Mean Flow Patterns Research Articles

Some characteristics of Limited-Area Model-Precipitation forecast of Indian monsoon and evaluation of associated flow features

The Florida State University based Limited-Area Model is in operational use at India Meteorological Department, New Delhi. This paper assesses the performance of the model in temporal and spatial scale to provide 24 hours rainfall forecast over Indian region during south-west monsoon (June to September) and north-east monsoon season (October and November) using three years data (1997–1999). Characteristic features of mean flow pattern produced by the forecast (24 hours) are also examined comparing them with the corresponding analysis fields. The study demonstrates that performance of the numerical model in predicting precipitation varies with month, geographic location and by synoptic regime. The model, in general, is able to reproduce the spatial pattern of monthly and seasonal rainfall but under-estimates orographic rainfall and over estimates rainfall associated with the monsoon low pressure systems. In October and November when the low pressure belt shifts to the southern latitudes of India, the model is able to capture the heavy rainfall zone confined over south Peninsula. Time series and categorical statistics show that there is a very good correspondence between observed and predicted rainfall. The other important characteristic features of south west monsoon are well captured by the model and are comparable with the analysis field. But some of the significant deficiencies noticed in the forecast are: higher mean sea-level pressure, particularly over the domain of heat low and relatively weaker lower tropospheric westerlies. The results of this paper will provide useful input on model performance to operational forecasters and model developers.

Testing for Erosion-Corrosion Under Disturbed Flow Conditions Using a Rotating Cylinder with a Stepped Surface

Abstract Erosion-corrosion is most severe in the vicinity of flow disturbances. In the past, erosion-corrosion under disturbed flow conditions has been studied experimentally in flow loops and numerically by performing flow simulations. In this study, a new, compact experimental setup was tested, intended for the study of erosion-corrosion under disturbed flow conditions, involving a rotating cylinder geometry with a sudden step. A thorough characterization of this new setup was initiated, involving wall mass-transfer measurements complemented with direct numerical simulation of the turbulent flow around it. A large variation of the wall mass-transfer rates behind the step was measured, similar in character to the one obtained in flow through a sudden pipe expansion. Flow simulations have confirmed that this flow geometry will create a qualitatively similar mean flow pattern as observed in a sudden pipe expansion flow involving flow separation and reattachment. Simulations also have shown that there is a ...

Wind Regime and Wind Power in North Patagonia, Argentina

The present paper discusses wind and wind power characteristics in a region north of 40° S (North Patagonia, Argentina) as well as wind power projects sustainability. Extreme wind episodes influence most economic and social regional activities. Mean flow pattern was studied in three North Patagonia sites: Neuquén, Mari Menuco and Cutral-Có, by sampling analysis of wind direction and intensity during a year. Gusts were quantified and strong winds were studied. Available meteorological power was also estimated. It was observed that North Patagonia is a place of great wind power potential. However, the energy available, which is related to prevailing strong winds, is also associated to high gust occurrence which certainly restricts exploitation.

Numerical Modeling for Fish Diversion Studies

A 3D numerical model, validated with data from field and laboratory measurements, is applied to simulate the flow in a 7-km-long reach of the Columbia River, upstream from Wanapum Dam, Wash. As a tool for analysis and design of alternative fish diversion schemes, the model provides mean flow patterns, turbulence levels, acceleration patterns, and other flow characteristics that are believed to be important to fish behavior. The paper describes the features of the model. Because of the pronounced three-dimensionality of the flow field and the need to capture it in the simulation, the river reach is divided into subreaches with different length scales and grid sizes. The model is applied successively to the three subreaches. An example is presented showing particle tracking through the flow field using a Monte Carlo technique.

Effect of general anesthesia on the severity of mitral regurgitation by transesophageal echocardiography

The effect of general anesthesia on the severity of mitral regurgitation (MR) was examined in 43 patients with moderate or severe MR who underwent preoperative and intraoperative transesophageal echocardiography. Systolic blood pressure, mean arterial pressure, and left ventricular end-diastolic and end-systolic dimensions were significantly lower during the intraoperative study, reflecting altered loading conditions. The mean color Doppler jet area and mean vena contracta decreased and the mean pulmonary venous flow pattern changed from reversed to blunted, reflecting a significant reduction in the severity of MR. Overall, 22 of the 43 patients (51%) improved at least 1 MR severity grade when assessed under general anesthesia. Thus, intraoperative transesophageal echocardiography may significantly underestimate the severity of MR. A thorough preoperative assessment is preferable when deciding whether to perform mitral valve surgery.

A meteorological overview of the Pacific Exploratory Mission (PEM) Tropics period

NASA's Pacific Exploratory Mission‐Tropics (PEM‐T) experiment investigated the atmospheric chemistry of a large portion of the tropical and subtropical Pacific Basin during August to October 1996. This paper summarizes meteorological conditions over the PEM‐T domain. Mean flow patterns during PEM‐T are described. Important circulation systems near the surface include subtropical anticyclones, the South Pacific Convergence Zone (SPCZ), the Intertropical Convergence Zone (ITCZ), and middle latitude transient cyclones. The SPCZ and ITCZ are areas of widespread ascent and deep convection; however, there is relatively little lightning in these oceanic regions. A large area of subsidence is associated with the subtropical anticyclone centered near Easter Island. PEM‐T occurred during a period of near normal sea surface temperatures. When compared to an 11 year climatology (1986–1996), relatively minor circulation anomalies are observed during PEM‐T. Some of these circulation anomalies are consistent with much stronger anomalies observed during previous La Nina events. In general, however, the 1996 PEM‐T period appears to be climatologically representative. Meteorological conditions for specific flights from each major operations area are summarized. The vertical distribution of ozone along selected DC‐8 flights is described using the DIAL remote sensing system. These ozone distributions are related to thermodynamic soundings obtained during aircraft maneuvers and to backward trajectories that arrived at locations along the flight tracks. Most locations in the deep tropics are found to have relatively small values of tropospheric ozone. Backward trajectories calculated from global gridded analyses show that much of this air originates from the east and has not passed over land within 10 days. The deep convection associated with the ITCZ and SPCZ also influences the atmospheric chemistry of these regions. Flights over portions of the subtropics and middle latitudes document layers of greatly enhanced tropospheric ozone, sometimes exceeding 80 ppbv. In situ carbon monoxide in these layers often exceeds 90 ppbv. These regions are located near, and especially south of Tahiti, Easter Island, and Fiji. The layers of enhanced ozone usually correspond to layers of dry air, associated with widespread subsiding air. The backward trajectories show that air parcels arriving in these regions originate from the west, passing over Australia and even extending back to southern Africa. These are regions of biomass burning. The in situ chemical measurements support the trajectory‐derived origins of these ozone plumes. Thus the enhanced tropospheric ozone over the central Pacific Basin may be due to biomass burning many thousands of kilometers away. Middle‐latitude portions of the PEM‐T area are influenced by transient cyclones, and the DC‐8 traversed tropopause folds during several flights. The flight area just west of Ecuador experiences outflow from South America. Thus the biomass burning that is prevalent over portions of Brazil influences this area.

Wind-induced mesoscale circulation off the Ebro delta, NW Mediterranean: a numerical study

It is well known that the general circulation on the Catalan continental slope is dominated by a quasi-permanent southwestward geostrophic jet associated to the so-called Catalan front [Millot, C., 1987. Circulation in the western Mediterranean sea. Oceanol Acta 10, 143–149; Font, J., Salat, J., Tintoré, J., 1988. Permanent features of the circulation in the Catalan Sea. Oceanol. Acta 9, 51–57]. On the continental shelf, however, the flow is modified by the action of friction which enhances also other nonlinear interactions. Several authors have hypothesized that the shelf circulation is anticyclonic north of the Ebro delta [Salat, J., Manriquez, M., Cruzado, A., 1978. Hidrografia del golfo de Sant Jordi. Campaña Delta (Abril 1970). Investigación Pesquera 42 (2), 255–272; Ballester, A., Castellvı́, J., 1980. Estudio hidrográfico y biológico de las plataformas continentales españolas: I. Efecto de los efluentes de una planta de energı́a nuclear en el Golfo de San Jorge (Febrero 1975–Octubre 1976). Informes Técnicos del Instituto de Investigaciones Pesqueras 76, 70 pp.]. A quasi-3D finite element code based on the shallow-water equations has been used to explore the effect of several mechanisms which might be responsible for such a local circulation pattern, and in particular of wind. The obtained numerical results suggest that the basic anticyclonic structure of the mean flow is controlled by the bathymetry and that the clockwise-rotating mean flow pattern is not a permanent circulation feature. It is seen that the characteristic local wind stress fields—computed through interpolation of the records of a local network of meteo stations—may `enhance' or `delete' the anticyclonic gyre depending on the sign of their relative vorticity. According to the analysis of a 2-yr record of local wind data, the net contribution of wind events with a duration longer than 24 h is to reinforce the anticyclonic circulation (over 70% of these wind fields supply negative relative vorticity to the study area).

Investigation of the mean and turbulent particle velocity fields in a spouted bed using radioactive particle tracking

AbstractA non invasive experimental Radioactive Particle Tracking method (R.P.T.) was used to determine the three‐dimensional mean and turbulent particle velocity fields in a spouted bed. The local statistical properties were evaluated with approximately one million data obtained on typically 6 hours of particle tracking. Radial profiles of first and second order moments of the particle velocity field are presented in the spout, the annulus and the fountain region on several axial sections. In addition to the complete description of the solids mean flow pattern, the relevance of a strong anisotropy behavior for particle turbulence field was well established. Thus, this study constitutes an original experimental investigation needed both for a better physical understanding and refined flow modelling of gas‐solid flows in spouted beds.

The tidal and subtidal variations in the transverse salinity and current distributions across a coastal plain estuary

The transverse structure in the current and salinity distributions across the mouth of Delaware Bay are examined over the tidal and subtidal time scales. Results show that the across-estuary variation in bathymetry, in the form of a channel-shoal configuration, has a very significant impact on the characteristics of transverse variability. The mean current over the channel is characterized by a strong outflow of low salinity water in the upper layer and a strong inflow of high salinity water in the lower layer, consistent with the density-induced gravitational circulation. The mean flow pattern in the shallow waters over the shoals is marked by transverse rather than vertical variation. The subtidal current and salinity fluctuations are primarily driven by the effect of local atmospheric forcing. The subtidal current fluctuations in the upper layer of the channel are frictionally coupled to the local wind, resulting in downwind currents. The subtidal current fluctuations in the lower layer of the channel, however, flow in the direction of local setup and against the wind. With a wind blowing down the estuary, the wind-induced current tends to reinforce the two-layer structure of the gravitational circulation and substantially enhance the vertical shear and surface to bottom salinity difference. The reverse occurs with a wind in the up-bay direction. The subtidal currents in the shallow areas to the right of the channel exhibit largely depth-independent response to the effect of local wind, with downwind currents at both the surface and the bottom. At tidal frequencies the currents show only a modest variation across the bay mouth. Tidal currents are highly deterministic, but the characteristics of the tidal variability in salinity exhibit significant changes over long time scales. These long-term changes in the intratidal salinity variability are caused by the nonlinear interactions between the tidal and subtidal motions. The residual salt flux through the bay mouth shows significant subtidal fluctuations. The leading factor responsible for producing such subtidal fluctuations is the advection of salt by the wind-induced subtidal currents, but the effect of tidal pumping also contributes significantly to the overall residual salt flux into the estuary.

Turbulent Thermal Convection with an Obstructed Sidewall

We present an experimental study of turbulent convection in a cell with staggered fingers on its sidewall using water as the convecting fluid. Our measurements reveal that $\mathrm{Nu}\ensuremath{\sim}{\mathrm{Ra}}^{\ensuremath{-}0.27}$, and that the temperature probability density function (PDF) at the cell center is a double-peaked function which can be fitted by the superposition of two Gaussians. Moreover, it is found that the size of the local temperature fluctuations scales with Ra and that the scaled PDFs for different Ra have an invariant form. Visualization studies show that the mean flow pattern in the cell is a twisted asymmetric four-roll circulation, which helped to explain the observed PDF.

Turbulent plane Couette flow subject to strong system rotation

System rotation is known to substantially affect the mean flow pattern as well as the turbulence structure in rotating channel flows. In a numerical study of plane Couette flow rotating slowly about an axis aligned with the mean vorticity, Bech & Andersson (1996a) found that the turbulence level was damped in the presence of anticyclonic system rotation, in spite of the occurrence of longitudinal counter-rotating roll cells. Moreover, the turbulence anisotropy was practically unaffected by the weak rotation, for which the rotation number Ro, defined as the ratio of twice the imposed angular vorticity Ω to the shear rate of the corresponding laminar flow, was ±0.01. The aim of the present paper is to explore the effects of stronger anticyclonic system rotation on directly simulated turbulent plane Couette flow. Turbulence statistics like energy, enstrophy and Taylor lengthscales, both componental and directional, were computed from the statistically steady flow fields and supplemented by structural information obtained by conditional sampling.The designation of the imposed system rotation as ‘high’ was associated with a reversal of the conventional Reynolds stress anisotropy so that the velocity fluctuations perpendicular to the wall exceeded those in the streamwise direction. It was observed that the anisotropy reversal was accompanied by an appreciable region of the mean velocity profile with slope ∼2Ω, i.e. the absolute mean vorticity tended to zero. It is particularly noteworthy that these characteristic features were shared by two fundamentally different flow regimes. First, the two-dimensional roll cell pattern already observed at Ro=0.01 became more regular and energetic at Ro=0.10 and 0.20, whereas the turbulence level was reduced by about 50%. Then, when Ro was further increased to 0.50, a disordering of the predominant roll cell pattern set in during a transient period until the flow field settled at a new statistically steady state substantially less affected by the roll cells. This was accompanied by a substantial amplification of the streamwise turbulent vorticity and an anomalous variation of the mean turbulent kinetic energy which peaked in the middle of the channel rather than near the walls. While the predominant flow structures of the non-rotating flow were longitudinal streaks, system rotation generated streamwise vortices, either ordered secondary flow or quasi-streamwise vortices. Eventually, at Ro=1.0, the turbulent fluctuations were completely suppressed and the flow field relaminarized.

Application of infrared thermography to the evaluation of local convective heat transfer on arrays of cubical protrusions

This paper reports on the experimental technique and measurement method used for the investigation of the local convective heat transfer from cubical protrusions placed on a vertical wall of a rectangular-sectioned wind tunnel. This experimental configuration simulates a simplified air-cooled vertical electronic circuit board, resembling a situation in an electronic cabinet. A controlled heat flux was generated by internal heating of the elements. An advanced infrared system was used to measure the surface temperatures of the cubical elements, which is crucial for determining the local convective heat transfer. The difficulties and uncertainties encountered in the course of measurements are discussed together with possible solutions of the problems. A selection of results, validated by means of an uncertainty analysis, is presented, showing a strong variation of the local convective heat transfer around the individual elements. Qualitative interpretation of the local heat transfer coefficients was facilitated by smoke visualization of the mean flow pattern around the protrusions.

Measurement of Turbulent Flow Fields in a Agitated Vessel with Four Baffles by Laser-Doppler Velocimetry. Mean Velocity Fields and Flow Pattern.

The three dimensional complex turbulent flow fields induced by a four flat blade paddle impeller in agitated vessel were measured by laser doppler velocimetry. Mixing vessel used was a closed cylindrical tank of 490 mm diameter with a flat bottom and four vertical buffles, giving water volumes of about 100 1. The impellers were at the midheight of the water level in the tank. A height of liquid (water) was equal to the vessel diameter. Three components of mean velocity were measured at three vertical sections θ=7.5°, 45°and 85°, in several horizontal planes. Mixing Reynolds number NRe was 1.2×105. It can be found from the results that circumferential mean velocity profiles show the symmetrical shape in the upper and lower sides of impeller. Secondary velocity components, such as axial and radial velocities, however, were not in symmetry. For this reason, the ratio of circulation flow volume which enter in upper and lower sides of impeller was roughly 7/3. In both the middle and buffle regions, mean flow velocities (flow patterns) were different, dependent of three vertical planes with different circumferential angle measured from buffle.

Measurements of air flow characteristics in a full-scale clean room

Detailed air turbulence characteristics in a full-scale and running fan—filter unit type clean room (4.20 m × 2.97 m × 2.70 m) with and without a clean bench loaded are investigated experimentally. The measurements of the three components of air velocity were conducted by means of a three-dimensional ultrasonic anemometer. Velocity vectors, turbulence intensity, turbulence kinetic energy, and probability density functions of velocity direction and velocity contours in the space domain are presented. The results show that the non-uniformity and deflection angle of air velocity are within 13% and 7.9 degrees, respectively. Moreover, the mean flow patterns, turbulence characteristics, and histogram features of the air flow are discussed. The detailed experimental data are not only useful for clean room design and equipment layout but also helpful for use in the evaluation of numerical modeling in clean room flow fields.

Deep-Water Formation and Meridional Overturning in a High-Resolution Model of the North Atlantic

The authors use different versions of the model of the wind- and thermohaline-driven circulation in the North and Equatorial Atlantic developed under the WOCE Community Modeling Effort to investigate the mean flow pattern and deep-water formation in the subpolar region, and the corresponding structure of the basin-scale meridional overturning circulation transport. A suite of model experiments has been carded out in recent years, differing in horizontal resolution (1° × 1.2°, 1/3° × 0.4°, 1/6° × 0.2°), thermohaline boundary conditions, and parameterization of small-scale mixing. The mass transport in the subpolar gyre and the production of North Atlantic Deep Water (NADW) appears to be essentially controlled by the outflow of dense water from the Greenland and Norwegian Seas. in the present model simulated by restoring conditions in a buffer zone adjacent to the boundary near the Greenland–Scotland Ridge. Deep winter convection homogenizes the water column in the center of the Labrador Sea to about 2000 m. The water mass properties (potential temperature about 3°C, salinity about 34.9 psu) and the volume (1.1×1053 km3) of the homogenized water are in fair agreement with observations. The convective mixing has only little effect on the net sinking of upper-layer water in the subpolar gyre. Sensitivity experiments show that the export of NADW from the subpolar North Atlantic is more strongly affected by changes in the overflow conditions than by changes in the surface buoyancy fluxes over the Labrador and Irminger Seas, even if these suppress the deep convection completely. The host of sensitivity experiments demonstrates that realistic meridional overturning and heat transport distributions for the North Atlantic (with a maximum of 1 PW) can be obtained with NADW production rates of 15–16 Sv, provided the spurious upwelling of deep water that characterizes many model solutions in the Gulf Stream regime is avoided by adequate horizontal resolution add mixing parameterization.

Climatology of the Simulated Great Plains Low-Level Jet and Its Contribution to the Continental Moisture Budget of the United States

The Great Plains region of the United States is characterized by some of the most frequent and regular occurrences of a nocturnal low-level jet (LLJ). While the LLJ is generally confined to the lowest Kilometer of the atmosphere, it may cover a substantial region of the Great Plains, and typically reaches maximum amplitudes of more than 20 m s−1. A two-month, springtime simulation with the Goddard Earth Observing System (GEOS-1) atmospheric general circulation model (AGCM) has produced a Great Plains LLJ with a vertical and temporal structure, directionality, and climatological distribution that compare favorably with observations. The diurnal cycle of the low-level flow is dramatic and coherent over a subcontinental area that includes much of the western United States and northern Mexico. This cycle can be interpreted as the nightly intrusion of the anticyclonic, subtropical gyre (associated with the Bermuda high) into the North American continent as surface friction decreases. The AGCM also simulates a pair of northerly LLJ maxima off the California coast, which seem to correspond to observations of a so-called “Baja Jet.” Other apparently related diurnal variations extending well into the upper troposphere are documented and compared with observations. The time-averaged climatological picture of the low-level flow is dominated over land by the nocturnal phase of the diurnal cycle, in which surface friction is minimal and wind speeds are strongest. This pattern, with its zones of strong convergence, is characteristic of an unsteady, strongly forced flow. Over the open ocean, the mean low-level flow is more reminiscent of a smooth, climatological pattern. Analysis of the simulated moisture budget for the continental United States reveals a horizontally confined region of strong southerly moisture transport with a strong diurnal cycle in the region of the Great Plains LLJ, as has been found in observations of water vapor transport. The LLJ plays a key role in that budget by transporting almost one-third of all the moisture that enters the continental United States with most of the influx from the LLJ (slightly less than two-thirds of it) entering during the 12 nighttime hours. However, it is the mean flow pattern and not covariances associated with the diurnal cycle that contribute most significantly to the total time-mean moisture transport. Covariances on the synoptic and longer timescales contribute only about one-fifth of the total time-mean transport of moisture in the jet region, and covariances on the diurnal timescale are negative and negligible despite the strong diurnal signal in the wind.

Effects of Increased Horizontal Resolution in a Simulation of the North Atlantic Ocean

Global mean and eddy fields from a four-year experiment with a 1/6° × 1/5° horizontal resolution implementation of the CME North Atlantic model are presented. The time-averaged wind-driven and thermohaline circulation in the model is compared to the results of a 1/3° × 2/5° model run in very similar configuration. In general, the higher resolution results are found to confirm that the resolution of previous CME experiments is sufficient to describe many features of the large-scale circulation and water mass distribution quite well. While the increased resolution does not lead to large changes in the mean flow patterns, the variability in the model is enhanced significantly. On the other hand, however, not all aspects of the circulation have improved with resolution. The Azores Current Frontal Zone with its variability in the eastern basin is still represented very poorly. Particular attention is also directed toward the unrealistic stationary anticyclones north of Cape Hatteras and in the Gulf of Mexico.

Laminar boundary layer separation: Instability and associated phenomena

A feature of laminar separated flows which occurs naturally, even at rather small Reynolds numbers, is their instability. Thus, a mean flow pattern in a separated flow as well as its unsteady properties depend primarily on the instability and other ‘transitional’ phenomena associated with a laminar separation. This review presents the results of studies of this feature. Several transitional problems like those found in transition researches for other boundary-layer flows are also considered. These are: the receptivity of separated flows to external excitation, linear instability and non-linear wave interactions in a separation region. Feedback effects which are important for such flows are also discussed. Finally, the control of laminar separated flows based on the effect of external excitation on the transition is touched upon.

The Structure and Nonlinear Evolution of Synoptic-Scale Cyclones. Part II: Wave–Mean Flow Interaction and Asymptotic Equilibration

Abstract The lift cycle of a baroclinic wave in nearly inviscid, adiabatic flow and its interaction with the zonal mean circulation is here described with primary emphasis focused upon the later stages of evolution. A comparison of simulations performed on the f plane with similarly initialized flows on the β plane reveals differences related to the enhanced northward displacement of the cyclone on the β plane. The zonal mean flow pattern is displaced northward for simulations on the β plane and southern jets are comparatively larger in magnitude than their counterparts in simulations on the f plane. On the β plane, wave energy is absorbed at latitudes equatorward of linear disturbance, whereas on the f plane, wave energy is absorbed at this same latitude. Associated with the northward propagation of the cyclone on the β plane is the splitting of zones of Eliassen-Palm flux divergence near the surface and the appearance of southward heat fluxes after cyclone cutoff. Diagnostic analyses of these flows, ver...

Modelling the movement of pollutants in the UK shelf seas

The EUROSPILL oil and chemical spill model has been extended to include the transport due to residual currents in the UK shelf seas. Wind-driven circulation patterns for the winter and summer have been derived, and the annual mean (long-term) flow pattern estimated. Current speeds during the winter are typically of the order of 0.05–0.10 m s −1, the summer values show a similar spatial pattern but are considerably weaker. Trajectories over a period of 100 days are presented for a selection of release points arranged on a regular latitude/longitude grid. Estimates of patch size after 300 days are given for releases from sites of heavy shipping and offshore exploration. Use of the method for accident simulation is demonstrated by considering the likely spread of Lindane spilled in the English Channel during March 1989.