence model and a non-equilibrium wall function was applied to the River Klarälven in the southwest part of Sweden. The objectives were to study the nature of the ,ow in the river bifurcation and to investigate the short-term sediment transport patterns in the river. The effectiveness of three-dimensional ,ow models depends upon: (1) how well the river geometry and it surface roughness are modelled; and (2) the choice of the closure model. Improvements were obtained by modelling the river in two parts: the entire river reach, and a selected part. Composite Manning coef,cients were used to account for roughness properties. The method requires a calibration process that ensures the water surface pro,les match the ,eld data. The k - , model under-predicted both the extent of ,ow separation zones and the number of secondary ,ow regions having a spiral motion, in comparison with the RNG k - , model. The 3-D model could predict with good accuracy both the general and secondary ,ow ,elds in the river. The results agreed well with the 3-D velocity measurements using an acoustic Doppler current pro,ler. A conceptual model was developed that accounts for the development of secondary ,ows in a river bifurcation having two bends. The main ,ow feature in the river cross-sections was the existence of multiple counter-rotating spiral motions. The number of spiral motions increased as the river bends were approached. The river bends also caused vorticity intensi,cation and increased the vertical velocities. The application of the 3-D ,ow model was extended by solving the sediment continuity equation. The sediment transport patterns were related to the secondary ,ow ,elds in the river. The sediment transport patterns at the river bifurcations are characterized by the growth of a sandbank. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Seismic characterization of vertical fractures described as general linear-slip interfaces

GEOPHYSICAL PROSPECTING, Issue 2 2003
Vladimir Grechka
ABSTRACT Fluid flow in many hydrocarbon reservoirs is controlled by aligned fractures which make the medium anisotropic on the scale of seismic wavelength. Applying the linear-slip theory, we investigate seismic signatures of the effective medium produced by a single set of ,general' vertical fractures embedded in a purely isotropic host rock. The generality of our fracture model means the allowance for coupling between the normal (to the fracture plane) stress and the tangential jump in displacement (and vice versa). Despite its low (triclinic) symmetry, the medium is described by just nine independent effective parameters and possesses several distinct features which help to identify the physical model and estimate the fracture compliances and background velocities. For example, the polarization vector of the vertically propagating fast shear wave S1 and the semi-major axis of the S1 -wave normal-moveout (NMO) ellipse from a horizontal reflector always point in the direction of the fracture strike. Moreover, for the S1 -wave both the vertical velocity and the NMO velocity along the fractures are equal to the shear-wave velocity in the host rock. Analysis of seismic signatures in the limit of small fracture weaknesses allows us to select the input data needed for unambiguous fracture characterization. The fracture and background parameters can be estimated using the NMO ellipses from horizontal reflectors and vertical velocities of P-waves and two split S-waves, combined with a portion of the P-wave slowness surface reconstructed from multi-azimuth walkaway vertical seismic profiling (VSP) data. The stability of the parameter-estimation procedure is verified by performing non-linear inversion based on the exact equations. [source]

Ground Water Discharge and Nitrate Flux to the Gulf of Mexico

GROUND WATER, Issue 3 2004
Carolyn B. Dowling
Ground water samples (37 to 186 m depth) from Baldwin County, Alabama, are used to define the hydrogeology of Gulf coastal aquifers and calculate the subsurface discharge of nutrients to the Gulf of Mexico. The ground water flow and nitrate flux have been determined by linking ground water concentrations to 3H/3He and 4He age dates. The middle aquifer (A2) is an active flow system characterized by postnuclear tritium levels, moderate vertical velocities, and high nitrate concentrations. Ground water discharge could be an unaccounted source for nutrients in the coastal oceans. The aquifers annually discharge 1.1 ± 0.01 × 108 moles of nitrate to the Gulf of Mexico, or 50% and 0.8% of the annual contributions from the Mobile-Alabama River System and the Mississippi River System, respectively. In southern Baldwin County, south of Loxley, increasing reliance on ground water in the deeper A3 aquifer requires accurate estimates of safe ground water withdrawal. This aquifer, partially confined by Pliocene clay above and Pensacola Clay below, is tritium dead and contains elevated 4He concentrations with no nitrate and estimated ground water ages from 100 to 7000 years. The isotopic composition and concentration of natural gas diffusing from the Pensacola Clay into the A3 aquifer aids in defining the deep ground water discharge. The highest 4He and CH4 concentrations are found only in the deepest sample (Gulf State Park), indicating that ground water flow into the Gulf of Mexico suppresses the natural gas plume. Using the shape of the CH4 -He plume and the accumulation of 4He rate (2.2 ± 0.8 ,cc/kg/1000 years), we estimate the natural submarine discharge and the replenishment rate for the A3 aquifer. [source]

Effects of circular motion on judgment of rotation direction and depth order in visual motion

JAPANESE PSYCHOLOGICAL RESEARCH, Issue 4 2005
MITSUHIKO HANADA
Abstract:, The rotation direction and depth order of a rotating sphere consisting of random dots often reverses while it is viewed under orthographic projection. However, if a short viewing distance is simulated under perspective projection, the correct rotation direction can be perceived. There are two motion cues for the rotation direction and depth order. One is the speed cue; points with higher velocities are closer to the observer. The other is the vertical motion cue; vertical motion is induced when the dots recede from or approach the observer. It was examined whether circular motion, which does not have any depth information but induces vertical velocities, masks the vertical motion cue. In Experiment 1, the effects of circular motion on the judgment of the rotation direction of a rotating sphere were examined. The magnitude of the two cues (the speed cue and the vertical velocity cue) as well as the angular speed of circular motion was varied. It was found that the performance improved as the vertical velocity increased and that the speed cue had slight effects on the judgment of the rotation direction. It was also found that the performance worsened as the angular speed of the circular motion was increased. In Experiment 2, the effects of circular motion on depth judgment of a rotating half sphere were investigated. The performance worsened as the angular speed of the circular motion increased, as in Experiment 1. These results suggest that the visual system cannot compensate perfectly for circular motion for the judgment of the rotation direction and depth order. [source]

Evaluation of a large-eddy model simulation of a mixed-phase altocumulus cloud using microwave radiometer, lidar and Doppler radar data

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 618 2006
J. H. Marsham
Abstract Using the Met Office large-eddy model (LEM) we simulate a mixed-phase altocumulus cloud that was observed from Chilbolton in southern England by a 94 GHz Doppler radar, a 905 nm lidar, a dual-wavelength microwave radiometer and also by four radiosondes. It is important to test and evaluate such simulations with observations, since there are significant differences between results from different cloud-resolving models for ice clouds. Simulating the Doppler radar and lidar data within the LEM allows us to compare observed and modelled quantities directly, and allows us to explore the relationships between observed and unobserved variables. For general-circulation models, which currently tend to give poor representations of mixed-phase clouds, the case shows the importance of using: (i) separate prognostic ice and liquid water, (ii) a vertical resolution that captures the thin layers of liquid water, and (iii) an accurate representation the subgrid vertical velocities that allow liquid water to form. It is shown that large-scale ascents and descents are significant for this case, and so the horizontally averaged LEM profiles are relaxed towards observed profiles to account for these. The LEM simulation then gives a reasonable cloud, with an ice-water path approximately two thirds of that observed, with liquid water at the cloud top, as observed. However, the liquid-water cells that form in the updraughts at cloud top in the LEM have liquid-water paths (LWPs) up to half those observed, and there are too few cells, giving a mean LWP five to ten times smaller than observed. In reality, ice nucleation and fallout may deplete ice-nuclei concentrations at the cloud top, allowing more liquid water to form there, but this process is not represented in the model. Decreasing the heterogeneous nucleation rate in the LEM increased the LWP, which supports this hypothesis. The LEM captures the increase in the standard deviation in Doppler velocities (and so vertical winds) with height, but values are 1.5 to 4 times smaller than observed (although values are larger in an unforced model run, this only increases the modelled LWP by a factor of approximately two). The LEM data show that, for values larger than approximately 12 cm s,1, the standard deviation in Doppler velocities provides an almost unbiased estimate of the standard deviation in vertical winds, but provides an overestimate for smaller values. Time-smoothing the observed Doppler velocities and modelled mass-squared-weighted fallspeeds shows that observed fallspeeds are approximately two-thirds of the modelled values. Decreasing the modelled fallspeeds to those observed increases the modelled IWC, giving an IWP 1.6 times that observed. Copyright © 2006 Royal Meteorological Society [source]

Three-dimensional simulation of the ASTEX Lagrangian 1 field experiment with a regional numerical weather prediction model

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 597 2004
Robert Sigg
Abstract The Atlantic Stratocumulus Transition Experiment (ASTEX) first Lagrangian experiment (Lagrangian 1) is here simulated with a modified version of the regional forecast model HIRLAM (High Resolution Limited Area Model). The main modification is that moist turbulent fluxes are accounted for in the model. Trajectory calculations show good agreement with earlier estimations. The initially rather shallow stratocumulus topped marine boundary layer is deepening along the trajectory, and in the end cumulus clouds are formed that penetrate the boundary-layer top. The model predicts this change in cloudiness, but the boundary layer is too shallow in the model. A simulation with modified initial conditions shows improved results, but is still too slow in increasing the boundary-layer depth. Additional factors that influence the boundary-layer growth are: the increase in sea surface temperatures, lower modelled wind speeds, low entrainment rates due to coarse vertical resolution, and synoptic-scale subsidence. An anticyclone at the surface moved slightly northward during the simulation. The anticyclone was accompanied at 500 hPa by a deepening cyclone and, therefore, one would expect synoptic subsidence in the area of the Lagrangian 1. The modelled negative vertical wind component at the boundary-layer top oscillates, and this is examined using spectral analysis. The results show that the vertical velocity is influenced by cumulus clouds on time-scales up to 15 h with a peak at 9 h. The horizontal and vertical wavelengths of the vertical velocity disturbances are estimated from model output to be 400,500 km and 6,10 km, respectively. Using the estimated vertical wavelength and linear theory for hydrostatic inertia,gravity waves, a horizontal wavelength of 350,550 km was calculated for a frequency of 9 h. The model results thus indicate that these types of waves are responsible for the undulating vertical velocity. Finally, an estimation of the synoptic-scale vertical velocity is calculated by filtering out all scales smaller than 15 h from the vertical velocity signal. This results in subsidence both at the beginning and the end of the Lagrangian with vertical velocities between ,0.1 and ,0.4 cm s,1. Copyright © 2004 Royal Meteorological Society [source]

Investigation of Fluid and Coarse-Particle Dynamics in a Two-Dimensional Spouted Bed

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 9 2004
T. Swasdisevi
Abstract The aerodynamics of particles and gas flow in a two-dimensional spouted bed (2DSB) with draft plates is investigated with the aid of the discrete element method. The geometry of the 2DSB with draft plates is set as close as possible to the experimental apparatus of Kudra [1] and Kalwar [2]. The physical properties of the coarse particles are similar to those of shelled corn. The calculated minimum spouting velocity and pressure drop agree well with the correlations of Kudra [1] and Kalwar [2]. In the spout region, the particle vertical velocities are found to decrease as the height increases. The fluid velocity in the downcomer region decreases as the superficial gas velocity increases. The particle circulation rate increases when the friction coefficient decreases or the separation height increases. At the minimum spouting velocity, the bed height does not affect the particle circulation rate in the 2DSB with draft plates. The draft plates not only reduce the minimum spouting velocity and pressure drop but also increase the maximum spoutable bed height. The effect of taking out the draft plates on the spouting phenomenon is investigated and the effect of putting in a deflector on the possible breakage of the particles is also estimated. [source]

Seismic anisotropy of shales

GEOPHYSICAL PROSPECTING, Issue 5 2005
C.M. Sayers
ABSTRACT Shales are a major component of sedimentary basins, and they play a decisive role in fluid flow and seismic-wave propagation because of their low permeability and anisotropic microstructure. Shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid, lithology and pore pressure from seismic data, and to understand time-to-depth conversion errors and non-hyperbolic moveout. A single anisotropy parameter, Thomsen's , parameter, is sufficient to explain the difference between the small-offset normal-moveout velocity and vertical velocity, and to interpret the small-offset AVO response. The sign of this parameter is poorly understood, with both positive and negative values having been reported in the literature. , is sensitive to the compliance of the contact regions between clay particles and to the degree of disorder in the orientation of clay particles. If the ratio of the normal to shear compliance of the contact regions exceeds a critical value, the presence of these regions acts to increase ,, and a change in the sign of ,, from the negative values characteristic of clay minerals to the positive values commonly reported for shales, may occur. Misalignment of the clay particles can also lead to a positive value of ,. For transverse isotropy, the elastic anisotropy parameters can be written in terms of the coefficients W200 and W400 in an expansion of the clay-particle orientation distribution function in generalized Legendre functions. For a given value of W200, decreasing W400 leads to an increase in ,, while for fixed W400, , increases with increasing W200. Perfect alignment of clay particles with normals along the symmetry axis corresponds to the maximum values of W200 and W400, given by and . A comparison of the predictions of the theory with laboratory measurements shows that most shales lie in a region of the (W200, W400)-plane defined by W400/W200,Wmax400/Wmax200. [source]

Seismic characterization of vertical fractures described as general linear-slip interfaces

Effects of circular motion on judgment of rotation direction and depth order in visual motion

Computational investigation of the mechanisms of particle separation and "fish-hook" phenomenon in hydrocyclones

AICHE JOURNAL, Issue 7 2010
B. Wang
Abstract The motion of solid particles and the "fish-hook" phenomenon in an industrial classifying hydrocyclone of body diameter 355 mm is studied by a computational fluid dynamics model. In the model, the turbulent flow of gas and liquid is modeled using the Reynolds Stress Model, and the interface between the liquid and air core is modeled using the volume of fluid multiphase model. The outcomes are then applied in the simulation of particle flow described by the stochastic Lagrangian model. The results are analyzed in terms of velocity and force field in the cyclone. It is shown that the pressure gradient force plays an important role in particle separation, and it balances the centrifugal force on particles in the radial direction in hydrocyclones. As particle size decreases, the effect of drag force whose direction varies increases sharply. As a result, particles have an apparent fluctuating velocity. Some particles pass the locus of zero vertical velocity (LZVV) and join the upward flow and have a certain moving orbit. The moving orbit of particles in the upward flow becomes wider as their size decreases. When the size is below a critical value, the moving orbit is even beyond the LZVV. Some fine particles would recircuit between the downward and upward flows, resulting in a relatively high separation efficiency and the "fish-hook" effect. Numerical experiments were also extended to study the effects of cyclone size and liquid viscosity. The results suggest that the mechanisms identified are valid, although they are quantitatively different. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source]

Impact of horizontal model resolution and orography on the simulation of a western disturbance and its associated precipitation

METEOROLOGICAL APPLICATIONS, Issue 2 2004
A. P. Dimri
A nonhydrostatic version of Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model (MM5) is used to study the effects of the horizontal model resolution and orography while simulating an active western disturbance (WD) that affected northwest India from 21 to 25 January 1999. Two numerical experiments are conducted with six combinations of two factors: horizontal model resolution and topography. National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysed data are used for the initial and boundary conditions. Simulation results indicate that the distribution and the rate of simulated precipitation due to a WD over northwest India is highly sensitive to the horizontal model resolution and topography. The model with finer resolution (30 km) is better able to estimate effects of mesoscale forcing on precipitation over the selected domain. The amount of precipitation simulated over the coarse domain is much less than the observed precipitation owing to the model's unrealistic representation of orographic effects and mesoscale forcing. Simulated terrain, vertical velocity, wind and streamline at different horizontal model resolutions are presented. The detailed structure and distribution of wind speed are simulated in the finer domain. Simulated vertical velocity and precipitation are less in the second experiment when a flat topography is used across the domain, which indicates that topography plays a significant role in modulating the WD. Sensitivity of the horizontal model resolution for precipitation is assessed and it is found that the finer domain of the model simulation gives better results. Copyright © 2004 Royal Meteorological Society. [source]

Scaling turbulent atmospheric stratification.

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 631 2008
III: Space, time stratification of passive scalars from lidar data
Abstract In this third and final part of the series, we concentrate on the temporal behaviour of atmospheric passive scalars. We first recall that,although the full (x, y, z, t) turbulent processes respect an anisotropic scale invariance,that due to advection,the generator will generally not be a diagonal matrix. This implies that the scaling of (1-D) temporal series will generally involve three exponents in real space: 1/3, 1/2, 3/5, for spectra ,, = 5/3, 2, 11/5, with the first and last corresponding to domination by advection (horizontal and vertical respectively), and the second to pure temporal development (no advection). We survey the literature and find that almost all the empirical ,, values are indeed in the range 5/3 to 2. We then use meteorological analyses to argue that, although pure temporal development is unlikely to be dominant for time-scales less than the eddy turnover time of the largest structures (about 2 weeks), an intermittent vertical velocity could quite easily explain the occasionally observed ,, , 2 spectra. We then use state-of-the-art vertically pointing lidar data of backscatter ratios from both aerosols and cirrus clouds yielding several (z, t) vertical space,time cross-sections with resolution of 3.75 m in the vertical, 0.5,30 s in time and spanning 3,4 orders of magnitude in temporal scale. We first test the predictions of the anisotropic, multifractal extension of the Corrsin-Obukhov law in the vertical and in time, separately finding that the cirrus and aerosol backscatters both followed the theoretical (anisotropic) scalings accurately; three of the six cases show dominance by the horizontal wind, the others by the vertical wind. In order to test the theory in arbitrary directions in this (z, t) space, and in order to get more complete information about the underlying physical scale, we develop and apply a new Anisotropic Scaling Analysis Technique (ASAT) which is based on a nonlinear space,time coordinate transformation. This transforms the original differential scaling into standard self-similar scaling; there remains only a ,trivial' anisotropy. This method is used in real space on 2-D structure functions. It is applied to both the new (z, t) data as well as the (x, z) data discussed in part II. Using ASAT, we verify the theory to within about 10% over more than three orders of magnitude of space,time scales in arbitrary directions in (x, z) and (z, t) spaces. By considering the high- (and low-) order structure functions, we verify the theory for both weak and strong structures; as predicted, their average anisotropies are apparently the same. Putting together the results for (x, z) and (z, t), and assuming that there is no overall stratification in the horizontal (x, y) plane, we find that the overall (x, y, z, t) space is found to have an effective ,elliptical dimension' characterizing the overall space,time stratification equal to Deff, st = 3.21 ± 0.05. Copyright © 2008 Royal Meteorological Society [source]

Testing a cumulus parametrization with a cumulus ensemble model in weak-temperature-gradient mode

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 626 2007
D. J. Raymond
Abstract This paper prototypes a method for calibrating a cumulus parametrization against a cumulus ensemble model. The key to this technique is to run the cumulus model and the parametrization in identical ,test cells' that provide forcing typical of that seen over tropical oceans. In particular, the mean temperature profile is relaxed to a reference profile that is assumed to be characteristic of the environment of the convection. This is done by calculating the mean vertical velocity needed to balance heating due to convection, latent-heat release, and radiation with adiabatic cooling. This ,weak-temperature-gradient' vertical-velocity profile is then used to advect moisture vertically and, via mass continuity, through the sides of the test cell, entraining reference-profile air as needed. As an example, a toy cumulus parametrization used previously is altered to reproduce the dependence of rainfall rate on surface wind speed shown by the cumulus ensemble model. This alteration greatly changes the behaviour of simulated large-scale disturbances in an aquaplanet equatorial beta-plane model. In particular, increasing the slope of the curve of rainfall rate against wind speed results in the development of much greater synoptic-scale variance. Copyright © 2007 Royal Meteorological Society [source]

A comparison of cloud-resolving model simulations of trade wind cumulus with aircraft observations taken during RICO

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 624 2007
S. J. Abel
Abstract This paper presents results from simulations of trade wind cumulus with the Met Office Large Eddy Model (LEM) based on observed environmental profiles from the Rain In Cumulus over the Ocean (RICO) field experiment. Comparisons of updraught core parameters are made with the in situ data collected onboard the three research aircraft that participated in RICO. The default set-up of the LEM was unable to produce sufficient amounts of rainwater content when compared to measurements by the aircraft. As a main aim of RICO was to quantify the importance of precipitation in the trade cumulus regime, we test the sensitivity of the model to changes in the rain microphysics, the large-scale forcing, and horizontal resolution. By changing these model variables we are able to obtain reasonably good agreement with the observations of updraught core vertical velocity, cloud and rainwater contents. Furthermore, the LEM produces comparable surface precipitation rates to those derived from radar measurements during RICO in a previous study. This gives us some confidence in the ability of the LEM to simulate realistic precipitating trade cumulus. Copyright © 2007 Royal Meteorological Society [source]

A coupled dispersion and exchange model for short-range dry deposition of atmospheric ammonia

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 618 2006
Benjamin Loubet
Abstract The MODDAS-2D model (MOdel of Dispersion and Deposition of Ammonia over the Short-range in two dimensions) is presented. This stationary model couples a two-dimensional Lagrangian stochastic model for short-range dispersion, with a leaf-scale bi-directional exchange model for ammonia (NH3), which includes cuticular uptake and a stomatal compensation point. The coupling is obtained by splitting the upward and downward components of the flux, which can be generalized for any trace gas, and hence provides a way of simply incorporating bi-directional exchanges in existing deposition velocity models. The leaf boundary-layer resistance is parametrized to account for mixed convection in the canopy, and the model incorporates a stability correction for the Lagrangian time-scale for vertical velocity, which tends to increase the Lagrangian time-scale in very stable conditions compared with usual parametrizations. The model is validated against three datasets, where concentrations of atmospheric NH3 were measured at several distances from a line source. Two datasets are over grassland and one is over maize, giving a range of canopy structure. The model correctly simulates the concentration in one situation, but consistently overestimates it at further distances or underestimates it at small distances in the two other situations. It is argued that these discrepancies are mainly due to the lack of length of one of the line sources and non-aligned winds. Analysis shows that the surface exchange parameters and the turbulent mixing at the source level are the predominant factors controlling short-range deposition of NH3. Copyright © 2006 Royal Meteorological Society [source]

Modelling tropical atmospheric convection in the context of the weak temperature gradient approximation

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 608 2005
David J. Raymond
Abstract A cumulus ensemble model is used to simulate the interaction between tropical atmospheric convection and the large-scale tropical environment in the context of Sobel and Bretherton's (2000) weak temperature gradient approximation. In this approximation, gravity waves are assumed to redistribute buoyancy anomalies over a broad area of the tropics, thus maintaining the local virtual-temperature profile close to the large-scale mean. This result is implemented in the model by imposing the advective effects of a hypothetical mean vertical velocity which is just sufficient to counteract the local heating induced by convection and radiation. The implied vertical advection in the moisture equation and entrainment of air from the surrounding environment have major effects on the evolution of convection in the model. The precipitation produced by the model mimics the results of a very simple model of tropical precipitation introduced by Raymond (2000), in that the mean rainfall rate predicted by the cumulus ensemble model is, to a good approximation, a function only of the mean column precipitable water. The evolution of the precipitable water, and hence the precipitation rate, is a result of the imbalance between the surface flux of moist entropy into the domain and the radiative loss of entropy out of the top of the domain. This evolution leads to a statistically steady solution in which the resulting precipitation rate is a unique function of the entropy flux imbalance. These results support the hypothesis that tropical precipitation averaged over distance scales of a few hundred kilometres and time scales of a day is a consequence only of local thermodynamic factors. Copyright © 2005 Royal Meteorological Society [source]

Three-dimensional simulation of the ASTEX Lagrangian 1 field experiment with a regional numerical weather prediction model

Turbulent length-scales in the marine atmospheric mixed layer

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 566 2000
P. Durand
Abstract The spectra of turbulence signals can be characterized by several independent scales. To provide a parametrization of these spectra requires knowledge of the relationships between these scales. This paper focuses on three independent scales: the integral scale (which is influenced by the low-frequency behaviour of the spectra); the wavelength of the spectrum peak (which characterizes the energy-containing domain); and the dissipation scale (which is relevant for the inertial subrange). First, we present definitions of these various scales, and the possible relationships between them. The profiles of the scales were computed from airborne measurements made in the atmospheric mixed layer over the open ocean, in a region where horizontal homogeneity can be assumed, at least for several tens of km. Furthermore, the diurnal cycle being very weak in this oceanic area, and aircraft moving at high speed through the air mass, stationarity is well verified on the runs, and Taylor's hypothesis may be used. The meteorological conditions correspond to a slightly unstable mixed layer, with weak to moderate winds. In a first part, we analyse the integral scales of various parameters on a 180-km run and demonstrate that these parameters cannot be computed with any soundness from horizontal-wind, temperature and moisture signals, because of the continuous increase in the spectral energy when moving towards lower frequencies. For the same reasons, the spectrum peak and the corresponding wavelength cannot be determined for these parameters. The computation of the integral and energy-containing scale is therefore restricted to the vertical velocity, and to the various covariances. The turbulence field is characterized by a stretching of the eddies along the mean wind direction which results in greater integral and energy-containing scales (but not in greater dissipation scales) when computed for along-wind runs than for the cross-wind runs. The profiles of the various scales increase with altitude and are well defined in the lower half of the mixed layer, but are much more scattered in the upper half. This behaviour is related to the source of turbulence, which lies in the surface buoyancy flux in the lower half of the mixed layer, and comes from higher altitude sources in the upper half. The integral scales have values comparable with those found in previous work, except for parameters related to temperature fluctuations, which have lower values. The ratio of the energy-containing scale to the integral scale, which determines the sharpness of the ,spectral knee', varies considerably from one parameter to another, and sometimes with altitude. This demonstrates that a single unique parametrization cannot be defined for turbulence spectra. As a consequence, the eddy-exchange coefficients, which depend on a characteristic length-scale, should vary from one parameter to another. This would then have to be taken into account in model parametrization based on mixing length-scales. [source]

Shear-driven magnetic buoyancy oscillations

ASTRONOMISCHE NACHRICHTEN, Issue 8 2009
V. Vermersch
Abstract The effects of uniform horizontal shear on a stably stratified layer of gas is studied. The system is initially destabilized by a magnetically buoyant flux tube pointing in the cross-stream direction. The shear amplifies the initial field to Lundquist numbers of about 200,400, but then its value drops to about 100,300, depending on the value of the sub-adiabatic gradient. The larger values correspond to cases where the stratification is strongly stable and nearly isothermal. At the end of the runs the magnetic field is nearly axisymmetric, i.e. uniform in the streamwise direction. In view of Cowling's theorem the sustainment of the field remains a puzzle and may be due to subtle numerical effects that have not yet been identified in detail. In the final state the strength of the magnetic field decreases with height in such a way that the field is expected to be unstable. Low amplitude oscillations are seen in the vertical velocity even at late times, suggesting that they might be persistent (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]