2010-09-06 20:52:25 UTC
Author(s): Riemer M (Riemer, Michael)1,2, Jones SC (Jones, Sarah C.)1,2,
Davis CA (Davis, Christopher A.)3
Source: QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Volume: 134
Issue: 630 Pages: 69-91 Part: Part A Published: JAN 2008
Abstract: The interaction of a tropical cyclone undergoing extratropical
transition (ET) with the midlatitude synoptic-scale flow is investigated
using full-physics numerical experiments with idealized initial conditions.
The emphasis is on the impact on the midlatitude flow downstream of the ET
event. The midlatitude flow is represented by a balanced straight jet
stream. As the tropical cyclone approaches the jet, a ridge-trough couplet
and a distinct jet streak form in the upper-level flow. A midlatitude
cyclone develops rapidly downstream of the ET system and the further
evolution is characterized by downstream baroclinic development.
Based on Hovmoller diagrams, the upper-level development is interpreted as
the excitation and subsequent dispersion of a Rossby wave train on the
potential vorticity gradient associated with the jet. The characteristics of
this wave train are sensitive to the structure of the jet and to moist
processes in the midlatitudes. The tropical cyclone undergoing ET acts as a
sustained forcing for the wave train and the structure of the ET system
impacts the development most significantly one to two wavelengths downstream
Piecewise inversion of potential vorticity, complemented by the partitioning
of the flow into its rotational and divergent parts, is applied to assess
the impact of the ET system quantitatively. Both the cyclonic circulation
and the outflow of the tropical cyclone are important contributors to the
formation and amplification of the ridge-trough couplet. The outflow anomaly
reduces the eastward motion of the ridge-trough couplet significantly and
thus promotes phase-locking between the tropical cyclone and the upper-level
pattern. Copyright (c) 2008 Royal Meteorological Society.
Note the words "sustained forcing".
As I kept on saying ertel potential vorticity is important!