Jet Streak Research Articles

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...

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.

A general circulation model study of a tropopause‐folding event at middle latitudes

AbstractThe UK Universities' Global Atmospheric Modelling Programme General Circulation Model (UGCM) has been run at high horizontal resolution to investigate a tropopause‐folding event at middle latitudes. Geopotential‐height, potential‐vorticity and specific‐humidity distributions were diagnosed from the model and showed a realistic representation of the synoptic development of the cut‐off low associated with the tropopause fold. the model also successfully portrayed the frontogenesis associated with tropopause folding and represented both the upper‐level frontal region and the corresponding jet‐streaks as they progressed through the region of the developing trough. A series of vertical cross‐sections at the ends of the jet streaks demonstrated that a fold could be captured by the UGCM using potential vorticity and specific humidity, although more small‐scale features were evident in the specific‐humidity contours than in the corresponding potential‐vorticity fields. the corresponding baroclinic zone was also resolved, although both the slope and thickness were rather larger than sometimes observed. Both the spatial and temporal extent of the fold compared well with observations. In addition the model simulated the ageostrophic winds in the entrance, exit and middle‐streak regions. A direct circulation was obtained at the entrance and an indirect circulation at the exit region of the jet streak, in good agreement with theory and with observational data. A combination of confluence, cold advection and curvature was responsible for the forcing of the ageostrophic motion at the jet entrance, whereas the forcing at the jet exit was attributed mainly to diffluence and curvature. Q vectors were employed to investigate the vertical motion in the vicinity of the fold, and showed the precise positions of descent corresponding to the fold along the entire length of the jet streak. Finally a tropopause fold around a jet that was not subject to the development of strong baroclinic instabilities was reproduced by the model: it was smaller in size than the fold of the cyclonic trough. A Q‐vector analysis confirmed the position of the small fold along the jet and revealed large‐scale ascending motion that may have been responsible for the small size of fold. the possibility of using the model to yield a realistic estimate of the transfer of air from the stratosphere to the troposphere in a tropopause‐folding event at mid‐latitudes is discussed.

Circulations between Mesoscale Convective Systems along a Cold Front

A frontal squall line that passed over the central United States on 14–15 June 1985 consisted of several primary mesoscale convective systems (MCSs) with a prominent gap (several hundred kilometers wide) between two of them over central Kansas and Oklahoma. An upper-level jet streak crossed the front at right angles, with the jet exit region centered in the gap. Surface, upper-air, radar, and satellite data are used to examine conditions contributing to this squall-line gap. All surface indicators (e.g., a sharp wind shift line and strong low-level moisture convergence) pointed toward the development of new deep convection in the gap. Computations of vertical velocity using sounding data showed that the low-level vertical motion was indeed upward there; however, strong subsidence existed aloft. The demarcation between subsidence aloft and ascent at low levels was near the 0°C level, where there was a concurrent inflow into the MCSs. A strong inversion existed near this level, which effectively capped vigorous congestus clouds that formed in the gap. The circulations in the gap appeared to be first influenced by the upper-level jet and then the convection itself. Initial convection with the first MCS developed in the left-exit region of the jet. The second MCS evolved from initial cells that formed over strongly heated, higher terrain approximately 500 km to the southwest. The subsequent circulations between the MCSs—converging outflows aloft and at the surface, and diverging flow near the 0°C level—resemble these generated by gravity waves emanating from an MCS-like convective heat source. The gravity wave-induced subsidence aloft acted in opposition to the jet streak forcing, maintaining a strong cap and preventing deep convection in the squall-line gap. The distinction between highly broken squall lines (such as this one) and those with a nearly contiguous band of cells in the presence of strong low-level convergence appears to be related to the degree of convective inhibition in the environment and how it is modulated by a variety of processes including the convection itself.

Simulation of Extreme New Zealand Precipitation Events. Part II: Mechanisms of Precipitation Development

Abstract The extreme precipitation event that occurred on 27 December 1989 over the South Island of New Zealand was simulated using the DAR hydrostatic mesoscale model nested within the ECMWF analyses. The model simulated nearly half of the peak observed rainfall for this storm (greater than 700 mm) and captured the location and timing of the intense precipitation. The heavy precipitation developed while a deep layer of moist subtropical air along a cold front ascended the high terrain of the South Island. The intense orographic ascent was associated with a low-level jet core with wind speeds of over 20 m s−1 ahead of the cold front. An upper-level trough and jet streak entrance region were also present upstream of the South Island during the event, aiding the ascent over the mountains and deepening the layer of moist air. The air crossing the mountain was nearly saturated throughout the troposphere and had only weak moist vertical stability near the cold front. Almost all of the simulated precipitation f...

A Composite Study of the Quasi-Periodic Subtropical Wind Maxima over the South Pacific during November 1984-April 1985

A composite of 10 cases of zonal wind maxima at 200 hPa over the subtropical region stretching from Australia to the central Pacific is examined for the six-month period, November 1984-April 1985. This region is unique in that distinct westerly jets frequently form and propagate eastward at latitudes between 20° and 35°S in the summer season. Some statistical tests were applied and suggest that the flow patterns are quasi periodic, consisting of a tendency for new jet streaks to develop over the eastern Australian region approximately every one to two weeks. These jets then take about 10 days to propagate across the western Pacific before dissipating or, perhaps, moving toward higher latitudes. Their average propagation speed is approximately 4 m s−1. An examination of the case-to-case variability of the jets provides additional evidence that they are significant features. A diagnosis of the trough/ridge systems at 200 and 850 hPa, together with calculations of the vertically integrated mean and shear kinetic energies suggests that baroclinic processes dominate in the entrance and center regions of the jet, whereas barotropic processes dominate in the exit and downstream regions.

Planetary and synoptic-scale interactions during the life cycle of a mid-latitude blocking anticyclone over the North Atlantic

The formation of a blocking anticyclone over the North Atlantic has been examined over its entire life-CyCle using the Zwack-Okossi (Z-O) equation as the diagnostic tool. This blocking anticyclone occurred in late October and early November of 1985. The data used were provided by the NASA Goddard Laboratory for Atmospheres on a global 2.O degree latitude by 2.5 degree longitudinal grid. The horizontal distribution of the atmospheric forcing mechanisms that were important to 500 mb block formation, maintenance and decay were examined. A scale-partitioned form of the Z-O equation was then used to examine the relative importance of forcing on the planetary and synoptic scales, and their interactions. As seen in previous studies, the results presented here show that upper tropospheric anticyclonic vorticity advection was the most important contributor to block formation and maintenance. However, adiabatic warming, and vorticity tilting were also important at various times during the block lifetime. In association with precursor surface cyclogenesis, the 300 mb jet streak in the downstream (upstream) from a long-wave trough (ridge) amplified significantly. This strengthening of the jet streak enhanced the anti-cyclonic vorticity advection field that aided the amplification of a 500 mb short-wave ridge. Tile partitioned height tendency results demonstrate that the interactions between the planetary and sn,noptic-scale through vorticity advection was the most important contributor to block formation. Planetary-scale, synoptic-scale. and their interactions contributed weakly to the maintenance of the blocking anticyclone with the advection of synoptic-scale vorticity by the planetary-scale flow playing a more important role. Planetary-scale decay ofthe long-wave ridge contributed to the demise of this blocking event.

Climatological features of blocking anticyclones Northern Hemisphere

Blocking anticyclones have long been of interest to the atmospheric science community because of their profound effect on local and regional climates. Previous climatologies of blocking anticyclones have been performed using subjective or objective techniques to locate individual blocking events using observational data sets typically of greater than 10 years. In this study, a 3-year climatology of Northern Hemisphere blocking anticyclones was developed using ECMWF analyses to derive a comprehensive set of blocking anticyclone characteristics, including location, frequency, duration, intensity, size, seasonal and regional distribution, and relationship to precursor cyclones and jet streaks. Results show that preferred blocking regions were located over the eastern Atlantic Ocean, the Pacific Ocean, and the Ukraine/western Russia and that most blocks occurred in winter, as seen in other climatological studies. Block half-wavelengths, which averaged about 3000 km, were positively correlated with block intensity at the 99% confidence level. However, block duration, which averaged 8.6 days, was only weakly correlated with both size and intensity. Also, this study reveals that all 63 blocking anticyclones were preceded by an identifiable surface cyclone, which began its most rapid deepening 36 h or more prior to block onset. However, only 34 of these cyclones could be characterized as “explosively” developing, with half of these preceding winter season blocks and none preceding summer season blocks. A positive correlation was found between the intensity of blocking anticyclones and the intensity of the precursor cyclone development, significant at the 95% confidence level. This correlation was also found for events occurring over the oceanic regions. Finally, the intensity of the precursor cyclone development was correlated with other blocking characteristics and no significant relationships were found.

The 5–6 December 1991 FIRE IFO II Jet Stream Cirrus Case Study: Possible Influences of Volcanic Aerosols

In presenting an overview of the cirrus clouds comprehensively studied by ground-based and airborne sensors from Coffeyville, Kansas, during the 5–6 December 1992 Project FIRE IFO II case study period, evidence is provided that volcanic aerosols from the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur-based particles that became effective cloud-forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded within the upper-level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol-enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations include abnormally high ice crystal concentrations (up to ∼600 L−1), complex radial ice crystal types, and relatively large haze particles in cirrus uncinus cell heads at temperatures between −40° and −50°C. Implications for volcanic-cirrus cloud climate effects and usual (nonvolcanic aerosol) jet stream cirrus cloud formation are discussed.

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

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

An Investigation of Cold Vortices in the Upper Troposphere over the Western North Pacific during the Warm Season

Abstract Sixty cold lows in the warm months (June–October) of 1982–87 (1984 missing) over the western North Pacific were studied by using rawinsonde data, airplane reports, and satellite cloud-tracked winds. The general characteristics of cold lows were analyzed. Various cloud types were derived from the cloud observations of the Geostationary Meteorological Satellite of Japan. Cloud distribution was investigated over different sectors of the vortex, over different regions of the jet streak, and at different life stages of the vortex. Structure of the vorticity and divergence fields was also studied for selected cases of cold lows. Cold lows generally moved westward with a mean speed of 4.1 m s−1 and a mean lifetime of 6.3 days. The frequency of occurrence exhibited significant interannual and monthly variations and reached a maximum in August and a minimum in October. One of the interesting findings of this study is the existence of a type NW (northwest) and/or a type S (south) jet streak for the majorit...

Diagnosing an Operational Numerical Model UsingQ-Vector and Potential Vorticity Concepts

Abstract A quasigeostrophic (QG) diagnostic model is used to evaluate the nested grid model's (NGM) predictions for a December cyclone whose impact on northeastern Colorado was underpredicted. Although the NGM predicted deepening of the associated 500-mb low, the model was 12 h slow in the onset of deepening and moved the storm too far east too quickly. Synthetic soundings, generated from 12-h predicted data initialized 24 h before cyclogenesis became apparent, were submitted to the same QG diagnostic algorithms used to analyze verifying rawinsonde data. Comparisons reveal that the NGM apparently 1) transported too much potential vorticity, westerly momentum, and cold air into the lower troposphere along the axis of the jet stream; 2) moved the first of two short-wavelength jet streaks too far northeastward and with too much strength; 3) failed to predict the strength of the following jet maximum; and 4) failed to develop an apparent tropopause fold. It is established that these errors were not caused by ...

Stratospheric/tropospheric exchange affecting the northern wetlands regions of Canada during summer 1990

The Arctic Boundary Layer Expedition (ABLE) 3B was conducted over the northern wetlands region of Canada during July and August 1990. Several Stratospheric/tropospheric exchange events were noted by zenith‐looking airborne lidar and in situ measurements of ozone and other trace gas species. Isentropic trajectories and potential vorticity analyses are utilized to determine the frequency of stratospheric inputs which would have affected the tropospheric column over the Moosonee and Schefferville regions and to describe the favored pathways of transport of stratospheric air arriving at these locations. At the 310 K potential temperature level (middle troposphere), trajectories having “aged stratospheric” values of potential vorticity at some point in their 5‐day history arrived at Moosonee or Schefferville roughly 40% of the time during the ABLE 3B study period, most often via large‐scale subsidence enroute from “stratospheric input regions” over the Arctic Ocean or northern and central Canada. At 325 K (upper troposphere), “fresh” stratospheric input was evident on about 80% of the trajectories, most often associated with jet streaks within the polar and Arctic jet streams. A case study is presented which illustrates both of these general stratospheric input processes.

A Comparative Study of Large-Amplitude Gravity-Wave Events

Abstract Two large-amplitude gravity waves were observed over the midwestern United States on 5 and 14 January 1989 during the University of Illinois Winter Precipitation Program. On both days, an extensive amount of data was recorded, including data from two radars and a radiosonde facility. The waves originated near Missouri, registered pressure fluctuations as large as 10 mb, and produced distinct precipitation bands along their updraft regions. The waves were long-lived and maintained their identity over 1000 km, a distance several times their wave-lengths. The synoptic features at the surface were dissimilar. A deep cyclone was present on 5 January, while a leeside trough was present on 14 January. However, the middle- and upper-tropospheric flow patterns were similar. In both cases, the axis of a trough was immediately upstream of the gravity-wave genesis area and a jet streak had just propagated through the base of the trough toward a downstream ridge. Soundings taken near the gravity waves were re...

A Case Study of Coupling between Low- and Upper-Level Jet–Front Systems: Investigation of Dynamical and Diabatic Processes

Abstract An example of coupling between an upper-level and a low-level jet–front system is analyzed using the mesoscale hydrostatic model SALSA. The case study chosen is the cold front sampled during the intensive observation period 2 of the Mesoscale Frontal Dynamic Project FRONTS 87 experiment (11 November 1987). Two prominent features of the cold front are a well-developed undulation and a frontal band ahead of the undulation. Dynamic and diabatic processes of the coupling are investigated through the results of two numerical simulations: full physics and adiabatic. In particular, the roles of ageostrophic circulations are investigated through the decomposition of the ageostrophic wind into its isentropic components: the advective inertial and the diabatic inertial components, and the isallobaric component. In both simulations it is shown that the low-level branch of the indirect and transverse ageostrophic circulation, associated with the exit region of the upper-level jet streak, is the origin of the...

Climatological and structural differences between wet and dry troughs over southern Africa in the early summer

Differences in the convective potential of troughs passing over the plateau of southern Africa in the early summer are assessed using operational synoptic weather data and radiosonde time-height sections. Wet and dry trough cases are chosen on the basis of the intensity and distribution of rainfall, sharp thermodynamic changes across the plateau and the passage of a geopotential wave. Composite differences are computed and indicate a high-low geopotential anomaly in the east-west direction, and a threefold increase in precipi-table water from 15 mm in dry cases to 28 mm in wet cases. The 500 hPa structure obtained by differencing wet and dry composites is dominated by low geopotentials and cyclonic vorticity over the plateau near 25°S, 25°E, and high geopotentials and anticyclonic vorticity to the south over the oceans near 40°S, 30°E. The dipole anomaly suggests a diffluent tilted baroclinic wave in the subtropical jet stream in convective cases. A double jet streak structure in the wet events enhances upper divergence which contributes to widespread uplift in the mid-troposphere. A case study comparison highlights the importance of pre-frontal moist influx, the kinematic trigger and thermodynamic instability.

The Effect of Jet-Streak Curvature on Kinematic Fields

Abstract A simple two-layer primitive equation (PE) model is used to study the effect of curvature on jet-streak kinematics, specifically vertical motion. Three types of vertical motion are studied: kinematic (PE) vertical motion, quasigeostrophic (QG) vertical motion, and vertical motion associated with an unbalanced component of the flow, partially due to inertial-gravity waves (IGW). The latter vertical motion is computed as the difference between the PE vertical motion and “balanced” vertical motion derived from Krishnamurti's balanced omega equation. In addition, the upper-level ageostrophic flow is discussed as it relates to the patterns of divergence associated with various jet curvatures. The PE model was run out to 12 h to we how straight-line (SL), cyclonic (CY), and anticyclonic (AC) curvature affects jet-streak kinematics. At the initial time, a two-cell pattern of vertical motion was found for the CY and AC jet streaks as opposed to the four-cell pattern associated with the SL jet streak. Als...

The Impact of Dynamic Data Assimilation on the Numerical Simulations of the QE II Cyclone and an Analysis of the Jet Streak Influencing the Precyclogenetic Environment

A mesoscale numerical model is combined with a dynamic data assimilation via Newtonian relaxation, or 'nudging', to provide initial conditions for subsequent simulations of the QE II cyclone. Both the nudging technique and the inclusion of supplementary data are shown to have a large positive impact on the simulation of the QE II cyclone during the initial phase of rapid cyclone development. Within the initial development period (from 1200 to 1800 UTC 9 September 1978), the dynamic assimilation of operational and bogus data yields a coherent two-layer divergence pattern that is not well defined in the model run using only the operational data and static initialization. Diagnostic analysis based on the simulations show that the initial development of the QE II storm between 0000 UTC 9 September and 0000 UTC 10 September was embedded within an indirect circulation of an intense 300-hPa jet streak, was related to baroclinic processes extending throughout a deep portion of the troposphere, and was associated with a classic two-layer mass-divergence profile expected for an extratropical cyclone.

Polar Airstream Cyclogenesis in the Australasian Region: A Composite Study Using ECMWF Analyses

Abstract Cold-air disturbances that form upstream of the main polar frontal cloud band and evolve into comma shapes are observed in almost any satellite image of the midlatitudes of the Southern Hemisphere. This study describes general features of the three-dimensional structure and dynamics of these polar airstream developments using composites from 30 typical cases from the Australasian area constructed from analyses prepared by the European Centre for Medium-Range Weather Forecasts (ECMWF). The major finding from this study is that an upstream upper-level southwesterly jet streak, present in each of the 30 cases at genesis, is necessary for development. Composites of initial stages showed the surface vorticity center coupled with the poleward exit region of this upper jet and directly beneath the cyclonic-vorticity advection maximum by the 300-hPa flow relative to the translating disturbance. This net import of cyclonic vorticity acts initially to amplify the upper trough. Near the surface, the environ...

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.