Upper-tropospheric Jet Research Articles

Interannual meridional displacement of the East Asian upper-tropospheric jet stream in summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS’s meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June–July–August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.

Investigation of inertia-gravity waves in the upper troposphere/lower stratosphere over Northern Germany observed with collocated VHF/UHF radars

Abstract. A case study to investigate the properties of inertia-gravity waves in the upper troposphere/lower stratosphere has been carried out over Northern Germany during the occurrence of an upper tropospheric jet in connection with a poleward Rossby wave breaking event from 17-19 December 1999. The investigations are based on the evaluation of continuous radar measurements with the OSWIN VHF radar at Kühlungsborn (54.1 N, 11.8 E) and the 482 MHz UHF wind profiler at Lindenberg (52.2 N, 14.1 E). Both radars are separated by about 265 km. Based on wavelet transformations of both data sets, the dominant vertical wavelengths of about 2-4 km for fixed times as well as the dominant observed periods of about 11 h and weaker oscillations with periods of 6 h for the altitude range between 5 and 8 km are comparable. Gravity wave parameters have been estimated at both locations separately and by a complex cross-spectral analysis of the data of both radars. The results show the appearance of dominating inertia-gravity waves with characteristic horizontal wavelengths of 300 km moving in the opposite direction than the mean background wind and a secondary less pronounced wave with a horizontal wavelength in the order of about 200 km moving with the wind. Temporal and spatial differences of the observed waves are discussed.

The surface of maximum wind as an alternative to the isobaric surface for wind climatology

The surface of maximum wind (SMW) is introduced as an analysis frame for climate studies of fast upper tropospheric winds. As defined here, the vertical domain for the SMW includes the troposphere above 500 hPa and upper tropospheric jet streams that may protrude into the lower stratosphere. We use NCEP‐NCAR Reanalysis data for summers 1958–2004 in the vicinity of the tropical easterly jet (TEJ) to show how the spatial and temporal variability of the SMW relate to jet stream cores and the tropopause. We then compare the SMW climatology of the TEJ to an isobaric climatology of the TEJ, demonstrating that the SMW climatology reveals descent and slowing of the TEJ over the period of record, whereas the isobaric climatology provides only an overestimate of the TEJ slowing trend.

Generation of Mesoscale Gravity Waves in Upper-Tropospheric Jet–Front Systems

Abstract Multiply nested mesoscale numerical simulations with horizontal resolution up to 3.3 km are performed to study the generation of mesoscale gravity waves during the life cycle of idealized baroclinic jet–front systems. Long-lived vertically propagating mesoscale gravity waves with horizontal wavelengths ∼100–200 km are simulated originating from the exit region of the upper-tropospheric jet streak, in a manner consistent with past observational studies. The residual of the nonlinear balance equation is found to be a useful index in diagnosing flow imbalance and predicting wave generation. The imbalance diagnosis and model simulations suggest that balance adjustment, as a generalization of geostrophic adjustment, is likely responsible for generating these mesoscale gravity waves. It is hypothesized that, through balance adjustment, the continuous generation of flow imbalance from the developing baroclinic wave will lead to the continuous radiation of gravity waves.

Arctic oscillation signals in the East Asian summer monsoon

The present study examines the relationship between the Arctic Oscillation (AO) and the East Asian summer monsoon. Two rainfall data sets are used. One is obtained from 10 stations along the Yangtze River to the southern Japan and the other from gridded global land rainfall data for the period 1900–1998. All data are high‐pass filtered before analyzing to highlight the interannual variability. Results show that the AO significantly influences on year‐to‐year variations in the East Asian summer monsoon rainfall. When AO leads by one month, the correlation between May–July AO and summer total rainfall is −0.44. When AO leads by two months, correlation becomes −0.32. Of all monthly, May AO shows the strongest connection to the summer monsoon rainfall. Correlation coefficient between them is −0.45. The large‐scale atmospheric circulation patterns in East Asia in association with the AO are also evident. A positive phase of the AO in late spring is found to lead to a northward shift in the summertime upper tropospheric jet stream over East Asia. This northward shift of the jet stream is closely related to anomalous sinking motion in 20°–40°N and rising motion in surrounding regions. These changes give rise to a drier condition over the region extending from the Yangtze River valley to the southern Japan and a wetter condition in the southern China. Possible mechanisms connecting the late spring AO and summer monsoon rainfall are suggested.

Statistical Modeling of Monthly Anomalies of Atmospheric Precipitations for the Region of the Ukraine and Black Sea

The data array accumulated for the period 1946–1995 is used to select the most significant predictor of monthly anomalies of precipitation in the region of the Ukraine and Black Sea, namely, a large-scale circulation in a sector covering the North Atlantic and Europe. Three large-scale predictors (geopotential at a level of 500 GPa, sea-level pressure, and the difference between the geopotentials at levels of 500 and 1000 GPa) give almost identical results: their correlation with anomalies of precipitation is equal to 0.7–0.8 in winter and 0.4–0.5 in summer. The predominant mechanism of the influence of circulation on precipitations at low frequencies is the response of the trajectories of European cyclones to changes in the upper-tropospheric jet currents in the Atlantic-European sector described as the superposition of the North-Atlantic oscillation and the Eurasian mode. The decomposition of the fields of precipitations and circulation in the eigenmodes of the canonical correlation analysis opens a possibility of direct computation of the monthly average fields of precipitations for the entire territory of the Ukraine and Black Sea region according to the large-scale geopotential fields.

Seasonal variation of gravity wave activity at 5–20 km observed with VHF radar at White Sands Missile Range, New Mexico

Wind observations at altitudes from 5 to 20 km obtained from the VHF wind profiler radar at the White Sands Missile Range (WS), New Mexico, are used to construct a climatology of gravity wave intensities over WS during the nearly 6‐year period from January 1991 through September 1996. The kinetic energy densities for both high‐frequency gravity waves (ground‐based period of 6 min to 2 hours) and low‐frequency gravity waves (period from 2 to 21 hours) are considered. These results are compared to earlier results from the middle and upper atmosphere (MU) radar in Japan. Comparisons of gravity wave variability at high and low frequency reveal both similarities and differences between the two sites. The primary differences occur at high frequency. The seasonal cycle in the high‐frequency gravity waves is less pronounced and less regular at WS compared to MU. The magnitude of the high‐frequency kinetic energy density is generally larger at WS compared to MU, and the relationship between the upper tropospheric jet stream winds and the high‐frequency gravity wave activity is not nearly so strong at WS as at MU. The primary high‐frequency gravity wave excitation mechanism at WS appears to change with season. There is evidence of a connection in winter and spring between the wind speeds in the middle troposphere and the high‐frequency gravity wave activity, suggesting a topographic source for these waves. Strong summertime high‐frequency variability at WS in the upper troposphere is most likely linked to waves launched by deep convection during the monsoon season in New Mexico. The results for low‐frequency gravity waves are more consistent between WS and MU. Similarities appear in the kinetic energy density, the nature of the seasonal cycle, and the correlation between the kinetic energy density and the mean winds.

GCM and Observational Diagnoses of the Seasonal and Interannual Variations of the Pacific Storm Track during the Cool Season

Abstract Previous work has found that the Pacific storm track intensity during the cool season is negatively correlated with the upper-tropospheric jet strength. In the seasonal march, such a variation manifests itself as the midwinter suppression of the storm track intensity, while in interannual variations the storm track intensity during midwinter is found to be weaker during years in which the Pacific jet is particularly strong. In this paper, GCM simulations and observational data have been analyzed to shed light on the physical mechanisms responsible for such variations. By examining the eddy energy budget and eddy structure, two different mechanisms have been found to contribute to the reduction in storm track activity associated with increases in jet intensity and baroclinicity in midwinter. For the seasonal variations, it was found that the major difference between fall and midwinter lies in the changing role of diabatic heating. During fall and spring, diabatic heating acts to generate eddy pote...

Multiscale Overview of a Violent Tornado Outbreak with Attendant Flash Flooding

On 1 March 1997 violent tornadoes caused numerous fatalities and widespread damage across portions of central and eastern Arkansas and western Tennessee. In addition, the associated thunderstorms produced very heavy rainfall and flash flooding, with a few locations receiving up to 150 mm (6 in.) of rainfall in 3 h. The initial environment appeared favorable for strong tornadoes with unseasonably warm moist air at lower levels resulting in significant instability (convective available potential energy values between 1400 and 1800 J kg−1) where 0–2-km storm-relative helicities exceeded 300 m2 s−2 and the middle-tropospheric storm-relative flow was conducive for tornadic supercells. The most destructive tornadoes developed along a preexisting surface boundary where lower-tropospheric moisture convergence and frontogenesis were enhanced. Tornadoes and heaviest rainfall only ensue after upward motion associated with the direct circulation of an upper-tropospheric jet streak became collocated with lower-tropospheric upward forcing along the surface boundaries. From a flash flood perspective the event occurred in a hybrid mesohigh-synoptic heavy rain pattern as thunderstorms developed and moved along surface boundaries aligned nearly parallel to the mean wind. In addition, strong flow and associated moisture flux convergence in the lower troposphere favored the formation of cells to the southwest or upstream of the initial convection with thunderstorms, including a a tornadic supercell, traversing over the same area. The available moisture and ambient instability also supported both vigorous updrafts and high precipitation rates.

寒季に東アジアで見られる鉛直流逆転の形成メカニズムについて

The formation mechanism of a systematic vertical-velocity reversal over East Asia was studied during the cold season by using MU radar data and global analysis data. Although active gravity waves may modify the vertical wind pattern in the stratosphere, clear reversal patterns are also observed in the periods when gravity waves are quiet. The frontogenesis function is positive in all the reversal periods. These facts suggest that the reversal is a part of the frontogenetically forced secondary circulations associated with an upper tropospheric jet streak. Calculation of the geostrophically forced secondary circulation following frontogenesis theory supports this conclusion.

Energetics of an Intensifying Jet Streak during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA)

Abstract A characteristic life cycle of upper-tropospheric cyclogenetic precursors involves the development of an elongated region of lower dynamic tropopause that forms in association with an intensifying midtropospheric jet/front. Transverse divergent circulations associated with the jet/front steepen and depress the dynamic tropopause prior to the onset of lower-tropospheric cyclogenesis. A representative event that occurred during the second intensive observation period (IOP 2) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA, December 1988–February 1989) is analyzed from the perspective of local energetics. The goals of the analysis are (i) to document the evolution of the three-dimensional eddy kinetic energy (EKE) distribution during this event and (ii) to identify the mechanisms leading to EKE growth in the upper-tropospheric jet streak associated with the precursor disturbance prior to cyclogenesis, as well as in the developing lower-tropospheric cyclone. Computation of...

Role of Multiple-Level Tropospheric Circulations in Forcing ENSO Winter Precipitation Anomalies

Regionally organized winter (DJF) precipitation anomalies over North America are presented in association with cold and warm phases of the El Niño–Southern Oscillation. Variations in low-level flow from moisture sources and in upper-level dynamic properties are diagnosed for each anomalous precipitation region using DJF composites of upper- and lower-tropospheric winds, sea level pressure, divergence, and vorticity advection. Variance analysis determines which precipitation and circulation patterns are robust and evaluates the predictability of the extreme phase composites with respect to the climatology. In cold phases, the Pacific Northwest and the lower Mississippi Valley regions have above-normal precipitation totals. The heavier precipitation over the Pacific Northwest is the result of a multilevel onshore flow forced by an amplified Pacific high off the California coast and a single zonal upper-tropospheric jet. In the Lower Mississippi and Tennessee Valleys, the equatorward entrance region of a west–east-oriented jet core combines with anomalous positive vorticity advection (PVA) and a convergent low-level flow off the Gulf of Mexico, resulting in heavier precipitation. Drier than normal conditions occur in coastal Alaska and California due to negative vorticity advection anomalies aloft, low-level divergence, and weaker onshore flow. The increased precipitation over the southern Mississippi Valley and the Pacific Northwest, and the drier conditions over Florida, are statistically robust. During warm phases, more precipitation occurs in the coastal regions of Alaska, California, and the Gulf of Mexico, whereas drier conditions persist over the Pacific Northwest. Alaskan precipitation is enhanced by strong, convergent, onshore flow associated with a deeper Aleutian low, upper-level PVA, and the northern branch of a split jet pattern. A strong southern subtropical branch of the split jet provides dynamic support for uplift of moisture along the Gulf Coast. The drier conditions in the Pacific Northwest and the wet conditions in Texas and Florida are predictable relative to the climatology and are statistically robust.

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...

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.

A Climatology of Cyclogenesis for the Southern Hemisphere

Abstract This study surveys life cycle characteristics of cyclones in the Southern Hemisphere (SH). An objective technique for locating instances of cyclone formation, intensification, maturation, and decay is applied to a 7-year dataset of SH cyclone positions, obtained from an automated finding and tracking scheme based on twice-daily ECMWF analyses during 1980–86. Geostrophic vorticity ζg at 1000 hPa is used to locate cyclones and determine their intensity. Cyclones form and develop within middle latitudes and migrate eastward and poleward during their lives. Preferred genesis and cyclogenesis regions include eastern coasts of Australia and South America in winter, and leeward of the Andes and near the baroclinic zones associated with the principal upper-tropospheric jet streams year round. Over open water, winter cyclones tend to form and intensify near zones of strongest SST gradient. Rapid cyclogenesis is most frequent east of South America, southeast of Africa, south of Australia, and near New Zeal...

The Mesoscale Forcing of a Midlatitude Upper-Tropospheric Jet Streak by a Simulated Convective System. Part I: Mass Circulation and Ageostrophic Processes

The mutual forcing of a midlatitude upper-tropospheric jet streak by organized mesoscale adiabatic and diabatic processes within a simulated convective system (SCS) is investigated. Using isentropic diagnostics, results from a three-dimensional numerical simulation of an SCS are examined to study the isallobaric flow field, modes of dominant ageostrophic motion, and stability changes in relation to the mutual interdependence of adiabatic processes and latent heat release. Isentropic analysis affords an explicit isolation of a component of isallobaric flow associated with diabatic processes within the SCS. Prior to convective development within the simulations, atmospheric destabilization occurs through adiabatic ageostrophic mass adjustment and low-level convergence in association with the preexisting synoptic-scale upper-tropospheric jet streak. The SCS develops in a baroclinic zone and quickly initiates a vigorous mass circulation. By the mature stage, a pronounced vertical couplet of low-level convergence and upper-level mass divergence is established, linked by intense midtropospoheric diabatic heating. Significant divergence persists aloft for several hours subsequent to SCS decay. The dominant role of ageostrophic motion within which the low-level mass convergence develops is the adiabatic isallobaric component, while the mass divergence aloft develops principally through the diabatic isallobaric component. Both compnents are intrinsically linked to the convectively forced vertical mass transport. The inertial diabatic ageostrophic component is largest near the level of maximum heating and is responsible for the development of inertial instability to the north of SCS, resulting in this quadrant being preferred for outflow. The inertial advective component, the dominant term that produces the new downstream wind maximum, rapidly develops north of the SCS and through mutual adjustment creates the baroclinic support for the new jet streak.

The Mesoscale Forcing of a Midlatitude Upper-Tropospheric Jet Streak by a Simulated Convective System. Part II: Kinetic Energy and Resolution Analysis

Abstract A kinetic energy (KE) analysis of the forcing of a mesoscale upper-tropospheric jet streak by organized diabaaic processes within the simulated convective system (SCS) that was discussed in Part I is presented in this study. The relative contributions of the ageostrophic components of motion to the generation of KE of the convectively generated jet streak are compared, along with the KE generation by the rotational (nondivergent) and irrotational (divergent) mass transport. The sensitivity of the numerical simulations of SCS development to resolution is also briefly examined. Analysis within isentropic coordinates provides for an explicit determination of the influence of dramatic processes on the generation of KE. The upper-level production of specific KE is due predominantly to the inertial advective ageostrophic component (IAD), and as such represents the primary process through which the KE of the convectively generated jet streak is realized. A secondary contribution by the inertial diabatic...

Nonlinear Barotropic Instability of Upper-Tropospheric Tropical Easterly Jet on the Sphere

Abstract The nonlinear evolution of perturbation superimposed on the barotropic, unstable observed mean easterly jet at 100 hPa is studied over the sphere. The nondivergent barotropic nonlinear global spectral model with rhomboidal truncation at zonal wavenumber 21 is integrated for 120 days for initial random and linear unstable perturbations. The model includes a Rayleigh friction and restoring mechanism for zonal wind to its initial distribution. Time variations of eddy and zonal kinetic energy, zonal-wave and wave-wave interactions, and eddy and zonal kinetic energy dissipations are examined. The growth of perturbation begins with exponential increase in its kinetic energy for a short period, followed by a linear increase. The perturbations undergo oscillations before approaching a steady state. During the initial exponential phase, the nonlinear interactions further destabilize the jet, and this effect is more pronounced for the random initial perturbation. It is found that oscillations in kinetic en...

Midwinter Suppression of Baroclinic Wave Activity in the Pacific

Abstract Seasonal variations in baroclinic wave activity and jet stream structure in the Northern Hemisphere are investigated based upon over 20 years of daily data. Baroclinic wave activity at each grid point is represented for each day by an envelope function, the lowpass-filtered time series of the squared highpass-filtered geopotential height. Baroclinic wave activity over the Atlantic exhibits a single maximum in January, whereas in the Pacific it exhibits peaks in late autumn and in early spring and a significant weakening in midwinter, which is more evident at the tropopause level than near the surface. This suppression occurs despite the fact that the low-level baroclinity and the intensity of the jet stream are strongest in midwinter.Based on the analysis of 31-day running mean fields for individual winters, it is shown that over both the oceans baroclinic wave activity is positively correlated with the strength of the upper-tropospheric jet for wind speeds up to ∼45 m s−1. When the strength of t...

Diagnosing Coupled Jet-Streak Circulations for a Northern Plains Snow Band from the Operational Nested-Grid Model

On 17 March 1989, moderate to heavy snow developed in a 100- to 200-km-wide band extending from South Dakota to northern Michigan. The 4- to 8-inch snowfall within this band was not associated with major cyclogenesis, and developed 500 to 600 km north of a stationary surface front. A diagnostic analysis based on an application of the General Meteorological Package (GEMPAK 5.0) to a numerical simulation from the operational nested-grid model (NGM) is utilized to show that the development of this snow band is related to the interaction of two upper-tropospheric jet streaks and their associated transverse circulation patterns. The eastward propagation of a jet streak from the West Coast toward the middle United States and to the south of a slower-moving jet along the U.S.-Canadian border led to a merger of the ascent maxima associated with the direct and indirect circulations of the northern and southern jets, respectively. The snow band developed as the ascending branches of the jet-streak circulation patterns merged, with the eastward propagation of the heaviest snow linked to the motion of the coupled circulation pattern. The study also demonstrates the usefulness of the operational NGM for providing the higher-resolution datasets required to relate the evolution of jet-streak circulation patterns to the development of mesoscale precipitation bands.