 |
| Titel |
Tropical cyclogenesis in a tropical wave critical layer: easterly waves |
| VerfasserIn |
T. J. Dunkerton, M. T. Montgomery, Z. Wang |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 9, no. 15 ; Nr. 9, no. 15 (2009-08-06), S.5587-5646 |
| Datensatznummer |
250007561
|
| Publikation (Nr.) |
 copernicus.org/acp-9-5587-2009.pdf |
|
|
|
|
|
| Zusammenfassung |
| The development of tropical depressions within tropical waves over the
Atlantic and eastern Pacific is usually preceded by a "surface low along
the wave" as if to suggest a hybrid wave-vortex structure in which flow
streamlines not only undulate with the waves, but form a closed circulation
in the lower troposphere surrounding the low. This structure, equatorward of
the easterly jet axis, is identified herein as the familiar critical layer
of waves in shear flow, a flow configuration which arguably provides the
simplest conceptual framework for tropical cyclogenesis resulting from
tropical waves, their interaction with the mean flow, and with diabatic
processes associated with deep moist convection. The recirculating Kelvin
cat's eye within the critical layer represents a sweet spot for tropical
cyclogenesis in which a proto-vortex may form and grow within its parent
wave. A common location for storm development is given by the intersection
of the wave's critical latitude and trough axis at the center of the cat's
eye, with analyzed vorticity centroid nearby. The wave and vortex live
together for a time, and initially propagate at approximately the same
speed. In most cases this coupled propagation continues for a few days after
a tropical depression is identified. For easterly waves, as the name
suggests, the propagation is westward. It is shown that in order to
visualize optimally the associated Lagrangian motions, one should view the
flow streamlines, or stream function, in a frame of reference translating
horizontally with the phase propagation of the parent wave. In this
co-moving frame, streamlines are approximately equivalent to particle
trajectories. The closed circulation is quasi-stationary, and a dividing
streamline separates air within the cat's eye from air outside. The critical
layer equatorward of the easterly jet axis is important to tropical
cyclogenesis because its cat's eye provides (i) a region of cyclonic
vorticity and weak deformation by the resolved flow, (ii) containment of
moisture entrained by the developing gyre and/or lofted by deep convection
therein, (iii) confinement of mesoscale vortex aggregation, (iv) a
predominantly convective type of heating profile, and (v) maintenance or
enhancement of the parent wave until the vortex becomes a self-sustaining
entity and emerges from the wave as a tropical depression. The entire
sequence is likened to the development of a marsupial infant in its mother's
pouch. These ideas are formulated in three new hypotheses describing the
flow kinematics and dynamics, moist thermodynamics and wave/vortex
interactions comprising the "marsupial paradigm". A survey of 55 named
tropical storms in 1998–2001 reveals that actual critical layers sometimes
resemble the ideal east-west train of cat's eyes, but are usually less
regular, with one or more recirculation regions in the co-moving frame. It
is shown that the kinematics of isolated proto-vortices carried by the wave
also can be visualized in a frame of reference translating at or near the
phase speed of the parent wave. The proper translation speeds for wave and
vortex may vary with height owing to vertical shear and wave-vortex
interaction. Some implications for entrainment/containment of vorticity and
moisture in the cat's eye are discussed from this perspective, based on the
observational survey. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|