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The impact of various potential‐vorticity anomalies on multiple frontal cyclogenesis events
Author(s) -
Zhang DaLin,
Cheng William Y. Y.,
Gyakum John R.
Publication year - 2002
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/003590002320603449
Subject(s) - cyclogenesis , potential vorticity , extratropical cyclone , geology , climatology , cyclone (programming language) , baroclinity , frontogenesis , anomaly (physics) , trough (economics) , advection , atmospheric sciences , vorticity , meteorology , mesoscale meteorology , physics , vortex , condensed matter physics , economics , macroeconomics , field programmable gate array , computer science , computer hardware , thermodynamics
The influences of various potential vorticity (PV) anomalies on multiple frontal cyclogenesis events are examined using 60 h simulations of a family of six frontal cyclones that developed over the western Atlantic between 0000 UTC 13 March and 1200 UTC 15 March 1992. The cyclogenesis events are characterized by pronounced low‐level baroclinicity along a large‐scale cold front moving over the warm ocean and by multiple propagating perturbations in the two PV rings on the cyclonic‐shear side of an upper‐level jet stream. It is found that the tracks of the frontal cyclones follow closely the distribution of the PV rings, whereas the cyclogenesis occurs in the cold‐frontal zone, and in close proximity to the propagating PV perturbations in the central portion of an upper‐level parent trough. A piecewise PV inversion reveals that the low‐level thermal advection (or the bottom thermal anomaly) contributes the most to the depths of two major frontal cyclones, followed by latent‐heat release and the upper‐level PV anomalies, whereas the opposite order is true for the remaining four weaker frontal cyclones. A series of sensitivity experiments is performed to test the impact of removing the upper‐level PV anomalies individually, or collectively, on the development of a major frontal cyclone (MFC) as an initial‐value problem. It is found that the MFC genesis and its final intensity depend on how much its track departs from the upper‐level PV perturbations. Removing a PV anomaly associated with a major short‐wave trough delays the MFC genesis by 18 h and weakens its final depth by 13 hPa. The largest departure occurs when two upstream PV anomalies are removed collectively, delaying the MFC genesis by 36 h and weakening its final depth by 21 hPa. It appears that the MFC genesis in the first 36 h depends more on the PV anomalies in an inner PV ring, whereas its subsequent development depends more on the PV reservoir in an outer PV ring. It is shown that most of the frontal cyclones can still develop in the absence of diabatic heating, indicating the important role of dry dynamics in controlling the multiple frontal cyclogenesis events. However, the ‘dry’ MFC fails to form a closed isobar when the trough‐related PV anomaly is removed. The associated PV inversion and sensitivity simulation results suggest that even though the low‐level thermal advection plays an important role in deepening the frontal cyclones, it is the upper‐level PV anomalies that provide the necessary forcing for its amplification and the frontal cyclogenesis. It is concluded that the upper‐level propagating multiple PV anomalies play a major role in triggering, but a secondary role in determining the final depth of the multiple frontal cyclones. It is the subsequent growth of the bottom thermal anomaly that accounts for most of the final depth of the frontal cyclones. Copyright © 2002 Royal Meteorological Society