A Technique for Diagnosing Three-Dimensional Ageostrophic Circulations in Baroclinic Disturbances on Limited-Area Domains

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  • 1 Department of Atmospheric Science, State University of New York at Albany, Albany, New York
  • | 2 Earth and Environmental Science Division Los Alamos National Laboratory, Los Alamos. New Mexico
  • | 3 Department of Atmospheric Science, State University of New York at Albany, Albany, New York
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Abstract

The methodology developed by Keyser et al. for representing and diagnosing three-dimensional vertical circulations in baroclinic disturbances using a two-dimensional vector streamfunction, referred to as the psi vector, is restricted to f-plane channel-model geometry. The vertical circulation described by the psi vector consists of the irrotational (or divergent) part of the ageostrophic wind and the vertical velocity. A key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations, allowing separation of a three-dimensional vertical circulation into two two-dimensional components, and thus objective assessment of the extent to which a three-dimensional vertical circulation is oriented in a preferred direction. Here the methodology for determining the psi vector is modified to be suitable for real-data applications. The modifications consists of reformulating the diagnostic equations to apply to conformal map projections and to limited-area domains; despite the desirability of incorporating topography, this is deferred to future research. The geostrophic wind is defined in terms of constant Coriolis parameter, rendering it nondivergent and thus confining the horizontal divergence to the ageostrophic wind. The ageostrophic wind is partitioned into harmonic, rotational, and divergent components. This three-field with its counter-part determined from the psi-vector calculation.

The modified psi-vector methodology is illustrated for two well-documented East Coast midlatitude cyclones. The first case (the President's Day storm: 1200 UTC 19 February 1979) considers an interpretation that ascent in the vicinity of a curved upper-level jet-front system may be viewed as a superposition of contributions from cross-stream divergent ageostrophic flow associated wit a jet streak and from alongstream divergent ageostrophic flow associated with a baroclinic wave. The second case (the megalopolitan strom: 1200 UTC 11 February 1983) addresses the hypothesis of Uccellini and Kocin that vertical circulations transverse to meridionally displaced upper-tropospheric jet streaks are coupled in a lateral sense. In both of these cases, the diagnoses reveal that the cross-stream component of the divergent ageostrophic circulations isolates meaningful mesoscale signatures coinciding with regions of precipitation and ascent in the vicinity of upper-level jet-front systems whereas the alongstream component is indicative of synoptic-scale vertical motion. Furthermore, it is found that the cross-contour ageostrophic flow, necessary for a Lagrangian rates of change of kinetic energy in jet entrance and exit regions, is due primarily to the nondivergent (i.e., harmonic plus rotational) ageostrophic wind. This result suggests that the practice of linking cross-contour ageostrophic winds and vertical motions in jet entrance and exit regions in the qualitative assessment of energy tranformations in these regions may be problematic in the case of upper-level jet-front system situated in three-dimensional flows.

Abstract

The methodology developed by Keyser et al. for representing and diagnosing three-dimensional vertical circulations in baroclinic disturbances using a two-dimensional vector streamfunction, referred to as the psi vector, is restricted to f-plane channel-model geometry. The vertical circulation described by the psi vector consists of the irrotational (or divergent) part of the ageostrophic wind and the vertical velocity. A key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations, allowing separation of a three-dimensional vertical circulation into two two-dimensional components, and thus objective assessment of the extent to which a three-dimensional vertical circulation is oriented in a preferred direction. Here the methodology for determining the psi vector is modified to be suitable for real-data applications. The modifications consists of reformulating the diagnostic equations to apply to conformal map projections and to limited-area domains; despite the desirability of incorporating topography, this is deferred to future research. The geostrophic wind is defined in terms of constant Coriolis parameter, rendering it nondivergent and thus confining the horizontal divergence to the ageostrophic wind. The ageostrophic wind is partitioned into harmonic, rotational, and divergent components. This three-field with its counter-part determined from the psi-vector calculation.

The modified psi-vector methodology is illustrated for two well-documented East Coast midlatitude cyclones. The first case (the President's Day storm: 1200 UTC 19 February 1979) considers an interpretation that ascent in the vicinity of a curved upper-level jet-front system may be viewed as a superposition of contributions from cross-stream divergent ageostrophic flow associated wit a jet streak and from alongstream divergent ageostrophic flow associated with a baroclinic wave. The second case (the megalopolitan strom: 1200 UTC 11 February 1983) addresses the hypothesis of Uccellini and Kocin that vertical circulations transverse to meridionally displaced upper-tropospheric jet streaks are coupled in a lateral sense. In both of these cases, the diagnoses reveal that the cross-stream component of the divergent ageostrophic circulations isolates meaningful mesoscale signatures coinciding with regions of precipitation and ascent in the vicinity of upper-level jet-front systems whereas the alongstream component is indicative of synoptic-scale vertical motion. Furthermore, it is found that the cross-contour ageostrophic flow, necessary for a Lagrangian rates of change of kinetic energy in jet entrance and exit regions, is due primarily to the nondivergent (i.e., harmonic plus rotational) ageostrophic wind. This result suggests that the practice of linking cross-contour ageostrophic winds and vertical motions in jet entrance and exit regions in the qualitative assessment of energy tranformations in these regions may be problematic in the case of upper-level jet-front system situated in three-dimensional flows.

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