Characteristics of Potential Vorticity Mixing by Breaking Rossby Waves in the Vicinity of a Jet
Characteristics of potential vorticity (PV) mixing by breaking Rossby waves have been investigated, using simple models based on method of contour dynamics/contour surgery and analyzing upper-tropospheric data obtained from a high-resolution general circulation model (GCM) and National Meteorological Center (NMC). Particular emphasis is placed upon influence of planetary-scale flow structure on evolution of synoptic-scale waves and how the latter may feed back on the former when there is a diffluence in the planetary-scale flow. Experiments with the simple models show many interesting results. Synoptic-scale waves with sufficiently large amplitudes in circular basic flows, break only outward. Inward breaking occurs only when the basic flow is contrived so that the magnitude of rate of strain just inside the vortex is greater than that outside. The cross-jet asymmetry in the magnitude of rate of strain and that in the location of critical lines are closely related. The relationship between the cross-jet asymmetry in wave breaking and that in the magnitude of rate of strain or location of critical lines is confirmed by experiments with straight-jet basic flows; waves on the jet tend to break more readily toward the side of closer critical line, which is also the side of greater magnitude of rate of strain. When the basic flow has zonal asymmetry, waves tend to amplify in the region of diffluence, due to the reduced local zonal velocity and stretching by the basic meridional flow associated with the diffluence. Finally, asymmetry in wave breaking can be induced by imposing stationary or steadily-propagating waves on the basic flow. The induced asymmetry is quite simple: the anticyclonic phase of "synoptic-scale" waves breaks more readily where there is anticyclonic curvature in "planetary-scale" flow, and vice versa. Output of a GCM run has been analyzed using Ertel's potential vorticity, q, on isentropic surfaces in the upper troposphere. The analyses show that the breaking asymmetry induced by planetary-scale curvature in the basic flow found in experiments with the simple models is also present in much more complex flows, although not as clear-cut. Strongly diffluent low-frequency flow structure is defined as "blocking" and is examined for its relationship with high-frequency synoptic-scale eddies. Five episodes of blocking are examined, using q forcing analyses and contour advection with surgery (CAS). The results demonstrate that the primary direct forcing of the blocking signatures is low-frequency flow advection, manifesting itself as a quasi-stationary Rossby wave amplification, breaking, and dissipation, throughout its life cycle. This life cycle is observed in time series of low-frequency q as well. Synoptic-scale eddies are breaking such that they tend to reinforce the diffluence in the low-frequency flow. This pattern of high-frequency forcing is in agreement with results reported by other investigators. These results are confirmed by analyses of NMC data during two observed blocking events with one minor exception; high-frequency forcing has an important direct contribution in forcing the low-frequency diffluence during the first 6 or 7 days of one episode. This exception suggests that there may be more than one mechanism to initiate a blocking. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.).
- Pub Date:
- January 1995
- Physics: Atmospheric Science