Richardson number profiles through shear instability wave regions observed in the lower planetary boundary layer

Remote sensing of the lower planetary boundary layer in the vicinity of a meteorological tower on many occasions reveals the existence of shear instability (Kelvin-Helmholtz) waves. In general, such waves are found within shallow strata which are marked by strong thermal stability and large vertical wind shear. The independent and concurrent measurements of the vector wind and temperature, made on a 152-m high tower, allow the construction of wind and temperature profiles. From such measurements, the Richardson number profile is constructed as well as the instability regime according to Drazin's criterion. The results show that regions of shear-instability waves as depicted by the remote sensor (an acoustic sounder) agree well with Drazin's instability regime, and that within such regions the Richardson number is indeed 0.25.     

Radar and sodar probing of waves and turbulence in statically stable clear-air layers

This paper reviews the remote sensing of waves and turbulence in statically stable atmospheric layers, utilizing sodar and microwave radar echoes from the small-scale inhomogeneities in gaseous refractive index caused by localized fluctuations in temperature, humidity, and velocity. Scattering theory and sounding methodology are reviewed briefly, and the relative performance of typical radar and sodar systems compared. The main section of the paper takes the form of a summary and discussion of experimental progress since 1969, showing how the echo patterns obtained may be applied to the interpretation of multiple layering, gravity waves, internal fronts and the details of dynamic instability and the genesis of turbulence in stably stratified shear layers. In addition, methods for the measurement of the intensity of the small-scale ( /2) variability of wind, temperature and water vapor from the observed radar or sodar echo intensities, and the use of Doppler techniques for the measurement of mean velocity and turbulence are discussed.SODAR from SOund Detection And Ranging in analogy to RAdio Detection And Ranging.     

Numerical Simulation of Strongly Stratified Flow Over a Three-Dimensional Hill

A numerical study of stably stratified flow over a three-dimensional hill is presented. Large-eddy simulation is used here to examine in detail the laboratory experimental flows described in the landmark work of Hunt and Snyder about stratified flow over a hill. The flow is linearly stratified and U/Nh is varied from 0.2 to 1.0. Here N and U are the buoyancy frequency and freestream velocity respectively, and h is the height of the hill. The Reynolds number based on the hill height is varied from 365 to 2968. The characteristic flow patterns at various values of U/Nh have been obtained and they are in good agreement with earlier theoretical and experimental results. It is shown that the flow field cannot be predicted by Drazin's theory when recirculation exists at the leeside of the hill even at UNh 1. The wake structure agrees well with a two-dimensional wake assumption when U/Nh 1 but lee waves start to influence the wake structure as U/Nh increases. The dividing-streamline heights obtained in the simulation are in accordance with experimental results and Sheppard's formula. The energy loss along the dividing streamline due to friction/turbulence approximately offsets the energy gained from pressure field. When lee waves are present, linear theory always underestimates the amplitude and overestimates the wavelength of three-dimensional lee waves. The simulated variations of drag coefficients with the parameterK (=ND/ U) are qualitatively consistent with experimental data and linear theory. Here D is the depth of the tank.     

Laboratory observations of shear-layer instability in a stratified fluid

Instability and transition to turbulence is studied in a shear layer between two streams of different salinities and velocities. The viscous layer between the two streams is 15–18 times thicker than the diffusion layer. When the initial layer Richardson number is low, the instability organizes the shear-layer vorticity into discrete lumps as in the homogeneous case. Here the similarity ends. The homogeneous shear layer continues to grow by the combination of neighboring vertical lumps (vortex pairing). In the stratified shear layer, the pairing process is stopped completely, and the vorticity is redistributed along the interface. This redistribution generates interfacial waves. The interfacial waves and the residual turbulence eventually decay, and the shear flow approaches a laminar state.     

A study of intermittent turbulence with cases-99 tower measurments

Characteristics of intermittent turbulence events in the stably stratified nocturnal boundary layer are investigated with data collected in the CASES-99 tower array of 300-m radius. The array consists of a central 60-m tower with eddy covariance measurements at eight levels and six satellite towers with eddy covariance measurements at 5 m. A significant increase in the magnitude of vertical wind velocity () and spectral information are used to define the onset of an intermittent turbulence event. Normally, only a subset of 5 m-levels in the tower network experience an intermittent turbulence event concurrent with one at the 5 m-level on the main tower. This behaviour reveals the small horizontal extent of most events. Intermittent turbulence events at the main tower 5-m level are normally confined to a layer much thinner than the 60-m tower height. The turbulent kinetic energy budget is evaluated for intermittent turbulence events observed at the 5-m level on the main tower. Generally, the onset of an intermittent turbulence event is not closely related to the reduction of the gradient Richardson number below 0.25, the critical Richardson number of turbulence generation for linear instability. Possible explanations including the influence of advected turbulence patches are discussed.     

Remote sensing of the dynamic stability of the atmospheric boundary layer

Summary The relative strength of the stabilizing effect of buoyancy and the destabilizing effect of velocity shear in a stratified shear flow, such as a stable atmospheric boundary layer, is measured by the gradient Richardson number, Ri_g. The boundary layer static stability, as described by the buoyancy frequency, N, can be calculated from the virtual potential temperature gradient derived from RASS temperature profiles. The mean wind profiles from a sodar can be used to calculate the mean vertical velocity shear. In combination these profilers are potentially a powerful tool for the remotely sensing the dynamic stability of the boundary layer. However, experience shows that the combinations of two experimentally derived quantities, like N and shear, may give highly variable results. On the other hand, a simple sensitivity analysis shows that reasonable estimates of Ri_g are achievable over a range of conditions in the stable nocturnal boundary layer. To test this conclusion, high spatial and temporal resolution temperature and velocity soundings were obtained above 50m in the stable nocturnal boundary layer using a 920MHz continuous wave Radio Acoustic Sounding System (RASS) and 1.875kHz and 5.00kHz Doppler sodars. Examples of the evolution of Ri_g are presented from 24 hours of observations of the boundary layer in Canberra, on the tablelands in south- eastern Australia. Most of the boundary layer had Ri_g between 0.1 and 1. Thus, it was marginally dynamically stable, even with the gradient Richardson number calculated from finite differences over a vertical interval of 68m. A comparison of the results from the two sodars showed that the velocity shear increased significantly when the vertical differencing interval was decreased from 68m to 20m.     

TURBULENCE STRUCTURE IN A STRATIFIED BOUNDARY LAYER UNDER STABLE CONDITIONS

Turbulence structure in stably stratified boundary layers isexperimentally investigated by using a thermally stratified wind tunnel. Astably stratified flow is created by heating the wind tunnel airflow to atemperature of about 50 °C and by cooling the test-section floor to asurface temperature of about 3 °C. In order to study the effect ofbuoyancy on turbulent boundary layers for a wide range of stability, thevelocity and temperature fluctuations are measured simultaneously at adownwind position of 23.5 m from the tunnel entrance, where the boundarylayer is fully developed. The Reynolds number, Re, ranges from 3.14× 104 to 1.27 × 105, and the bulk Richardson number, Ri,ranges from 0 to 1.33. Stable stratification rapidly suppresses thefluctuations of streamwise velocity and temperature as well as the verticalvelocity fluctuation. Momentum and heat fluxes are also significantlydecreased with increasing stability and become nearly zero in the lowest partof the boundary layer with strong stability. The vertical profiles ofturbulence quantities exhibit different behaviour in three distinct stabilityregimes, the neutral flows, the stratified flows with weak stability(Ri = 0.12, 0.20) and those with strong stability (Ri= 0.39,0.47, 1.33). Of these, the two regimes of stratified flows clearly showdifferent vertical profiles of the local gradient Richardson number Ri,separated by the critical Richardson number Ri cr of about 0.25. Moreover,turbulence quantities in stable conditions are well correlated with Ri.     

Richardson's Law in Large-Eddy Simulations of Boundary-Layer Flows

Relative dispersion in a neutrally stratified planetary boundary layer (PBL) is investigated by means of large-eddy simulations (LES). Despite the small extension of the inertial range of scales in the simulated PBL, our Lagrangian statistics turn out to be compatible with the Richardson t³ law for the average of square particle separation, where t is time. This emerges from the application of non-standard methods of analysis through which a precise measure of the Richardson constant was also possible. Its value is estimated as C₂ 0.5, in close agreement with recent experiments and three-dimensional direct numerical simulations.     

MCC and gull flight behavior

Wintertime observations of mesoscale cellular convection (MCC) over the East China Sea have resulted in criteria that have a remarkable similarity to those reported by Woodcock (1975) in the study of thermals and gull flight behavior. It has been determined that the surface wind speed (V) and the air-sea temperature difference (T) prescribe unique and compatible conditons for both the occurrence of MCC and soaring by sea gulls. Specifically, the onset of MCC when V is between 5 and 9 m s^–1 is inversely proportional to T in the range 5 to 7 °C. Elsewhere, the onset of MCC occurs under conditions of direct proportionality between V and T. Necessary conditions for the occurrence of MCC due to heating from below are T 5 °C and V 5 m s^–1. The boundaries of the convective regime for MCC are discussed and interpreted in accordance with the regime for sea-gull soaring and similarity concepts.     

The Value Of The β Coefficient In The Relaxed Eddy Accumulation Method In Terms Of Fourth-Order Moments

The commonly measured value of in the relaxed eddy accumulationmethod of about 0.56is shown to arise from the non-Gaussiannature of turbulence. Fourth-orderGram–Charlier functions forthe two-dimensional probability distributionsof variation in the horizontal component of wind velocityand concentrations of water vapour, carbondioxide and methane with respect to thevertical component of wind velocity are used to examinethe value of .An analytical solution for ispresented in terms of fourth-order moments.Under mean conditions, this solution givesa value for of0.557. Variation of is shown to be controlledprimarily by the ratio of the mean ofc'w3 (where c'is relevant to the entity of interest andw' is vertical component of windvelocity) to the correlationcoefficient between the entity concentrationand vertical component of wind velocity.     

Microscale ordered motions and atmospheric structure associated with thin echo layers in stably stratified zones

The interpretation of ultra-high resolution radar observations of thin clear-air echo strata is made with the aid of fine-scale aircraft measurements. The echo layer, generally comprising two sub-strata each 5 m thick and spaced 7–10 m apart, is found within a 10–20 m deep section of a strong inversion where the thermal stability and shear are maximized, and the Richardson number is close to 0.25. Mechanical turbulence is restricted entirely to this layer; the variance of the N-S velocity component, ³, is the strongest, consistent with the orientation of the shear vector in this stratum. Spectra and cospectra of a 9-s slant run through the echo stratum show remarkably ordered motions. A strong negative peak of <w> covariance at 80-m scale, accompanied by a zero of <uw> covariance and bulges in the longitudinal () and vertical (w) velocity spectra, is identified with breaking Kelvin-Helmholtz waves oriented in the N-S direction along the shear vector. A synthesis of the temperature and velocity structures from measurements along the flight path confirms the ordered motion deduced from the spectra and reveals a group of K-H waves of 80-m length and 10-m height at the height of the radar echo. Microscale K-H ripples of 3–4 m length are also deduced to be present in the 0.5 m thick interfacial region where the thermal gradient and shear are strongly enhanced by the larger shearing K-H wave.Two possible sources of the echoes are proposed: (1) scatter from fully developed turbulence within the interfacial zone in an inertial subrange falling entirely in sub-meter scales; and (2) the incoherent summation of specular reflections from properly oriented portions of the microscale K-H ripples. While the authors favor the first of these mechanisms, both require stringent conditions of the physical microstructure which are beyond the available observations. Fossil turbulence is precluded as an echo mechanism.This paper is based in part on the doctoral dissertation by the senior author.Present affiliation: Air Force Cambridge Research Laboratories, Bedford, Mass., U.S.A.     

Wind-Tunnel Study Of Atmospheric Stable Boundary Layers Over A Rough Surface

Wind-tunnel simulations of theatmospheric stable boundary layer (SBL) developedover a rough surface were conducted by using athermally stratified wind tunnel at the Research Institutefor Applied Mechanics (RIAM), Kyushu University. Thepresent experiment is a continuation of the workcarried out in a wind tunnel at Colorado StateUniversity (CSU), where the SBL flows were developed over asmooth surface. Stably stratified flows were createdby heating the wind-tunnel airflow to a temperature ofabout 40–50°and by cooling the test-section floor toa temperature of about 10°. To simulate therough surface, a chain roughness was placed over thetest-section floor. We have investigated the buoyancyeffect on the turbulent boundary layer developed overthis rough surface for a wide range of stability,particularly focusing on the turbulence structure andtransport process in the very stable boundary layer.The present experimental results broadly confirm theresults obtained in the CSU experiment with the smoothsurface, and emphasizes the following features: thevertical profiles of turbulence statistics exhibitdifferent behaviour in two distinct stability regimes with weak and strong stability,corresponding to the difference in the verticalprofiles of the local Richardson number. The tworegimes are separated by the critical Richardsonnumber. The magnitudes in turbulence intensities andturbulent fluxes for the weak stability regime aremuch greater than those of the CSU experiments becauseof the greater surface roughness. For the very stableboundary layer, the turbulent fluxes of momentum andheat tend to vanish and wave-like motions due to theKelvin–Helmholtz instability and the rolling up andbreaking of those waves can be observed. Furthermore,the appearance of internal gravity waves is suggestedfrom cross-spectrum analyses.     

Variability Of The Stable And Unstable Atmospheric Boundary-Layer Height And Its Scales Over A Boreal Forest

Radiosondes releases during the NOPEX-WINTEX experiment carried out in late winter in Northern Finland were analysed for the determination of the height h of the atmospheric boundary layer. We investigate various possible scaling approaches, based on length scales using micrometeorological turbulence surface measurements and the background atmospheric stratification above h. Under stable conditions, the three previously observed turbulence regimes delineated by values of z/L (L is the Obukhov length) appears as a blueprint for understanding the departures found for the suitability of the Ekman scaling based on L_E = u_/f (u is the friction velocity and f the Coriolis parameter). The length scale L_N = u_/N (where N is the Brunt–Väisälä frequency) appears to be a useful scale under most stable conditions, especially in association with L. Under unstable conditions, shear production of turbulence is still significant, so that the three scales L, L_N and L_E are again relevant and the dimensionless ratios _N = L_N/L and L_N/L_E = N/f describe well the WINTEX data. Furthermore, in the classical scaling framework, the unstable domain may also be divided into three regimes as reflected by the dependence ofu_/f on instability (z/L).     

Acceleration effects for linearly-stratified flow over long ridges

Summary The time-dependent motion of long ridges through a linearly stratified fluid otherwise at rest is investigated in a series of laboratory experiments. Similarity conditions for relating such flows to the atmosphere are deduced from the equations of motion and boundary conditions for the respective systems.Experiments concerning end-wall effects in towing experiments with linearly stratified fluid systems are conducted. For obstacles extending across the entire width of the tow tank it is shown that the upstream conditions are continually changing so that a final steady state motion may never be realized. Isolated topographies are shown to induce significantly less effect on the far upstream fluid motions. Case studies for the flow past long ridges for which the motion at large times is to be that of single, double and triple mode lee-waves and breaking lee vortices are conducted for impulsively started and uniformly accelerated and decelerated obstacle transverses. The final flow configuration under certain situations is shown to be relatively insensitive to the starting conditions. In other cases the final flow can be highly dependent on the time history of the ridge traverse through the tank. For example, for the case in which a breaking lee vortex is expected as the final flow, small initial uniform accelerations from a zero velocity lead to the formation of a strong rotor along the free surface of the tank and in the lee of the obstacle. This rotor is maintained in an approximate equilibrium position as the ridge speed reaches a value for which a breaking lee vortex (having no rotor) should be expected; i.e., the type of flow obtained for impulsively started or rapidly accelerating ridges, other parameters being fixed.The phenomenon of the oscillation of the structure of the wake flow between a relatively smooth laminar lee-wave pattern and lee waves that break into turbulence is investigated for impulsively started ridges. By defining the parameterN _w as the number of waves downstream of the first trough that are clearly identifiable it is shown that the tendency for wake breakdown into turbulence increases with increasing internal Froude number, other parameters being fixed. No definitive period was found relating the alternating nature of the wake between breakdown, into turbulence, relaminarization and so on.With 20 Figures     

Internal gravity waves in a stably stratified boundary layer

Often, a combination of waves and turbulence is present in the stably stratified atmospheric boundary layer. The presence of waves manifest itself in the vertical profiles of variances of fluctuations and in low-frequency contributions to the power spectra. In this paper we study internal waves by means of a linear stability analysis of the mean profiles in a stably stratified boundary layer and compare the results with observed vertical variance profiles of fluctuating wind and temperature along a 200 m mast. The linear stability analysis shows that the observed mean flow is unstable for disturbances in a certain frequency and wavenumber domain. These disturbances are expected to the detectable in the measurements. It is shown that indeed the calculated unstable frequencies are present in the observed spectra. Furthermore, the shape of the measured vertical variance profiles, which increase with height, is explained well by the calculated vertical structure of the amplitude of unstable Kelvin-Helmholtz waves, confirming the contribution of waves to the variances. Because turbulence and waves have quite distinct transport properties, estimates of diffusion from measurements of variances would strongly overestimate this diffusion. Therefore it is important to distinguish between them.     

An explicit three-dimensional nonhydrostatic numerical simulation of a tropical cyclone

Summary A nonhydrostatic numerical simulation of a tropical cyclone is performed with explicit representation of cumulus on a meso- scale grid and for a brief period on a meso- scale grid. Individual cumulus plumes are represented by a combination of explicit resolution and a 1.5 level closure predicting turbulent kinetic energy (TKE).The results demonstrate a number of expected and unexpected important scale interaction processes. Within the central core of the developing cyclone, meso- convective regions grow and breakdown into propagating inertiagravity waves throughout the lifecycle of the cyclone. In the early stages, the amplitude of pressure fluctuations associated with the meso- scale convection exceed the central pressure of the cyclone and strongly modulate its intensity. With each meso- scale pulsation, the cyclone core increases in strength, measured by the central pressure deficit. The increasingly strong inertial frequency of the storm core acts to increasingly trap the convection induced heating within the core by balancing the tangential wind against the low central pressure, before the meso- scale convection breaks down and sends the warmth away as a propagating wave. Eventually, the slow manifold's amplitude exceeds the amplitude of the meso- scale oscillations and a stable eye region is formed. As inertial instability increases, increasingly high thermal warmth can be protected in the core, allowing persistent subsidence to form and to clear out the cyclone eye.On the outside of the eye wall, strong inertial stability gradients in the troposphere cause convective warming to split the inflow to the eye wal! and spawn outwardly propagating inertia gravity waves. These waves carry away all of the heating forced by convection that is not inertially trapped by the eye wall and act as a moderating influence on storm intensity.Inertia gravity waves are also spawned in the stratosphere at the top of the eye wall by the revolution of asymmetric cumulus structures. In all instances, the tropospheric waves are coupled to the propagating stratospheric waves which both move at 35 ms^–1, although there are many instances where the stratospheric waves seem to have no tropospheric counterpart. Hence the anvil top forcing and low level breakdown are linked.The outwardly propagating inertia gravity waves act to initiate outer bands of convection. This initiation is with the assistance of low level boundary layer variations of density related to previous convection and to virga falling from the anvil which moistens and destabilizes the mid levels of _e minimum. The convection initiated by these waves does not move substantially outward with the wave, although may appear to develop outward discontinuously.With 12 Figures     

The Adaptation of the Atmospheric Boundary Layer to a Change in Surface Roughness

The adaptation of the atmospheric boundary layer to a change in the underlying surface roughness is an interesting problem and hence much research, theoretical, experimental, and numerical, has been undertaken. Within the atmospheric boundary layer an accurate numerical model for the turbulent properties of the atmospheric boundary layer needs to be implemented if physically realistic results are to be obtained. Here, the adaptation of the atmospheric boundary layer to a change in surface roughness is investigated using a first-order turbulence closure model, a one-and-a-half-order turbulence closure model and a second-order turbulence closure model. Perturbations to the geostrophic wind and the pressure gradients are included and it is shown that the second-order turbulence closure model, namely the standard k - model, is inferior to a lower-order closure model if a modification to limit the turbulent eddy size within the atmospheric boundary layer is not included within the model.     

Bistability of the thermohaline circulation identified through comprehensive 2-parameter sweeps of an efficient climate model

The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic–Pacific moisture flux (F_a) and atmospheric moisture diffusivity (K_q). In a large ensemble of spinup experiments, the values of F_a and K_q are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor On and Off states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of F_a and K_q, reveal the co-existence of two stable THC states (Conveyor On and Off)—i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a Conveyor On state, irreversibly collapses to a Conveyor Off state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.     

Energy- and Flux-Budget Turbulence Closure Model for Stably Stratified Flows. Part II: The Role of Internal Gravity Waves

We advance our prior energy- and flux-budget (EFB) turbulence closure model for stably stratified atmospheric flow and extend it to account for an additional vertical flux of momentum and additional productions of turbulent kinetic energy (TKE), turbulent potential energy (TPE) and turbulent flux of potential temperature due to large-scale internal gravity waves (IGW). For the stationary, homogeneous regime, the first version of the EFB model disregarding large-scale IGW yielded universal dependencies of the flux Richardson number, turbulent Prandtl number, energy ratios, and normalised vertical fluxes of momentum and heat on the gradient Richardson number, Ri. Due to the large-scale IGW, these dependencies lose their universality. The maximal value of the flux Richardson number (universal constant ≈0.2–0.25 in the no-IGW regime) becomes strongly variable. In the vertically homogeneous stratification, it increases with increasing wave energy and can even exceed 1. For heterogeneous stratification, when internal gravity waves propagate towards stronger stratification, the maximal flux Richardson number decreases with increasing wave energy, reaches zero and then becomes negative. In other words, the vertical flux of potential temperature becomes counter-gradient. Internal gravity waves also reduce the anisotropy of turbulence: in contrast to the mean wind shear, which generates only horizontal TKE, internal gravity waves generate both horizontal and vertical TKE. Internal gravity waves also increase the share of TPE in the turbulent total energy (TTE = TKE + TPE). A well-known effect of internal gravity waves is their direct contribution to the vertical transport of momentum. Depending on the direction (downward or upward), internal gravity waves either strengthen or weaken the total vertical flux of momentum. Predictions from the proposed model are consistent with available data from atmospheric and laboratory experiments, direct numerical simulations and large-eddy simulations.     

Charakteristische Zirkulationstypen in mittleren Breiten der nördlichen Hemisphäre

Zusammenfassung In der vorliegenden Arbeit werden neue Klassifikationsprinzipien für Großwetterlagen entwickelt. Bisher wurde bei Wetterlagenklassifikationen das Druckfeld zugrunde gelegt, wobei quasistationären Druckzentren eine nicht berechtigte Vorrangstellung eingeräumt wurde. In der hier versuchten Klassifikation wird vom Strömungsfeld ausgegangen, das in elementare Formen zerlegt wird. Eine zu diesem Zweck durchgeführte statistische Untersuchung ergab, daß alle im Strömungsfeld auftretenden Zirkulationstypen auf drei Grundformen zurückgeführt werden können. Diese Grundformen sind:Driften, Wellen undWirbel.Die Untersuchung ergab im einzelnen, daß in mittleren Breiten der nördlichen Hemisphäre bei 49% aller untersuchten Fälle Driften, bei 23% Wellen und bei 28% Wirbel auftraten.In der hier durchgeführten Klassifikation wird das Druckfeld durch das Strömungsfeld und der Begriff Großwetterlage durch den umfassenderen Begriff des Zirkulationstyps ersetzt. Damit wird der unberechtigte Vorrang der Druckformen bei der Wetterlagenklassifikation aufgegeben. Die Klassifizierung der Zirkulationstypen ergibt sich schließlich durch Kombination der drei Zirkulationselemente: Drift, Welle und Wirbel.

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Summary New principles of classification for large-scale weather situations are outlined in this paper. Hitherto the pressure-field has been taken as a basis for such classifications by conceding a precedence of an unjustified position to semi-permanent centres of pressure. The new classification starts from the field of large-scale motions, which is dissected in elementary models. A statistical test yielded the possibility to reduce all types of atmospheric circulations in the following three elementary models:drifts, waves andeddies.In detail it was found out, that drifts occur in 49%, waves in 23% and eddies in 28% of all cases investigated.In the new classification the term pressure-field is substituted by field of motion and the expression large-scale weather situation by the more comprehensive conception type of circulation. By that the unjustified priority of pressure-centers in classifying weather situations is abolished. At last the classification of the types of circulation follows from a combination of the three elementary models: drift, wave and eddy.

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résumé La présente étude développe de nouveaux principes de classification des situations météorologiques. Alors que jusqu'ici on s'est fondé sur le champ de pression ce qui conduisait à attribuer aux centres d'action quasi stationnaires un rôle trop important, l'auteur part ici du champ de mouvement décomposé en formes élémentaires. Un examen statistique lui a montré que tous les types de circulation peuvent se ramener à trois formes fondamentales:courants, ondulations ettourbillons.Aux latitudes moyennes de l'hémisphère Nord les courants représentent le 49%, les ondulations le 23% et les tourbillons le 28%.Au champ de pression se substitue donc le champ de courant, et les situations météorologiques se groupent en types de circulation ce qui supprime le rôle prépondérant des formes isobariques. Le classement final des types de circulation résulte de la combinaison des trois types mentionnés:courants, ondulations ettourbillons.

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