P Vectors as a Diagnostic Tool for Synoptic-Scale Circulations
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Authors
Kuo, Hung-Chi
Nuss, Wendell A.
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Date of Issue
1995-03
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Abstract
The quasigeostrophic geopotential tendency equation is derived using P vectors in both pressure and entropy
coordinates. This vector form of the geostrophic forcing in the geopotential tendency equation is similar to the
Q-vector form of thew equation. It is shown that the horizontal components of Pare the advection of geostrophic
momentum, and the vertical component of P is the horizontal temperature advection. The P vectors are shown
to be related to Q and C vectors, as well as the ageostrophic circulation. The three-dimensional pseudocurl of
the P vector gives the C vector that equals the ageostrophic pseudovorticity in the quasigeostrophic model. The
horizontal components of the pseudocurl of P are perpendicular and proportional to the Q. The horizontal divergence
of the P vector is the geostrophic absolute vorticity advection while the three-dimensional divergence
of the P vector is the geostrophic quasigeostrophic potential vorticity advection. The ageostrophic wind can be
partitioned into the P vector (geostrophic advective) and isallobaric wind contributions. A numerical simulation of an idealized cyclone is used to characterize the distribution of the P vectors and
P-vector diagnostics in order to demonstrate their potential application to the diagnosis of synoptic-scale circulations.
The distribution of the P vectors clearly indicates the advections of geostrophic momentum and temperature
that characterize cyclogenesis. An examination of the P vectors and the isallobaric wind demonstrates that
the P vectors provide insight into the ageostrophic circulation of the cyclone. Diagnoses of the three-dimensional
P-vector divergence and curl are shown to produce useful depictions of cyclonic vortex spinup and the propagation
of both the large- and smaller-scale features of the system. These diagnostics can be interpreted from a
variety of perspectives, including the height tendency and the advection of quasigeostrophic potential vorticity ..
The use of P vectors to diagnose synoptic-scale circulations appears to provide potentially useful insights into
the dynamics of synoptic-scale disturbances not readily obtained from other diagnoses.
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Meteorology
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Monthly Weather Review, Volume 123, pp. 776-789, March 1995.
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.