[For additional information, go to Phil's main page on jet streaks here.]
Jet streaks, defined as localized wind speed maxima along the jet stream, are a common feature of the extratropical atmosphere, particularly at the level of the tropopause, and have assumed a prominent role in synoptic meteorology, largely in recognition of their association with cyclogenesis and severe weather. There is significant observational evidence (e.g., Hakim 1997; Pyle 1997) that in many cases jet streaks result from the superposition of coherent monopolar and dipolar vortices of mesoscale dimensions (vortex radius ~500 km) with the enhanced potential vorticity gradients that constitute the extratropical tropopause. Such vortices often are long lived, with documented lifetimes of up to several weeks, and may be associated with multiple jet streaks during their life cycles.
Dynamical interpretation of the structure and evolution of jet streaks from the perspective of coherent vortices (Cunningham and Keyser 1996, 1997) shows that idealized analytical and numerical solutions of barotropic vortex dipoles, both in isolation and in the presence of jet-like and wavelike background flows, offer plausible explanations for the structure, motion, and evolution characteristic of observed atmospheric jet streaks. Nevertheless, these solutions are unable to explain certain details of jet-streak dynamics, such as the role of divergent circulations, and the life cycles of jet streaks and the coherent structures with which they are associated.
Motivated by observations and guided by theoretical perspectives applicable to coherent structures, geophysical turbulence, and baroclinic-wave life cycles, we employ stratified quasigeostrophic (QG) and primitive equation (PE) models, formulated for f- and beta-plane geometries, to investigate the following aspects of jet-streak structure and dynamics: (i) the three-dimensional structure of jet streaks and their representation in terms of coherent structures; and (ii) the motion and evolution of jet streaks as represented by coherent structures, both in isolation and in the presence of background flows characteristic of synoptic- and planetary-scale environments in the atmosphere. In particular, monopolar and dipolar vortices are examined in isolation and in conjunction with progressively general barotropic and baroclinic background flows (i.e., zonal flows with meridional and/or vertical shear and wavelike flows) to assess the applicability of these vortex features as dynamical models for the structure, motion, and evolution of jet streaks in three dimensions. To provide an example of the representation of jet streaks in terms of coherent vortices, Fig. 1 shows selected fields associated with the QG vortex dipole solution due to Berestov (1979). This solution displays many of the features commonly attributed to jet streaks in the atmosphere (e.g., Uccellini 1990), and may be considered to be an idealized conceptual representation of a jet streak.
Berestov, A. L., 1979: Solitary Rossby waves. Izv. Acad. Sci. USSR
Atmos. Oceanic Phys., 15, 443-447.
Cunningham, P., and D. Keyser, 1996: Numerical modelling of jet-streak
dynamics. Preprints, Seventh Conference on Mesoscale Processes, Reading,
U.K., Amer. Meteor. Soc., 20-22.
Cunningham, P., and D. Keyser, 1997: Analytical and numerical modelling
of jet-streak dynamics. Preprints, Eleventh Conference on Atmospheric
and Oceanic Fluid Dynamics, Tacoma, WA, Amer. Meteor. Soc., 106-110.
Hakim, G. J., 1997: Extratropical cyclogenesis in terms of baroclinic
vortex dynamics. Ph.D. dissertation, University at Albany, State
University of New York, 210 pp.
Pyle, M. E., 1997: A diagnostic study of jet streaks: Kinematic
signatures and relationship to coherent disturbances. M.S. thesis,
University at Albany, State University of New York, 174 pp.
Uccellini, L. W., 1990: Processes contributing to the rapid development of
extratropical cyclones. Extratropical Cyclones, C. W. Newton and E. O.
Holopainen, Eds., Amer. Meteor. Soc., 81-105.