Jet Stream Flow Research Articles

South Asian Monsoon Variability and Arctic Sea Ice Extent Linkages During the Late Pliocene

AbstractThe influence of a warming‐induced reduction in Arctic sea ice extent (SIE) on the South Asian monsoon (SoAM) is not well understood. The mid‐Piacenzian Warm Period (MPWP, 3.264–3.025 million years ago, Ma), the most recent warm climate event when the Earth's CO2 levels were similar to the present, is considered an analog for future global warming. We present a multi‐proxy high‐resolution record of SoAM variability during the late Pliocene (∼3.4–2.7 Ma) using denitrification, productivity, weathering, and terrestrial influx proxies from the eastern Arabian Sea (International Ocean Discovery Program Expedition 355). The proxy‐based SoAM reconstruction is independently verified using a hybrid model‐proxy data set (Oscillayers). We identify two distinct intervals of monsoon intensification—during the MPWP and at 2.9 Ma. The SoAM variability appears to result from an interplay between dynamic (Insolation, Arctic SIE variability, and southern tropical Indian Ocean surface temperature variability due to Indonesian Throughflow changes) and thermodynamic effects (global temperature change due to pCO2 variability). Using a late Pliocene Arctic spring sea ice concentration reconstruction, we find that lower (higher) Arctic SIE leads to stronger (weaker) late Pliocene SoAM via asymmetric interhemispheric energy export and through modulating jet stream flow and meridional circulation. Our finding of strengthened SoAM during the MPWP corroborates model results and implies that more intense precipitation is expected over South Asia with continued climate warming.

COMPUTATIONAL STUDIES OF TURBULENT CROSS-FLOW JET USING LES

The interaction between a turbulent jet and free stream flow is computationally studied using the large-eddy simulation (LES) approach. The effect of the blowing-ratio on the turbulent mixing is also subject of investigation in this research. The study revealed that the LES approach is suitable for the computation of highly-turbulent flows and able to capture all the flow structures of the jet in cross-flow. The analysis showed that the blowing-ratio is one of the parameters that control the turbulent mixing. Thus, the increase of the blowing-ratio causes a deeper penetration of the jet into the free stream flow and hence, an increased turbulent mixing. The interaction between the jet and free stream flow causes a sequence of vortex rings of different length-scales. Thus, larger vortex rings were observed for higher blowing –ratios. A concluding remarks of the present research is that high-turbulent mixing requires higher blowing-ratios.

Early 20<sup>th</sup> Century Climate-Driven Shift in the Dynamics of Forest Tent Caterpillar Outbreaks

Using tree-ring analysis, we show that the dynamics of forest tent caterpillar [Malacosoma disstria (Hbn.)] outbreaks in Alberta, Canada shifted at the turn of the 20th century from cyclic, synchronous behaviour 1850-1910 to complex, asynchronous behavior 1910-1993. This shift in dynamics coincided with the emergence in 1910 of a latitudinal gradient in outbreak stability and periodicity reflecting a similar gradient in the periodicity of winter temperatures. We postulate that the synchronizing strength of winter temperatures has diminished as a result of climate warming, and that any synchronizing strength due to inter-population migration has been superseded by regionalized patterns of periodic forcing caused by weak low-frequency variability in winter temperatures. We speculate that a decrease in polar vorticity at the start of the 20th century led to increased meridional jet stream flow and more frequent arctic weather anomalies, resulting in a loss of synchronous decadal periodicity in outbreak occurrence. These changes in insect disturbance probabilities, including rising uncertainty, have profound consequences for forest disturbance risk management.

Interaction of an Upper-Tropospheric Jet with a Squall Line Originating along a Cold Frontal Boundary

AbstractOn 8 June 2003, an expansive squall line along a surface cold frontal boundary was sampled during the Bow Echo and Mesoscale Convective Vortex Experiment. The Naval Research Laboratory P-3 aircraft and the National Oceanic and Atmospheric Administration P-3 aircraft simultaneously sampled the leading and trailing edge of this squall line, respectively, with X-band Doppler radars. Data from these two airborne radar systems have been synthesized to produce a pseudo-quad-Doppler analysis of the squall line, yielding a detailed three-dimensional kinematic analysis of its structure. A simulation of the squall line was carried out using the Weather Research and Forecasting Model to complement the pseudo-quad-Doppler analysis. The simulation employed a 3-km, convection-allowing, nested domain centered over the pseudo-quad-Doppler domain, along with a 9-km parent domain to capture the larger synoptic-scale cyclone.The pseudo-quad-Doppler analysis reveals that the convective line was embedded within the upper-tropospheric jet stream, causing local decelerations and deviations in the jet-level flow. The vertical transport of low momentum air from the boundary layer via convective updrafts is shown to significantly decelerate jet-level flow. Pressure perturbations associated with the intrusion of low momentum air into the jet stream–level flow led to deviation of the jet stream flow around the squall line that resulted in counter-rotating ribbons of vertical vorticity parallel to the squall line. Model results indicate that disturbances in the jet stream structure persisted downwind of the squall line for several hours.

An Assessment of the South Asian Summer Monsoon Variability for Present and Future Climatologies Using a High Resolution Regional Climate Model (RegCM4.3) under the AR5 Scenarios

We assessed the present and future climatologies of mean summer monsoon over South Asia using a high resolution regional climate model (RegCM4) with a 25 km horizontal resolution. In order to evaluate the performance of the RegCM4 for the reference period (1976–2005) and for the far future (2070–2099), climate change projections under two greenhouse gas representative concentration pathways (RCP4.5 and RCP8.5) were made, the lateral boundary conditions being provided by the geophysical fluid dynamic laboratory global model (GFDL-ESM2M). The regional climate model (RCM) improves the simulation of seasonal mean temperature and precipitation patterns compared to driving global climate model (GCM) during present-day climate conditions. The regional characteristic features of South Asian summer monsoon (SASM), like the low level jet stream and westerly flow over the northern the Arabian Sea, are well captured by the RegCM4. In spite of some discrepancies, the RegCM4 could simulate the Tibetan anticyclone and the direction of the tropical easterly jet reasonably well at 200 hPa. The projected temperature changes in 2070–2099 relative to 1976–2005 for GFDL-ESM2M show increased warming compared to RegCM4. The projected patterns at the end of 21st century shows an increase in precipitation over the Indian Peninsula and the Western Ghats. The possibilities of excessive precipitation include increased southwesterly flow in the wet period and the effect of model bias on climate change. However, the spatial patterns of precipitation are decreased in intensity and magnitude as the monsoon approaches the foothills of the Himalayas. The RegCM4-projected dry conditions over northeastern India are possibly related to the anomalous anticyclonic circulations in both scenarios.

Climatic background of cold and wet winter in southern China: part I observational analysis

This study explores the climate background of anomalous wet and cold winter in southern China, focusing on results in January when most of its disastrous snowstorms and freezing rainfall events were observed. Based on the ERA-40 reanalysis and Climate Research Unit (CRU) observed precipitation and surface temperature monthly data for the period of 1959–2001, the difference between normalised monthly precipitation and temperature is used to define a simple index which reflects the intensity of the wet and cold condition in the region. It offers a good agreement with an index defined by daily weather station data observed in the region. Then, through simple correlation analyses we focus on exploring the dominant physical and dynamical processes leading to such climatic anomalies. While we acknowledge the contribution of the cold/dry air penetrated from the north, the importance of maintaining a warm and moist airflow from the south is highlighted, including an enhanced Middle East Jet Stream (MEJS) and southwesterly flow over Indochina Peninsula and South China Sea region. Strong vertical share of meridional wind, with enhanced northerly flow near the surface and southerly flow in the low to middle troposphere, leads to significant temperature and moisture inversions. These are consistent with results from synoptic analyses of the severe January 2008 event which was not included in the correlation calculations and thus suggest the 2008 event was not an unusual event although it was very intense. In the third part, we use a partial least-square statistical method to uncover dominant SST patterns corresponding to such climatic conditions. By comparing results for the periods of 1949–1978 and 1978–2007, we demonstrate the shift of dominant SST patterns responsible for the wet and cold anomalies. Shifting from “conventional” ENSO SST patterns to ENSO Modoki-like conditions in recent decades partially explains the unstable relationship between ENSO and Asian winter monsoon. Meanwhile, the importance of SST conditions in extra-tropic Pacific and Indian oceans is acknowledged. Finally, we developed a forecasting model which uses SST condition in October to predict the occurrence of the anomalous wet and cold January in the region and reasonable forecasting skill is obtained.

菱形角柱群管路内でのポケット部の有無による流れの変化(流体工学,流体機械)

The promotion of jet-stream diffusion may enhance the efficiency of mechanical performance in various industrial devices such as fluid machinery and combustion equipments. Both active and passive means were applied for the control of jet streams. Our previous studies disclosed that diamond-shaped cylinder bundles produce a self-excited oscillating jet-stream flow field having multiple uniform flow-rate groups. The present visualization work deals with flip-flop flow oscillation from a diamond-shaped cylinder bundle with wall concavity. It is disclosed that the wall concavity has induced a substantial change in the flow patterns in the exit jet-stream field with strong turbulence and jet-stream dispersion.

Numerical flow visualization of a Single Expansion Ramp Nozzle with hypersonic external flow

Numerical simulation of scramjet asymmetric nozzle flow is carried out to visualize and investigate the effects of interaction between engine exhaust and hypersonic external flow. The Single Expansion Ramp Nozzle (SERN) configuration studied here consists of flat ramp and a cowl with different combinations of ramp angle and cowl geometry. Using PARAS 3D, simulations are performed for a free stream Mach number of 6.5 that constitutes the external flow around the vehicle. Appropriate specific heats ratio has been simulated for the jet and free stream flow. External shock wave due to jet plume interaction with free stream flow, the internal barrel shock wave and the shear layer emanating from the cowl trailing edge and sidewalls are well captured. Wall static pressure distribution on the nozzle ramp for different nozzle expansion angles has been computed for both with and without side fence. Axial thrust and normal force have been evaluated by integrating the wall static pressure. Effect of cowl length variation and side fence on the SERN performance has also been studied and found to be quite significant. Based on this study, an optimum ramp angle at which the SERN generates maximum axial thrust is obtained. SERN angle of 20o was found to be optimum when the flight axis coincides with nozzle axis.

The Gulf of Mexico mid-tropospheric response to El Niño and La Niña forcing

This analysis further refines Ropelewski & Halpert's (1987; Mon Wea Rev 115:1606-1626) analysis which investigates the relationship between El Nino-La Nina/Southern Oscillation events and southern United States precipitation. Comparisons are made between eigenvector-derived mid-tropos- pheric (500 mb) flow patterns over North America during extreme El Nino and La Nina months and a base climatology. In addition, the patterns are correlated to regional precipitation anomalies for the southern United States to determine mean surface responses. Cool season (November to March) months are divided into all winter months (AWM), positive anomaly months (PAM), and negative anomaly months (NAM). The extreme anomaly months were determined as any month with a South- ern Oscillation Index (SOI) ± 1 standard deviation from the standardized mean. Therefore, the PAM and NAM anomalies represent the La Nina and El Nino extreme phases of the SOI, respectively. Results suggest that the positive (La Nina) SOI phase elicits a greater surface precipitation response than the El Nino phase. This is caused by substantial changes in the primary longwave flow during opposite SOI phases. During AWMs and NAMs, similar flow patterns, dominated by the Pacific/North American (PNA) teleconnection, prevail which induce similar regional precipitation responses. During PAMs, the mid-tropospheric flow shifts to a hybrid flow pattern which is between the PNA and the Tropical Northern Hemisphere teleconnections. Such displacement in the longwave flow variation centers ultimately affects jet stream flow and precipitation forcing, resulting in negative precipitation anomalies across the southern United States.

Paleoceanographic reconstructions of surface ocean conditions in the Greenland, Iceland and Norwegian seas through the last 14 ka based on diatoms

Prior reconstructions of the last glacial maximum and the Holocene did not include detailed paleoclimate data from the Greenland, Iceland and Norwegian (GIN) seas, which cover sensitive areas for oceanographic boundaries, sea ice distribution and deep water formation. In order to fill this gap, we studied nine well dated sediment cores from this area and reconstructed the paleoceanographic conditions of the surface GIN seas at 2000-year intervals since 13,400 BP based on the diatom record. Results show that a N.-S. extending sea-ice-free corridor had opened along Norway already at 13,400 BP, indicating a northward flow of a branch of the North Atlantic Drift, possibly caused by a change in the jet stream flow due to the decreased height of the North American ice sheet. A major change of climatic conditions occurred over the GIN seas as the insolation anomaly reached its maximum around 9000 BP, when the sea ice cover and the oceanic fronts retreated to a northwesterly position along Greenland, and the ocean temperatures rose. The first half of the Holocene is recorded as the warmest period during the last 13,400 BP in the GIN seas. The duration of this climatic optimum decreased both toward the north and toward the margins of the area. The second half of the Holocene is characterized by a cooling trend in step with the decreasing insolation. These results support the proposal of Imbrie et al. (1992) that the GIN seas region acts as an initial responder to insolation changes.

Sources of Mesoscale Variability of Gravity Waves. Part II: Frontal, Convective, and Jet Stream Excitation

Abstract We present studies of four cases of mesoscale variance enhancements of horizontal velocity and temperature due to frontal activity, nonfrontal convection, and wind shear. These data were obtained aboard commercial aircraft during the Global Atmospheric Sampling Program (GASP) in 1978 and 1979 and from the corresponding meteorological analyses and satellite imagery. Additional GASP data were used to permit a statistical assessment of the importance of various sources of enhanced variances. Our results, and those in a companion paper addressing the variance enhancements associated with topography, represent refinements of previous source analyses using the GASP dataset. Significant findings include mean variance enhancements of velocity and temperature due to convection and jet-stream flow ranging from ∼2 to 8 for 64-km and 256-km data segments, and enhancements for individual segments as high as ∼20 to 100. The mean 64-km variance enhancement for all variables and source types, relative to a quies...

The NASA spring 1984 Stratosphere‐Troposphere Exchange experiment: Science objectives and operations

We present an overview of the first experiment of the Stratosphere‐Troposphere Exchange Project (STEP), conducted in spring 1984 over the southwestern United States. The objective was to identify modes of transport during large‐scale cyclogenesis, both (1) within the stratosphere, from the anticyclonic to the cyclonic side of the jet, and (2) from the troposphere to the stratosphere. The primary platform was the NASA U‐2 high‐altitude research aircraft, supported by the regular twice‐daily radiosonde network. Measurements included temperature, pressure, horizontal wind, ozone, cosmogenic radionuclides, water vapor, and condensation nuclei. U‐2 flights were made on 4 days. On 3 days the flight path was in a vertical plane orthogonal to the jet stream flow, extending from the anticyclonic side of the jet, across it, and into the cyclonic vortex. The other flight was directed at processes associated with the upper level cirrus outflow from a tropical vortex. Aircraft direction was performed using analysis of NMC prognostications, commercial diagnostics and predictions, radiosonde data, and satellite infrared imagery. The cross‐jet flight most intensively analyzed (April 20–21, 1984) was coordinated with simultaneous flights in the troposphere and lower stratosphere made by the NASA CV‐990 and Electra as a part of the Global Tropospheric Experiment. This paper briefly reviews the U‐2 instrument payload, shows the flight paths of April 20–21 and May 6 in relation to the respective meteorological situations, and reviews the procedures used to predict large‐scale cyclogenesis and associated tropopause folding and to direct the aircraft. The following papers report the measurement data and interpret the results in terms of reversible and irreversible cross‐jet transport mechanisms.

Large-scale transport of a CO-enhanced air mass from Europe to the Middle East

On 14 November 1981, the shuttle-borne Measurement of Air Pollution from Satellites (MAPS) experiment observed a carbon monoxide (CO) enhanced air mass in the middle troposphere over the Middle East. The primary source of this polluted air was estimated by constructing adiabatic isentropic trajectories backwards from the MAPS measurement location over a 36 h period. The isentropic diagnostics indicate that CO-enhanced air was transported southeastward over the Mediterranean from an organized synoptic-scale weather regime, albeit of moderate intensity, influencing central Europe on 12 November. Examination of the evolving synoptic scale vertical velocity and precipitation patterns during this period, in conjunction with METEOSAT visible, infrared, and water vapor imagery, suggests that the presence of this disturbed weather system over Europe may have created upward transport of CO-enhanced air between the boundary-layer and midtropospheric levels, and subsequent entrainment in the large-scale northwesterly jet stream flow over Europe and the Mediterranean.

Modelling of the local distributions of velocity components and turbulence parameters in agitated vessels—method and results

Three correlations for the description of the velocity and turbulence parameter distributions are defined: (i) rotational, (ii) circulating, and (iii) jet stream flow. The two latter are based upon the classification of the stirred tank in regions of the cellular vortex and the jet stream from the impeller. These models are taken from equations of well-known flow situations (wall jet, rotating disc, free jet, etc.) and from generalizations obtained from many measurements. The resulting equations are simple in form. The model parameters can be calculated from geometrical data and power number. The comparison of the model with measured values shows good agreement for tanks with and without baffles, impellers with radial and axial flow, and for a wide range of Reynolds numbers. The accuracy obtained is comparable but less work is involved compared with the results obtained through the numerical solution of the Navier-Stokes equations. Thus the engineer has a simple method for calculation of the local distribution of velocity and turbulence parameters in stirred tanks. A methodical way for obtaining this model is demonstrated. With the models presented here it is possible, in a simple way, to estimate local distributions of velocity components and technically important parameters. Only geometrical data and power number are necessary. Power number is usually easily obtainable for many impeller types [2-4, 71]. Table 4 shows systematically the steps for application of the models presented. By comparison with experimental data the accuracy of these models was demonstrated. The model is applicable for radial and axial flow impellers, tanks with and without baffles and for a wide range of Reynolds numbers. Reasonable values are also obtained for rotating cylinders [24]. Compared with the numerical solution of the Navier-Stokes equation the model gives results of the same accuracy at a much smaller expense. Through the use of the cellular structure a simple scale-up on other systems is possible, for example, on multistage impellers. The model described has a greater range of application and fewer parameters than other models. This results from the model strategy which is distinguished by: - extensive analysis of the data; - application of well-known flow situations (e.g. jet models) and its transfer to the conditions in the agitated vessels; - application of simple balance equations; - consideration of physically useful conditions to estimate the model parameters. With this strategy it was possible to generalize existing data and to connect the velocity profiles with global parameters of flow. Further, the presented model can be the basis for the formulation of boundary and initial conditions for more detailed calculations based on the numerical solution of the Navier-Stokes equation. The method is also useful for the generation of flow models of other mixing devices and apparatus.

The numerical study of combined jet stream and external flow allowing for the effect of boundary layers and the turbulent mixing zone

The numerical study of combined jet stream and external flow allowing for the effect of boundary layers and the turbulent mixing zone

A case study of wave motion above a jet stream flow system observed by an instrumented balloon

A case study of wave motion above a jet stream flow system observed by an instrumented balloon

A case study of wave motion above a jet stream flow system observed by an instrumented balloon

An analysis is presented of the data obtained from an instrumented balloon rising through a jet stream. Emphasis is placed on the determination of the vertical atmospheric velocity. The analysis of the temperature and velocity perturbations shows evidence of a wave structure propagating upward. An estimation of the vertical flux of horizontal momentum is given. These preliminary results show clearly the ability of our helisonde to measure wave motions with a vertical velocity as low as 0.1 m s -1 . The method allows a direct measurement of wave propagation in the stratosphere. DOI: 10.1111/j.1600-0870.1984.tb00263.x

Impact tube measurement of flow patterns, velocity profiles and pumping capacities in mixing vessels

AbstractImpact‐static‐shielded tubes were found to be suitable for measuring time‐average velocities in the jet stream flow generated by the marine propeller. Pumping capacities and entrainment flow were then calculated from these velocity profiles. Pumping capacities were correlated by: Q = KpND3.When the tank contained water: Kp = 0.54. The same probe was found to be useful in evaluating qualitatively the flow patterns from turbine and paddle type impellers. The effect of several parameters on the flow patterns are reported.

アーク溶接現象に関する界面張力理論(第9章)

Authors investigated the influence of the various welding condition on the shape of penetration surface caused by TIG arc torch fixed perpendiculer to the mild steel plate. After discussing the results obtained we have come to the following conclusions.(1) The surface of penetration obtained was roughly one of revolution and its sectional profile was clearly divided into three groups, that is, peripheral, central and simple type. Penetration in p type consists of deep peripheral groove and shallow and flat bottom at the center part. Much deeper center hollow is added to the former in case of c type. Sectional profile curve in s type is of simply concaved arc.(2) Type of penetration changes abruptly from p to c as welding current is increased or arc length is decreased. Stype seems to be formed when the arc and mother drop is unstable.(3) Authors proposed as a probable hypothesis that the deep peripheral groove in p type is originated from the strong outerward surface flow of over-heated molten metal caused by the difference of surface tension under the arc.(4) If the contral depression of molten metal surface, caused by plasma jet stream or abovementioned surface flow, become deep enough to reach the bottom of penetration, then the direct action of the arc begins to act on the bare surface of base plate forming the central deep penetration(5) If the contact angle of molten metal on the solid metal has a definit value being not zero, occurence of the bare surface is not continuous to the welding condition, jumping up to a certain wide range from zero, which may satisfactorily explain the discontinuity of the change of penetration type.

THE STRUCTURE OF THE JET STREAM CORE

Abstract Typical jet stream characteristics are illustrated by discussions of an average, a strong and a weak jet stream. The basic data were obtained by aircraft of Project Jet Stream in flights carried out mainly over the southeastern United States. An empirical model, based on observations obtained during some fifty flights, illustrates the average wind and temperature fields in the vicinity of the jet stream core and the structure of the “jet stream front.” It is suggested that this model may be used to define jet stream flow in mid-latitudes. Average distributions of turbulence, clouds and contrails around the jet stream are presented.