Structure and evolution of upper-tropospheric jet streaks in a stratified quasigeostrophic model (original) (raw)
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Coherent Structures in the Extratropical Atmosphere: A Dynamical Interpretation of Jet Streaks
The objective of this prospectus is to motivate and formulate a research plan to investigate the dynamics of jet streaks, building on recent idealised analytical and numerical work conducted on this topic using barotropic models (Cunningham 1997, hereafter C97). The hypothesis to be investigated is that jet streaks often are associated with the superposition of mesoscale coherent vortices of both monopolar and dipolar nature on a larger-scale westerly background ow. Motivated by observations and guided by theoretical perspectives applicable to coherent structures, geophysical turbulence, and baroclinic-wave life cycles, a hierarchy of idealised dynamical models (nondivergent barotropic; shallow-water; strati ed quasigeostrophic and primitive equation), formulated for fand-plane geometries, will be employed to investigate the following general aspects of jet-streak structure and dynamics: (i) the three-dimensional structure of jet streaks and their representation in terms of coherent structures; (ii) the motion and evolution of jet streaks as represented by coherent structures, both in isolation and in the presence of background ows representative of characteristic synoptic-and planetary-scale environments in the atmosphere; and (iii) the nature of dynamical balance in jet streaks. Speci c details of this investigation and relevant issues and questions to be addressed regarding these general aspects will be discussed herein. Furthermore, issues requiring investigation that will not be addressed directly by this study, such as the origin of jet streaks and of the associated coherent structures, will be discussed brie y. It is suggested that this study will serve simultaneously to address the limitations of C97 and to provide the next level of generality beyond that study in a hierarchical dynamical explanation of jet streaks.
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
Monthly Weather Review, 1999
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 uppertropospheric jet streak associated with the precursor disturbance prior to cyclogenesis, as well as in the developing lower-tropospheric cyclone. Computation of the local EKE budget during ERICA IOP 2 indicates that the Reynolds stress plays an important role in jet streak intensification over North America. Analysis of the Reynolds stress reveals that the contribution of this term is determined primarily by the relative orientation of the perturbation horizontal wind velocity and the dilatation axis of the time-mean flow. In regions where the perturbation wind velocity is oriented within 45Њ of normal to the dilatation axis of the time-mean flow, the contribution of the Reynolds stress to the EKE tendency is positive. The presence of a ridge over western North America favors jet streak intensification through the Reynolds stress as northerly perturbation flow east of the ridge axis possesses a favorable orientation with respect to the dilatation axes of the time-mean flow over central North America. Local EKE increases accompany strengthening transverse divergent circulations, thus facilitating the downward advection of stratospheric potential vorticity and eventually resulting in the development of a mobile upper trough. This sequence is consistent with the preference for mobile upper-trough genesis over central North America in the presence of a northerly flow component, a finding documented previously by Sanders.
Vortex formation by unstable oceanic jets
ESAIM: Proceedings, 1999
In a two-layer quasi-geostrophic model, we investigate the mixed barotropic-baroclinic instability of a thin zonal jet. Given the jet velocity profile at the surface, the deformation radius and layer depths, the three parameters governing the instability are the ratio of layerwise maximum velocities (U 2 /U 1), the planetary vorticity gradient (β-effect), and the wavenumber (k) of the perturbation. First, simple criteria provide thresholds in this parameter space for the onset of instability. Growth rates of monochromatic perturbations are then computed and their most unstable wavelengths are compared with oceanic observations. With a nonlinear numerical model, high-Reynolds-number evolutions of the perturbed jet are computed in the (U 2 /U 1 , β, k) space: wave breaking and vortex formation occur at small β, while for larger values of β, the perturbation equilibrates into meanders. Multiple waves are also observed. Instability is minimum when the jet is confined in the upper layer (U 2 /U 1 = 0). Finally, vortex formation by this unstable jet is specifically studied for a localized perturbation.
Jet Streaks in the Gulf Stream
Journal of Physical Oceanography, 1999
Mesoscale alongstream speed changes of the Gulf Stream are diagnosed from an array of current meters at depths 400, 700, and 1000 m, near 68ЊW, during the development of steep [ratio of ''amplitude'' to ''wavelength'' O(1)] meanders. Speed maxima (jet streaks) are generally found between trough and crest axes in steep meanders with local speed minima near the trough and crest axes. Speed changes along streamlines can be quite dramatic. Speed changes along the jet axis, between jet streaks and local minima in excess of 0.60, 0.40, and 0.35 m s Ϫ1 , are observed at depth 400, 700, and 1000 m, respectively. This is in comparison with peak speeds in a frontal coordinates system mean of 1.22, 0.67, and 0.28 m s Ϫ1 , at depth 400, 700, and 1000 m, respectively, from a previous study.
Subtropical Jet Streaks over the South Pacific
Monthly Weather Review, 1997
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 energy of the jet streaks. In contrast, in 13 cases it appeared that little or no connection occurred between tropical convective heat sources and the accelerating jets. In fact, it seemed that midlatitude wave activity was the important factor during the acceleration phase of most of these 13 cases. For the remaining 10 cases, it was difficult to conclude whether tropical forcing was more important than middle latitude forcing; however, it appeared that tropical forcing, albeit weaker than the 17 aforementioned cases, did play a forcing role. An examination of the case composites in each of these three categories revealed that the energy cycle for the tropically forced cases consisted of a generation of divergent kinetic energy, a conversion of divergent to rotational kinetic energy, and a loss of rotational kinetic energy due to horizontal export and frictional dissipation. Except for the loss of rotational kinetic energy by dissipation, the main energy cycle for the nontropically forced accelerations was the reverse of that for tropically forced jets. Finally, for those 10 cases where the primary region of forcing was uncertain, the composited energy cycle generally consisted of a compromise between the tropically and nontropically forced composites, although there was a significant generation of divergent kinetic energy, as well as a conversion of divergent to rotational kinetic energy, as for all tropically forced cases.
The Divergence Fields Associated with Time-Dependent Jet Streams
Journal of the Atmospheric Sciences, 1999
This study examines the effect of temporal variations in either location or structure of both straight and curved jets on their associated divergence patterns. For each of these jets the time-dependent geopotential height field is prescribed analytically, from which the geostrophic and ageostrophic wind fields are extracted. The divergence fields are calculated separately for the steady state (which was commonly assumed in studies on this subject) and for the following time-dependent cases: progressing, retrograding, intensifying, and weakening jets. In this study, the ''intensification'' of a straight jet implies an increase in the wind speed while in the case of a curved jet the intensification refers to an increase in the meandering aspect, that is, amplitude divided by wavelength.
Depth of a strong jovian jet from a planetary-scale disturbance driven by storms
Nature, 2008
The atmospheres of the gas giant planets (Jupiter and Saturn) contain jets that dominate the circulation at visible levels 1,2 . The power source for these jets (solar radiation, internal heat, or both) and their vertical structure below the upper cloud are major open questions in the atmospheric circulation and meteorology of giant planets 1-3 . Several observations 1 and in situ measurements 4 found intense winds at a depth of 24 bar, and have been interpreted as supporting an internal heat source. This issue remains controversial 5 , in part because of effects from the local meteorology 6 . Here we report observations and modelling of two plumes in Jupiter's atmosphere that erupted at the same latitude as the strongest jet (236 N). The plumes reached a height of 30 km above the surrounding clouds, moved faster than any other feature (169 m s 21 ), and left in their wake a turbulent planetary-scale disturbance containing red aerosols. On the basis of dynamical modelling, we conclude that the data are consistent only with a wind that extends well below the level where solar radiation is deposited.