Gregory J. Hakim
National Center for Atmospheric Research, Boulder, CO
80307
Monthly Weather Review, 128, in 385--406.
This paper poses and tests the hypothesis that some of
the synoptic-scale and mesoscale tropopause-based disturbances that produce
organized vertical motion and induce surface cyclones in the extratropical
troposphere are vortex-like coherent structures. Based on the theory
of nonlinear waves and vortices, tests are constructed and applied to observations
of relative vorticity maxima for a 33-winter climatology at 500 hPa and
\3D composites for a single winter season. The method is designed
to determine the following disturbance properties: nonlinearity, QGPV--streamfunction
relationship, speed, and trapping of fluid particles. These properties
are determined for four disturbance-amplitude categories, defined here
in terms of 500 hPa relative vorticity.
The results show that, on average, 500 hPa relative vorticity
maxima are localized monopolar vortices with length scales radii) of approximately
500--800 km; there is a slight increase in length scale with disturbance
amplitude. Nonlinearity is O(1) or greater for all amplitude categories,
approaching O(10) for the strongest disturbances. Trapping of fluid particles,
estimated by the presence of closed contours of potential vorticity on
isentropic surfaces near the tropopause, requires greater than O(1) nonlinearity;
the threshold disturbance amplitude is approximately 8\vu in the vertical
component of 500 hPa relative vorticity and $-8$ K in anomaly tropopause
potential temperature. The vortices move westward with respect to
the background flow, with a slight northward drift. The observational evidence
does not support an interpretation of these features in terms of modons
or solitary waves.