Jet Streak Research Articles

Tropical-Midlatitude Interactions over Asia and the Western Pacific Ocean during the 1983/84 Northern Winter

Abstract Previous studies of cold surges during the northern winter monsoon suggested a short-term midlatitude-tropical interaction such that the variations of the midlatitude jet over East Asia correspond to variations in tropical convection. However, because cold surges occur during periods of intensified baroclinicity, it is possible that the strengthening of the jet may be due entirely to midlatitude baroclinic development rather than the enhanced local Hadley circulation forced by the increase in tropical convection. In this study objectively analyzed 200 mb wind data for the 1983/84 winter are examined to address the problem of determining cause and effect in short-term midlatitude-tropical interactions. Significant positive correlation between the midlatitude jet acceleration and tropical divergence was found in several regions in the Asia-Pacific-Indian Ocean region. Among six major intensifications of the East Asian jet streak maximum during December 1983, three were related to tropical cyclone a...

Numerical Simulations of a Transverse Indirect Circulation and Low-Level Jet in the Exit Region of an Upper-Level Jet

Uccellini and Johnson present a case study of a severe weather event in Ohio on 10–11 May 1973 to show evidence for coupling between an upper-tropospheric jet streak and a low-level jet within an indirect transverse circulation found in the exit region of the upper-level jet. The differential advection of moisture and temperature created by the shear between the upper- and low-level jets reduced convective stability, thereby enhancing the potential for severe convection. Two 12 h numerical simulations of the 10–11 May 1973 case are studied to determine 1) if a transverse indirect circulation with a low-level jet imbedded in its lower branch can be diagnosed in the exit region of the upper-level jet and studied using the model output at 3 h intervals and 2) if the initial magnitude and structure of the upper-level jet have a significant effect on the subsequent development of the low-level jet and the decrease in convectivc stability due to differential advection. In an adiabatic model simulation, an indirect transverse circulation having a low-level jet within its lower branch occurs in the exit region of the upper-level jet. The simulated vertical distribution of mass divergence and ageostrophic flow in the exit region agree with the diagnoses of Uccellini and Johnson. At upper levels, mass divergence (convergence) occurs on the cyclonic (anticyclonic) side of the exit region, while the opposite occurs at low levels. The, upper branch of the indirect circulation is dominated by the inertial–advective contribution to the ageostrophic wind which is related to the alongstream isotach gradient in the exit region. The lower branch is dominated by the wind tendency contribution to the ageostrophic wind. Ageostrophic shear associated with this circulation contributes to the development of differential moisture and temperature advection, which act to destabilize the preconvective environment. A second simulation using a smoothed, nondivergent initialization with a weaker upper-level jet streak and weaker alongside isotach gradient in the exit region of the upper-level jet produces a weaker indirect transverse circulation even though diabatic heating effects are present. The indirect circulation for this simulation is marked by smaller vertical motions, a weaker low-level return branch, and weaker low-level thermal and moisture advection associated with the low-level flow. Comparison of the two simulations suggests that the indirect circulation in the exit region of the upper-level jet is strongly responsive to dynamical processes associated with the initial structure of the jet streak.

The Presidents' Day Cyclone of 18–19 February 1979: Influence of Upstream Trough Amplification and Associated Tropopause Folding on Rapid Cyclogenesis

The Presidents' Day cyclone of 18-19 February, 1979 is analyzed based on conventional radiosonde data, infrared and visible satellite imagery, water vapor images and ozone measurement. The well-known synoptic-scale characteristics of the PJ-trough system are described, and evidence for the development of the tropopause fold are presented. Processes contributing to the formation of the fold are discussed in terms of an evaluation of the Sawyer-Eliassen circulation equation and a diagnostic evaluation of the ageostrophic winds and vertical motion near the polar jet streak. The documentation of stratospheric extrusions and their possible role in cyclogenesis is reviewed, and Eulerian and Lagrangian diagnostics are used to establish a connection between the tropopause fold associated with the polar jet streak and rapid cyclogenesis along the East Coast. Conservation of potential vorticity expressed in isentropic coordinates is used as a constraint from which to view cyclogenesis.

A Two‐Week Evaluation of Available Potential Energy and Kinetic Energy from NMC and ECMWF Analyses

Abstract Available potential energy and kinetic energy distributions for the Northern Hemisphere computed from the FGGE Level IIIa analyses produced at the National Meteorological Center (NMC) and the Level IIIb analyses prepared at the European Centre for Medium Range Weather Forecasting (ECMWF) are presented for a two‐week period 14–27 February 1979. The most significant difference between the two datasets appear in the eddy components, where the hemispheric ECMWF values are smaller than the NMC values. This difference is found at almost every latitude in the APE distributions and in tropical and polar latitudes in the KE distributions. Vertical cross sections and 250 mb height patterns for three individual days show smoother and weaker short wave features over the oceans in the ECMWF analyses. Jet streak maxima at 250 mb tended to be stronger in the NMC analyses. These results are consistent with those from previous studies which noted smaller energy values in the ECMWF analyses compared to NMC and GFD...

Mesoscale analysis of a polar trough interacting with a polar front

AbstractThe interaction of a polar trough with a polar front can lead to a characteristic satellite cloud pattern commonly referred to as an instant occlusion. the detailed structure and development of an event rather resembling an instant occlusion is analysed in this paper using routine data supplemented by additional radiosondes, soundings and cloud imagery from NOAA‐7 and Meteosat, and radar network pictures. the event presented differs from an instant occlusion as described by Anderson and others in that a band of polar front cloud developed simultaneously with the band of cloud associated with the polar trough rather than being a pre‐existing feature. Thus we refer to it under the different name of pseudo‐occlusion in order to keep this distinction in mind. A conceptual model of the pseudo‐occlusion is developed in which the key elements are (i) a moist, relatively warm, low‐level jet associated with the polar trough, referred to as the ‘polar trough conveyor belt’, and (ii) a major upper‐level jet streak associated with the polar front, referred to as the ‘polar front conveyor belt’. the two conveyor belts intersect almost at right angles. Precipitation associated with the polar front is predominantly from stratiform cloud, whereas that associated with the polar trough is a mixture of stratiform and convective. Just on the poleward side of the upper jet the cloud top drops abruptly. Here, dry air having recently descended beneath the tropopause fold, suppresses the depth of the moist air associated with the polar trough conveyor belt. This leads to a minimum in the intensity of precipitation between the rain areas associated with the trough and the front. Although the air overrunning the trough just on the poleward side of the polar front is dry as a result of earlier descent, the air in this location is likely to have begun ascending as part of the indirect circulation in the exit region of the upper level jet streak. the polar trough low‐level jet probably forms part of this circulation.

Diagnosis of a Jet Streak in the Vicinity of a Severe Weather Outbreak in the Texas Panhandle

This is a case study of the synoptic and mesoscale aspects of a severe-weather outbreak in the Texas Panhandle. We offer circumstantial evidence that the rising branch of a thermally indirect circulation in the exit region of an unusually intense upper-level jet streak played a role in storm formation and sustenance. The jet streak's vertical circulation could not be accounted for by straight dynamics alone; curvature was important, especially along the right side of the exit region. The geostrophic momentum approximation leads to a reasonable qualitative explanation of the ageostrophic circulation, while quasi-geostrophic theory does not.

The Interactive Role of Subsynoptic Scale Jet Streak and Planetary Boundary Layer Processes in Organizing an Isolated Convective Complex

Surface analyses and numerical simulation sensitivity studies are compared in order to determine the role played by deep, well-mixed, and well-heated boundary layers in perturbing a weak jet streak in proximity to the development of an isolated but intense convective complex associated with the Grand Island, Nebraska tornado outbreak of June 3-4, 1980. A brief description of the case is first presented, emphasizing three-hourly surface analyses, radar, and satellite data. The results of numerical experiments comparing differences in the runs with and without diurnal surface sensible heating are discussed and related to observations. The dynamical processes responsible for these simulation differences are discussed, and the significance of these differences are considered in terms of their effect on the preconvective environment.

Effects of Diabatic Heating on the Ageostrophic Circulation of an Upper Tropospheric Jet Streak

The interaction between the mass circulations within a mesoscale convective complex (MCC) and the entrance region of an upper tropospheric polar jet streak is examined to investigate mechanisms responsible for linking these two scales of motion. During NASA's fourth Atmospheric Variability Experiment (AVE IV), maximum wind speeds within a jet streak increased nearly 15 m s−1 over three to six hours as the jet streak propagated eastward over the Great Lakes region. Severe convection located on the rear anticyclonic flank of the jet streak within the direct circulation of the entrance region also intensified and increased in areal extent. The results analyzed within isentropic coordinates establish that latent heating in the MCC modified the direct mass circulation in the jet streak entrance region through the forcing of diabatic components of ageostrophic motion. The net isallobaric ageostrophic component in the entrance region, determined through the gradients of differential heating and mass flux, exceeded 4 m s−1. The mass divergence in the upper troposphere was due to the slight excess of the diabatic isallobaric mode over the opposing adiabatic mode, while mass convergence in the lower troposphere was due to the slight excess of the adiabatic isallobaric mode over the diabatic mode. The intensity of the other diabatically forced ageostrophic component, induced through vertical advection of momentum in a sheared environment, ranged from 5 to 10 m s−1 in the middle and upper troposphere of the jet's entrance region. Over much of the convective region, both the total isallobaric and the inertial diabatic ageostrophic components were directed from the anticyclonic to the cyclonic side of the jet streak at jet streak level in the same sense as pre-existing ageostrophic motion in the upper branch of the jet's direct mass circulation. This diabatically forced ageostrophic motion directed along the pressure gradient of the larger scale resulted in additional generation of kinetic energy which ultimately produced stronger winds in the jet streak downstream. A comparison between actual and geostrophic momentum forms for ageostrophic motion revealed discrepancies of 20 m s−1 that were mainly due to differences in the horizontal fields of inertial advective ageostrophic motion. This expected result points out that the rapid evolution of ageostrophic motion within the shorter time scales of MCCs limits the applicability of geostrophic momentum theory in prescribing the structure of ageostrophic motion.

Mechanics of a Strong Subsynoptic Gravity Wave Deduced from Satellite and Surface Observations

The three-dimensional structure and dynamics of a strong, tropospheric gravity wave event continuously observed from both satellite and surface facilities reveal the subsynoptic character of this 'wave of depression', suggesting that the gravity wave originated in the upper troposphere near a jet streak, and, being quasi-hydrostatic, was relatively nondispersive and long lived. Theory is applied in order both to explain and estimate gravity wave properties throughout the troposphere. Further improvements in the descriptions presented may require the development of solitary wave theory for deep depression waves in shear flow.

The Presidents' Day Cyclone of 18–19 February 1979: Synoptic Overview and Analysis of the Subtropical Jet Streak Influencing the Pre-Cyclogenetic Period

A detailed examination is undertaken of the subtropical jet (STJ) streak revealed by an analysis of the Presidents' Day cyclone of February 18-19, 1979. During its 24 h period, the STJ's flow became increasingly supergeostrophic and apparently unbalanced, while ageostrophic wind speeds increased to more than 30 m/sec in association with a significant cross-contour component, directed toward lower values of the Montgomery streamfunction, as the flow along the STJ became more divergent with time. These phenomena are linked to the increasing confluence in the entrance region of the jet streak and the decreasing wavelength of the trough-ridge system in which the jet streak was embedded. The upper level divergence and upward vertical motion near the axis of the STJ are found to be important factors in the development of the cyclone's first area of heavy snow.

Short-Term Planetary-Scale Interactions over the Tropics and Midlatitudes. Part II: Winter-MONEX Period

The FGGE/winter-Monex wind data from 850 and 200 mb are used to analyze the short-term teleconnections over the tropical and midlatitude Pacific and monsoonal surges in the winter of 1978-79. The data were taken twice a day, and included the total wind field, the computed rotational and divergent winds, the velocity potential, and the streamfunction. Cold surges were identified from shipboard data over the South China Sea and characterized an increase in the surface northerly wind and a surface pressure differential between Hong Kong and 30 deg N, 115 deg E. Midlatitude baroclinic disturbances triggered the cold surges, which displayed intensities dependent on the strength of the midlatitude anticyclogenesis and the intensity of the East Asia local Hadley cell. Fluctuations in the 200 mb jet streak over Japan indicated changes in eddy behavior over the North Pacific and the intensity of the East Asia local Hadley cell circulation and tropical convection. It was concluded that midlatitude synoptic systems could force equatorial convection that produces remote midlatitude responses.

A Cyclone/Anticyclone Couplet over North America: An Example of Anticyclone Evolution

A detailed case study has been made of a cyclone/anticyclone couplet over North America during early winter, 11–18 November 1969. The anticyclone was the dominant member of the couplet in this case. Objective analyses of the wind and mass fields were carried out in both the isobaric and isentropic coordinate systems. Calculated quantities which are discussed include quasi-geostrophic vertical velocity and height tendency, potential vorticity, and quasi-Lagrangian kinetic energy budgets. The movement of the cold, polar anticyclone out of its source region in Alaska southeastward to the Gulf of Mexico is seen to be thermally steered. During this stage the anticyclone moves toward the region of descent forced by the low level cold advection. As the anticyclone recurves and begins to move northeastward along the cut coast of the United States, it becomes a warm, dynamic system. The forcing directing the movement is that of differential vorticity advection. The static stability structure of the anticyclone changes in a manner consistent with this changeover in forcing in that the lower troposphere becomes more stable and the upper troposphere becomes somewhat less stable. Calculations show that the strong cyclogenetic diabatic forcing created by cold air flowing over warm water along the east coast is overwhelmed by the quasi-geostrophically driven height rises. The mechanism by which the cold air dome associated with the anticyclone moves southward intact is explained from two complementary perspectives. One viewpoint is that of asymmetries in the jet stream about the long wave trough with a jet streak to the west of the cold dome. The other is that a maximum in potential vorticity located above the coldest air will prevent the dome from subsiding. The migration of the jet streak to the eastern side of the cold dome and the movement of the potential vorticity maximum from its position over the cold air lead to collapse of the cold air and induce cyclogenesis on the eastern side of the cold air. Furthermore, the calculations show that the potential vorticity is not at all conserved in the region of the deepening cyclone. The kinetic energy budgets indicate that the anticyclone region serves as a source of upper level energy for the downstream cyclone. The collapse of the cold air converts potential to kinetic energy and this energy is fed into the jet streak on the eastern side of the long wave trough.

Numerical Simulation of Nonlinear Jet Streak Adjustment

Abstract The geostrophic adjustment process in a propagating jet maximum is studied through numerical experiments performed with a two-layer, nonlinear primitive equation model. Gravity-inertia modes generated by the jet streak from an initial three-dimensional balanced flow are isolated with the aid of diagnostic quasi-geostrophic and balance models. Them modes appear to represent gravitational subsynoptic “signal” rather than “noise,” and their behavior is examined as a function of initial core strength. Persistent unbalanced patterns relative to the jet core are discovered, whose amplitude increases approximately as the square of the Rossby number. Their behavior is suggestive of forced, rather than free, ageostrophic modes, and their existence implies that the “four-cell” vertical motion pattern characteristic of jet streaks in filtered models may be significantly altered in a real jet streak. The free response to initial data imbalances placed at varying locations in a jet streak environment is explo...

Short-Term Planetary-Scale Interactions over the Tropics and Midlatitudes during Northern Winter. Part I: Contrasts between Active and Inactive Periods

Objectively analyzed 200 mb winds of four winters are used to study the short-term (several days) teleconnections between planetary-scale circulation components over the monsoon region. The composited structures suggest that during very actively northeasterly monsoon (surge) periods, the midlatitude and tropical circulation components vary in a coherent way. The jet streak and local Hadley circulation over East Asia, the divergent flow over the maritime continent, and the equatorial Walker circulations over the Pacific and Indian Oceans all strengthen steadily, while the secondary jet streak over West Asia weakens. During inactive (break) periods the midlatitude circulation components all exhibit reversed changes, while the variations in the tropics are less coherent, although still showing reversed tendencies. The results basically verify the short-term teleconnection model proposed by Chang and Lau (1980), but more complex processes in the acceleration of the midlatitude jet streaks are indicated. In addition, the contrast between the very active and break monsoon periods suggests the relative importance of tropical versus midlatitudinal forcings in different regions of the monsoon circulation.

A Synoptic Analysis of the First AVE-SESAME '79 Period

Fields of divergence, vertical motion, stability, and surface pressure tendency are examined at 3 h intervals for the first regional scale AVE-SESAME '79 (Atmospheric Variability Experiment—Severe Environmental Storms and Mesoscale Experiment) day. Two areas of severe storms formed during the period from 1200 GMT 10 April through 1200 GMT 11 April. The Red River Valley outbreak began during the afternoon of 10 April, while a second area formed in southwestern Texas during the early evening hours. Results show the rapid changes in environmental conditions associated with these two storm areas. The propagation of an upper level jet streak into the region was a major factor in producing the Red River Valley outbreak. This streak was associated with the formation of a strong low-level jet and a small-scale surface pressure perturbation. The sudden development of a strong upper tropospheric wind maximum over Oklahoma and Kansas corresponded with major changes in kinematic parameters at that level. Instability over the Red River Valley was released by strong upward motion producing intense convection. Similar features were responsible for the storms in southwestern Texas. Although this area was quite unstable, forcing mechanisms appear somewhat weaker than in the earlier outbreak.

Research Aircraft Measurements of Jet Stream Geostrophic and Ageostrophic Winds

Abstract Radar altimeter and inertial navigation wind velocity measurements from research aircraft are shown to give the ageostrophic wind velocity distribution within a jet stream system near the tropopause. Results from one case study reveal cross-height-contour ageostrophic winds exceeding 20 m s−1, and centripetal-acceleration-induced ageostrophic winds approaching 100 m s−1 in the exit region of a jet streak which has entered into the trough region of a synoptic wave. Diagnostic calculations are made of the horizontal velocity divergence and the geostrophic and ageostrophic components of the observed wind velocity acceleration. The results suggest caution in the application of near-straight jet-streak dynamics and the geostrophic momentum approximation to jet-streak systems embedded within large-amplitude synoptic wave regimes.

Lagrangian Partial-Inertial Oscillations, and Subtropical andLow-Level Monsoon Jet Streaks

Abstract Jet streaks (local wind maxima) are prominent features of current systems both in the high and lowtroposphere. As particles in the core of the current flow through them, they experience cyclical velocityoscillations whose characteristics (period, dimensions and form of the wave described by a trajectory) arerelated to the way in which the geostrophic wind varies along the current. Although there is considerablevariation, the modal period is nearly semi-inertial (twice the inertial period). Two special cases are examined. First, jet streaks in the mean winter subtropical jet stream (STJ) aredemonstrated to be approximate semi-inertial oscillations and also planetary waves, a coincidence shownto be unique to the subtropical latitude. STJ waves are distinctive from middle-latitude Rossby waves inthe respects that they are dominated by departures from gradient-wind motion and also have dissimilardivergence distributions. Second, the mean July low-level monsoon jet over the Arabian Sea, with anoscilla...

On the Role of Upper Tropospheric Jet Streaks and Leeside Cyclogenesis in the Development of Low-Level Jets in the Great Plains

A review of 15 cases of low level jets (LLJ) which developed in the Great Plains is presented. For 12 out of the 15 cases, a systematic upper level flow pattern was isolated which includes the existence of a trough over the southwest United States and the propagation of upper level jet streaks from the Rocky Mountains toward the Great Plains. This flow pattern is responsible for lee side cyclogenesis or lee side troughing that produces the pressure gradients needed for the development of the LLJ. For the other three cases, a blocking ridge existed over the Great Plains and the upper level flow is relatively weak. It is during these situations that the classic, diurnal oscillating LLJ was observed. A more detailed review of four cases indicates that the subsynoptic scale adjustments associated with the upper level jet streak's forcing of lee side cyclogenesis could be an important factor in the development of LLJ's in the Great Plains.

A diagnostic study of the long‐term balance of kinetic energy of atmospheric large scale motion over the British Isles

AbstractCalculations of the various terms in the kinetic energy budget of the atmosphere were carried out for the area of the British Isles using aerological data for the period 1974–76. The residual term (‘dissipation’) necessary to balance the equation is interpreted as representing the forcing effect of sub‐grid scale processes. The results are compared with earlier calculations for the same area, and with corresponding results for the area of North America.The results show that the annual mean dissipation of kinetic energy in the boundary layer (below 850 mb) is 3.4 Wm−2, which is considerably larger than that found for the area of North America. The corresponding values for winter, spring, summer and autumn are 6.2, 2.3, 2.0 and 3.2 Wm−2, respectively.Results for the British Isles in the free atmosphere are very different from those for North America. Whereas there is, on average, generation of kinetic energy by pressure forces (−V.▽Φ > 0) and net export of kinetic energy over North America, the contrary is true over the British Isles.The mean residual term in the kinetic energy equation over the British Isles is relatively small between about 850 and 400 mb. However, higher up, the term is definitely positive, indicating (if interpreted physically) an energy input from sub‐grid scales to the synoptic scale: a situation opposite to that prevailing, in the annual mean, over North America. It is suggested that this ‘negative viscosity’ phenomenon may in general be typical for diffluent flows (such as are found in the exit areas of jet streams) and could be due to horizontal and vertical convergence of momentum flux associated with jet streaks and other sub‐grid scale phenomena.

Evidence of Jet Streak Vertical Circulations

Abstract A Man Computer Interactive Data Access System (McIDAS) has been used to great advantage in the monitoring and processing of weather data on a real-time basis. In particular, McIDAS facilitated the timely acquisition of data for the study of a Midwest cyclone which was characterized by a well-defined jet streak aloft. Moreover, study of this jet streak revealed patterns of ageostrophic motion and divergence which agree well with the theory of jet streak vertical circulations. Previously, there had been only limited documentation of these patterns using an actual case study. The most striking results of this study were as follows: 1).Machine computation of synoptic-scale ageostrophic motions and divergence patterns can beaccomplished with a high degree of reliability and consistency. 2)The weather over large areas of the United States can be affected by a single, well-definedjet streak. 3)Significant research can now be accomplished on a near real-time basis. Systems such as McIDAS hold the promise...