Jet Streak Research Articles

The Numerical Simulation of an Unbalanced Jetlet and Its Role in the Palm Sunday 1994 Tornado Outbreak in Alabama and Georgia

Abstract Meso-beta-scale numerical model simulations and observational data are synthesized in an effort to develop a multistage paradigm for use in forecasting tornadic convection in the southeastern United States. The case study to be utilized as an example of the multistage sequence of events is the Palm Sunday 1994 outbreak, which culminates with the development of an unbalanced mesoscale jet streak or jetlet that focuses a given region for significant values of low-level vertical wind shear, low-level confluence and vertical vorticity, midtropospheric cooling, and storm-relative helicity. The five-stage paradigm includes 1) the existence of a jet exit region accompanying a deep balanced thermally indirect circulation south of the outbreak and a return branch ageostrophic low-level southerly jet, both typically accompanying the subtropical jet stream and the leading edge of hot continental air; 2) the existence of a jet entrance region accompanying a deep balanced thermally direct circulation north of...

The Intensification of the Low-Level Jet during the Development of Mesoscale Convective Systems on a Mei-Yu Front

Abstract During the period of 21–25 June 1991, a mei-yu front, observed by the post–Taiwan Area Mesoscale Experiment, produced heavy precipitation along the western side of the Central Mountain Range of Taiwan. Several oceanic mesoscale convective systems were also generated in an area extending from Taiwan to Hong Kong. Numerical experiments using the Penn State–NCAR MM5 mesoscale model were used to understand the intensification of the low-level jet (LLJ). These processes include thermal wind adjustment and convective, inertial, and conditional symmetric instabilities. Three particular circulations are important in the development of the mei-yu front. First, there is a northward branch of the circulation that develops across the upper-level jet and is mainly caused by the thermal wind adjustment as air parcels enter an upper-level jet streak. The upper-level divergence associated with this branch of the circulation triggers convection. Second, the southward branch of the circulation, with its rising mot...

Tropical cyclones, 6?25 day oscillations, and tropical-extratropical interaction over the northwestern Pacific

This paper investigates tropical-extratropical interactions over the northwestern Pacific Ocean that involve tropical cyclones and subtropical jet streaks. Another aspect of this study is to examine the relation between 6–25 day convective variability and tropical cyclones. This investigation is conducted for the fall and early winter season, with a focus on the months, October through December (OND). In addition to outgoing longwave radiation (OLR) data, we use 10 years (1985–1994) of WCRP/TOGA archive II analyses produced by ECMWF to compute equivalent temperature,θe, precipitable water, W, and kinematic and kinetic energy transfer variables. These variables are composited for two classes of tropical cyclones, recurving cyclones (RCs) and non-recurving cyclones (NCRs), in order to examine the influence of tropical cyclones and baroclinic processes on changes in the jet streak intensity.

Subtropical tropopause break as a possible stratospheric source of ozone in the tropical troposphere

Abstract The understanding of the transport of trace chemical species between the stratosphere and the troposphere is necessary for global change prediction. Until recently it was believed that stratospheric inputs, through jet streaks and tropopause folding, should occur only at extratropical latitudes. A case study of a tropopause fold was reported at Pointe-a-Pitre by Gouget et al . (1996). We presently corroborate this first case study by new observations in the Indian Ocean suggesting that stratosphere-troposphere exchanges, induced by the subtropical jet, are actually occurring near the edge of the tropics. Key to these exchanges is the crucial region of the junction between the lowermost stratosphere and the tropical and extratropical tropospheres, defined by Holton (1996) as the intersection zone of the 2 PVU potential vorticity surface with the 380 K potential temperature level. In this paper, the wind and ozone climatological context is given using vertical radiosounding data from Reunion Island (France, 21 °S, 55 °E) and Irene (South Africa, 25 °S, 28 °E). A case of subtropical tropopause fold occurring between Madagascar and Reunion Islands is presented using ECMWF and TOMS data. Observations are found to be fairly accordant with a dynamical jet analysis and suggest that stratospheric air intrusions are possible during winter in the sub-tropics edges.

Study on Growth Mechanism of Self-Induced Sloshing, Using Numerical Simulation. 1st Report. Numerical Simulation of Self-Induced Sloshing Caused by Horizontal Plane Jet.

A self-induced sloshing is excited by the flow without any other external force. In a rectangular tank having a horizontal plane jet, the first-mode sloshing grew in a certain condition of the inlet jet velocity and the water level. In this study, the self-induced sloshing was simulated, using a two dimensional numerical simulation code. The code was based on the Boundary Fitted Coordinate (BFC) method with the height function. The results of the simulation agreed qualitatively with the experimental results, e. g. the sloshing occurrence condition. In order to investigate the growth mechanism of the self-induced sloshing, the fluctuation of the jet streak line was calculated using the simulated results. The dependency of the jet fluctuation on the self-induced sloshing was clarified qualitatively using the numerical simulation.

Numerical Simulations of a Gravity Wave Event over CCOPE. Part I: The Role of Geostrophic Adjustment in Mesoscale Jetlet Formation

Abstract Mesoscale model simulations are performed in order to provide insight into the complex role of jet streak adjustments in establishing an environment favorable to the generation of gravity waves on 11–12 July 1981. This wave event was observed in unprecedented detail downstream of the Rocky Mountains in Montana during the Cooperative Convective Precipitation Experiment. The high-resolution model simulations employ a variety of terrain treatments in the absence of the complicating effects of precipitation physics in order to examine the complex interactions between orography and adiabatic geostrophic adjustment processes. Results indicate that prior to gravity wave formation, a four-stage geostrophic adjustment process modified the structure of the mid- to upper-tropospheric jet streak by creating secondary mesoscale jet streaks (jetlets) to the southeast of the polar jet streak in proximity to the gravity wave generation region (WGR). During stage I, a strong rightward-directed ageostrophic flow i...

A Linear Theory for Jet Streak Formation Due to Zonal Momentum Forcing in a Stably Stratified Atmosphere

Abstract A perturbation potential vorticity (PV) theory is developed to investigate the three-dimensional, time-dependent, linear geostrophic adjustment of a stably stratified, Boussinesq atmosphere that is disturbed from (i) quiescent equilibrium due to a localized, unbalanced, zonal wind anomaly and (ii) geostrophic equilibrium of the uniform zonal flow due to an isolated couplet of acceleration–deceleration forcing. This prescribed zonal momentum forcing propagates downstream at a speed c that is less than the basic-state zonal flow speed U and physically represents the parameterized effects of nonlinear inertial advection. Transient, dispersive inertia–gravity waves in all fields are essentially removed during the early stage of the response associated with the initial value problem. The steady-state equilibrium that conserves the initial perturbation PV is a localized, geostrophic zonal jet with meridionally confluent (diffluent) flow in its entrance (exit) region. This jet is supported by a couplet ...

Subtropical Jet Streaks over the South Pacific

Abstract The main objective of this study is to obtain a better understanding of the upper-tropospheric subtropical westerly wind maxima over the Australian–South Pacific region in the summer half of the year, which have been documented in previous papers to occur with a periodicity of 1–2 weeks. The focus of the study is to quantify the relative importance of tropical versus nontropical forcing during the acceleration phase of the aforementioned westerly wind maxima. Outgoing longwave radiation, wind data, and kinetic energy budgets, partitioned into rotational and divergent components, are used to examine the significance of the forcing mechanisms during the 6-month summer periods from 1985 to 1989. Criteria are developed to identify strong episodes of zonal wind accelerations. In all, 40 cases were found that met these criteria, or approximately 10 cases per year. In summary, 17 of the 40 cases suggested that tropical forcing was primarily responsible for the observed increase in the rotational kinetic...

Synoptic-Scale Features Common to Heavy Snowstorms in the Southeast United States

Eighteen heavy snowstorms in the Southeast are examined to determine the synoptic-scale features common to these storms. Storm-relative composites in the temporal domain are created by assigning a ‘‘zero hour’’ to each storm based on the time of initial snowfall at Asheville, North Carolina. The composites indicate the importance of warm air advection (isentropic upglide) in producing upward motion within these storms. Of secondary importance in producing upward motion are the right entrance region of an upper-level jet streak, diabatic processes, and cyclogenetic lift. The composites also indicate that moisture is drawn off the Gulf of Mexico to feed these storms, while Atlantic moisture pools at low levels and may inhibit snowfall in the Piedmont region by limiting evaporative cooling. The surface cyclones, which deepen over the Atlantic near the Carolina coast, appear to play a small role in the development of Southeast snowstorms but often lead to heavy snowfall in the Northeast.

Processes Leading to the Formation of Mesoscale Waves in the Midwest Cyclone of 15 December 1987

Abstract On 15 December 1987, several long-lived, large-amplitude mesoscale wave disturbances accompanied a rapidly developing extratropical cyclone in the midwest United States. Previous observational and modeling studies have suggested that the disturbances had large amplitudes and long lifetimes as a result of a wave-CISK-type instability occurring within an imperfect wave duct and were initiated by convection. However. infrared (IR) satellite imagery and radar echoes shortly before the wave disturbances formed suggest that convection was not the primary feature in the wave genesis region at that time. Instead, a meso-β-scale comma-shaped cloud was present and appeared to evolve into the wave disturbances. The origins of the comma cloud can be traced back to a cloud streak and precipitation maximum in the left exit region of an approaching jet streak over northern Mexico 15 h earlier. In this study, satellite observations are examined in conjunction with numerical simulations of the case to explore a n...

Development of a Rapidly Deepening Extratropical Cyclone over Land. Part I: Kinematic Aspects

Abstract The relatively rare case of an explosive land cyclone that occurred on I5–17 November 1989 over the United States and Canada is investigated to determine the physical mechanisms responsible for its development. Hourly surface and 12-h upper-air data are analyzed for this storm for the 36-h period beginning on 0000 UTC 15 November. The system appears to develop through favorable positioning of the surface low with respect to a 5OO-hPa short-wave trough and 250-hPa jet streak and yields the greatest deepening of 13 hPa in 12 h. Through an analysis of the terms in the quasigeostrophic height tendency equation, quasigeostrophic theory is deemed to be qualitatively inaccurate in diagnosing the development of this system. The performance of the National Meteorological Center's (currently known as the National Centers for Environmental Prediction) operational models is viewed, and errors in model forecasts of surface low position and intensity are attributed in large part to faulty initialization. A com...

Development of a Rapidly Deepening Extratropical Cyclone over Land. Part II: Thermodynamic Aspects and the Role of Frontogenesis

Abstract The unusual case of a rapidly developing land cyclone that occurred on 15–17 November 1989 over the United States and Canada is investigated to determine the physical mechanisms responsible for its development. Hourly surface and 12-h upper-air data are analyzed for this storm for the 36-h period beginning on 0000 UTC 15 November. Findings reveal that surface-based frontogenetic forcing and the diabatic effects of latent heating were primarily responsible for the initial development of the system. Proper positioning of the surface low with respect to a 500-mb short-wave trough and 250-mb jet streak yields the greatest deepening of 13 mb in 12 h at a later time. It is suggested that frontogenetic mechanisms contribute to development of this system prior to any favorable organization of the large-scale upper-tropospheric dynamic forcing.

Jet Stream Winds: Comparisons of Aircraft Observations with Analyses

Wind measurements have been obtained from commercial aircraft crossing the 1992 winter subtropical jet streams over southwest and east Asia. Comparisons of these data with new, high-resolution analyses from four of the major operational centers show that the peak analyzed winds are still too weak by about 10%. In addition, about 17% of the cases show larger errors in which the analyses substantially miss the location or magnitude of individual jet streaks. Both the aircraft data and the highest-resolution analysis (European Centre for Medium-Range Weather Forecasts, equivalent equatorial quarter-wave grid spacing of 47 km) show evidence of orographically generated gravity waves but differ in their vertical damping.

An observational Study of the “Interstate 5” Dust Storm Case

On 29 November 1991 a series of collisions involving 164 vehicles occurred on Interstate 5 in the San Joaquin Valley in California in a dust storm that reduced the visibility to near zero. The accompanying high surface winds are hypothesized to result from intense upper-tropospheric downward motion that led to the formation of a strong upper front and tropopause fold and that transported high momentum air downward to midlevels where boundary layer processes could then mix it to the surface. The objectives of the research presented in this paper are to document the event, to provide support for the hypothesis that both upper-level and boundary layer processes were important, and to determine the structure of the mesoscale circulations in this case for future use in evaluating the navy's mesoscale data assimilation system. The strong upper-level descent present in this case is consistent with what one would expect for jet streak and frontal circulations in combination with quasigeostrophic processes. During the period examined, upper-level data and analyses portray a strong upper-tropospheric jet streak with maximum winds initially in excess of 85 m s−1 (≈170 kt) that weakened as it propagated southward around the base of a long-wave trough. The jet streak was accompanied by a strong upper front and tropopause fold, both of which imply intense downward motion. The vertical motion field near the time of the accidents had two maxima—one that was associated with a combination of quasigeostrophic forcing and terrain-induced descent in the lee of the Sierra and one that was associated with the descending branch of the secondary circulation in the jet streak exit region and the cold advection by both the geostrophic wind and the ageostrophic wind in the upper front. The 700-hPa wind speed maximum over and west of the San Joaquin Valley overlapped with the latter maximum, supporting the hypothesized role of downward momentum transport. Given the significant 700-hPa wind speeds over the San Joaquin Valley during daytime hours on the day of the collisions, boundary layer mixing associated with solar heating of the earth's surface was then able to generate high surface winds. Once the high surface winds began, a dust storm was inevitable, since winter rains had not yet started and soil conditions were drier than usual in this sixth consecutive drought year. Surface observations from a variety of sources depict blowing dust and high surface winds at numerous locations in the San Joaquin Valley, the Mojave and other desert sites, and in the Los Angeles Basin and other south coast sites. High surface winds and low visibilities began in the late morning at desert and valley sites and lasted until just after sunset, consistent with the hypothesized heating-induced mixing. The 0000 UTC soundings in California portrayed an adiabatic layer from the surface to at least 750 hPa, also supporting the existence of mixing. On the other hand, the high winds in the Los Angeles Basin began near sunset in the wake of a propagating mesoscale trough that appeared to have formed in the lee of the mountains that separate the Los Angeles Basin from the San Joaquin Valley.

The 13 March 1993 Severe Squall Line over Western Cuba

On 13 March 1993 a powerful prefontal squall line hit western and central Cuba. At 0000 UTC, a fast-developing extratropical cyclone with a well-defined cold front was located in the northeast Gulf of Mexico. Over the eastern Gulf of Mexico there was a 300-hPa jet streak interacting with a low-level jet. A hodograph typically associated with supercells or severe squall lines was observed at Key West, Florida. The air mass over the eastern Gulf of Mexico, Florida, and Cuba was conditionally unstable. Instability was enhanced by moist air at the surface interacting with a dry intrusion in the midtroposphere from the Mexican Plateau. Lifting induced by a strong short-wave trough favored the formation a compact squall line with embedded bow-echoes and line-echo wave patterns. Damaging winds were widespread with the squall line. Thus, the event fits the definition of a serial derecho. One of the two bow echoes that affected Cuba included a small but strong mesocyclone in its forward flank, as detected by radar and surface observations. The mesocyclone caused great damage in a 20-km-wide strip over the Havana and Havana City provinces, producing a family of downbursts in its southern portion. Estimated damage up to F2 on the Fujita scale was experienced in the effected area. Another bow echo struck the central region of Cuba producing similar damage. The squall line event described in this study is the most damaging one ever recorded in Cuba.

Behavior of One to Two Week Summertime Subtropical Wind Maxima over the South Pacific during an ENSO Cycle

Abstract The behavior of subtropical jet streaks over the Australian–South Pacific region is investigated for four consecutive 6-month summer seasons, November to April, during the period 1985–89. The study is an extension of a recent one by Ko and Vincent (hereafter referred to as KV), in which jet streaks during the summer season of 1984–85 were examined. Ko and Vincent found that the upper-tropospheric zonal wind behaved in a quasi-periodic manner, with maxima recurring over the western South Pacific at intervals between one and two weeks. In the present study, wind maxima are found to exhibit a 1–2-week periodic oscillation over the South Pacific in all four years, thus substantiating the finding by KV. In addition, a second set of jet streaks, with a similar periodicity, is found over the southern Australian region. It is also seen that the mean summertime position of the South Pacific jet and its accompanying jet streaks are much farther cast during the El Nino event of 1986–87 than they are in the ...

UHF radar observation of strato-tropospheric transfers on the anticyclonic side of a jet streak

Abstract. An observation by UHF ST radar of a subsidence pattern on the right side of the exit region of a jet streak is reported. The onset of the subsidence pattern occurred at 23:30 UTC on the 29 November 1991, when a downward motion was initiated above 14 km. The injections of stratospheric air in this region seem to have an intermittent nature; they occur during at least three intervals during the lifetime of the subsidence pattern. Comparison of these results with an ECMWF analysis suggests that it is an unfolding case. However, observation of turbulent intensities w' greater than 60 cm s–1 at the tropopause level also suggests the existence of a turbulent flux between the stratosphere and the troposphere. From the turbulence characteristics measured by the radar and the potential temperature profile obtained by radiosonde data, the eddy diffusivity at the tropopause level has been calculated. An eddy diffusion coefficient ranging between 5 and 7 m2 s–1 is found. From these values, and with the assumption of a climatological gradient of the volume mixing ratio of ozone in the lower stratosphere, it is possible to deduce a rough estimate of the amount of ozone injected from the stratosphere into the troposphere during this event. A rate of transfer of 1.5×1020 molecules of ozone per day and per square meter is found.

The “Striated Delta” Cloud System–A Satellite Imagery Precursor to Major Cyclogenesis in the Eastern Australian-Western Tasman Sea Region

A distinctive upper-tropospheric cloud pattern, frequently a precursor to major extratropical cyclogenesis within the eastern Australian–western Tasman Sea region, has been identified from an examination of Geostationary Meteorological Satellite imagery. This cloud feature is usually triangular- (or delta-) shaped, approximately symmetrical, composed of cirrus level cloud tops, and associated with transverse banding. Thirty-seven of these cloud systems, referred to as “striated deltas” (SDs), were detected within the study region between 1980 and 1991. Approximately 75% of these were associated with the development of storms of at least gale force intensity within 24 h of formation of the SD. Most of these intense developments were characterized by a 12-h period of explosive deepening. Three types of SD development are observed. The cloud system usually forms as a discrete feature; this process can be either rapid or gradual. The SD can also form as a subcomponent of a major meridional cloud band. Striated deltas associated with intense cyclogenesis are normally generated east of a high-amplitude 1000-500-hPa thickness trough and within diffluent north to northwesterly flow in the exit region of a strong thickness jet streak. The relationship between the SD and the surface pattern differs substantially between oceanic and continental event. Case studies in which major surface low deepening occurred are presented to demonstrate these variations. Events in which intense cyclogenesis does not occur are also examined. The synoptic and geographic environment in which such cloud systems form are found to vary from that observed in intense developments. A qualitative investigation into the source mechanism of the striations suggests that their most probable cause is gravity wave activity due to the geostrophic adjustment process.

The Influence of the Rocky Mountain on the 13–14 April 1986 Severe Weather Outbreak. Part I: Mesoscale Lee Cyclogenesis and Its Relationship to Severe Weather and Dust Storms

Abstract In this first of a two paper series, a sequence of dynamical processes involving the evolution of a mesoscale Ice cyclone and its subsequent interaction with a mesoscale tropopause fold downstream of the Rocky Mountains is investigated. These scale-interactive phenomena, which resulted from the jet streak interaction with the topography, were examined in detail using the observational data obtained from the Program for Regional Observing and Forecasting Services' mesonetwork and wind profilers, as well as conventional surface and rawin-sonde data and Total Ozone Mapping Spectrometer satellite data over the Colorado region for the severe weather event that occurred during 13–14 April 1986. Large-scale analysis indicated that as a baroclinic low pressure system approached the Rockies with its attendant upper-level jet streak, a typical prestorm environment over western Kansas formed in the early morning hours of 13 April. Hourly mesonet data analysis revealed the formation and eastward progression ...

The Influence of the Rocky Mountains on the 13–14 April 1986 Severe Weather Outbreak. Part II: Evolution of a Prefrontal Bore and Its Role in Triggering a Squall Line

Abstract In this paper, Part II of a series, the evolution of a prefrontal bore on the leeside of the Rockies and its subsequent propagation and initiation of convection farther downstream over eastern Colorado and western Nebraska are investigated. The observational evidence for this sequence of events was obtained from combined analyses of high-resolution GOES satellite imagery and Program for Regional Observing and Forecasting Services mesonetwork data over the Colorado region for the severe weather event that occurred during 13–14 April 1986. A 2D nonhydrostatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger convection. Analysis of satellite imagery and mesonet data indicated that an internal bore (ahead of a cold front), a moderate downslope windstorm, and a quasi-stationary hydraulic jump were generated within a few hours along the Iceslope as a Pacific cold front and its attendant upper-level jet streak advanced o...