ABSTRACT FOR THE NINTH EXTRATROPICAL CYCLONE WORKSHOP

CORRESPONDING AND PRESENTING AUTHOR: Philip Cunningham
TITLE: Graduate Student
AFFILIATION: Department of Atmospheric Science, State University of New York at Albany, 1400 Washington Avenue, ES-234, Albany, NY 12222
PHONE NUMBER: (518) 442-4515
EMAIL:cunning@atmos.albany.edu
TOPIC OF PRESENTATION: 2) Scale Interaction

Numerical Modelling of Jet Streak Dynamics

Philip Cunningham and Daniel Keyser

Department of Earth and Atmospheric Sciences
University at Albany
State University of New York
Albany, NY 12222, USA

Jet streaks are ubiquitous in extratropical flow and, because of their well-documented divergent vertical circulations and association with cyclogenesis and severe weather, they have received significant attention from the synoptic community (e.g., Bluestein 1993, pp. 394-407). However, while there exists a number of conceptual models relating jet streaks to vertical motion, little attention has been devoted to dynamical treatment of these features. Questions arise as to how jet streaks may be represented dynamically, and hence how they may be studied using a body of idealised conceptual and numerical models.

Previous diagnostic (e.g., Keyser et al. 1989, 1992) and numerical modelling (Moore and VanKnowe 1992) studies of jet streaks suggest that the ageostrophic flow in entrance and exit regions is mainly rotational, and hence that nondivergent barotropic dynamics may be of primary importance for jet streaks. This hypothesis will be investigated using a hierarchy of f- and beta-plane channel models: nondivergent barotropic vorticity equation; shallow-water primitive equation; two-layer adiabatic primitive equation. Systematic variation of nondimensional parameters (i.e., Rossby number and, in the latter two models, Froude number) in these models will allow investigation of the effects of divergence on jet streak motion and evolution.

Initially, the focus of this study will be on the dynamical specification of jet streaks and their subsequent motion and evolution. It has been suggested (e.g., Bluestein 1993, p. 237; Kocin and Uccellini 1990, p. 43; Weglarz 1994) that a jet streak may be thought of as a positive-negative couplet of potential vorticity. In addition, preliminary observations suggest that an isotach maximum located on the tropopause, as defined by the 1.5 PVU surface, may be associated with a cold-warm dipole of potential temperature on that surface. This dipolar vortex structure has been investigated extensively in the fluid dynamics literature, and may be studied analytically using discrete distributions of vorticity, such as point vortices and vortex patches (e.g., Lamb 1932, Arts. 155 and 158, respectively), and continuous distributions of vorticity, such as modons (e.g., Larichev and Reznik 1976). For our numerical study, we shall opt for a more general representation, consisting of a pair of elliptic vortices of opposing sign that may have differing strengths and sizes and that we shall call a "dipolar elliptic vortex" (DEV).

Assuming that a jet streak may be reasonably represented by a DEV, jet streak motion and evolution may be examined using PV thinking, whereby each vortex is advected and deformed by the flow due to itself and its opposing DEV member. An initial "control" experiment will be described which considers a simple DEV, with vortices of identical strength and size but opposing sign, in isolation on an f-plane. Further experiments will be considered that allow for more general DEVs, in which the vortices may be of differing strength and size, and for more general background environments, allowing for meridional gradients of PV, both planetary and otherwise. In addition, a DEV will be superposed on a Rossby-Haurwitz wave to investigate the effects of curvature and to assess Shapiro's (1982) conceptual model of jet streak motion through synoptic-scale waves. The two-layer model will be used to address the effects of vertical shear, and in particular how and under what conditions jets in upper and lower layers remain vertically coherent. For all these experiments, ageostrophic circulations can be diagnosed in the shallow water and two-layer models to assess the dominance of the rotational part, and hence to evaluate the importance of divergent circulations to jet streak motion and evolution.

REFERENCES:

Bluestein, H. B., 1993: Synoptic-Dynamic Meteorology in Midlatitudes, Volume II. Oxford University Press, 594 pp.

Keyser, D., B. D. Schmidt, and D. G. Duffy, 1989: A technique for representing three-dimensional vertical circulations in baroclinic disturbances. Mon. Wea. Rev., 117, 2463-2494.

Keyser, D., B. D. Schmidt, and D. G. Duffy, 1992: Quasigeostrophic diagnosis of three-dimensional ageostrophic circulations in an idealized baroclinic disturbance. Mon. Wea. Rev., 120, 698-730.

Kocin, P. J., and L. W. Uccellini, 1990: Snowstorms Along the Northeastern Coast of the United States: 1955 to 1985. Meteor. Monogr., 22, No. 44, 280 pp.

Lamb, H., 1932: Hydrodynamics. Cambridge University Press, 738 pp.

Larichev, V. D., and G. M. Reznik, 1976: Two-dimensional solitary Rossby waves. Dokl. Akad. Nauk SSSR, 231, 12-13.

Moore, J. T., and G. E. VanKnowe, 1992: The effect of jet-streak curvature on kinematic fields. Mon. Wea. Rev., 120, 2429-2441.

Shapiro, M. A., 1982: Mesoscale weather systems of the central United States. CIRES/NOAA Tech. Rep., University of Colorado, 78 pp.

Weglarz, R. P., and Y.-L. Lin, 1994: Geostrophic adjustment and jetogenesis forced by impulsive and propagating zonal momentum sources. Part I: Three- dimensional response of a rotating homogeneous atmosphere of finite depth. J. Atmos. Sci., submitted.

Phil Cunningham
cunning@atmos.albany.edu