Theoretical and Applied Climatology
, Volume 76, Issue 3, pp 189202
First online:
Rotation of midlatitude binary cyclones: a potential vorticity approach
 B. ZivAffiliated withThe Open University of Israel, TelAviv, Israel
 , P. AlpertAffiliated withDepartment of Geophysics and Planetary Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
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¶Two cyclonic vortices close to each other, a ‘binary cyclone’ or ‘binary system’, tend to rotate cyclonically relative to one another and to merge, i.e. the “Fujiwhara effect”. The point vortex model that represents barotropic binary cyclones predicts their rotation features as follows. The rotation rate is proportional linearly to the sum of the cyclones’ intensities and inversely to the square of their separation distance while the more intense cyclone rotates slower. Our earlier observational analysis of 1423 midlatitude binary cyclones (Ziv and Alpert, 1995) showed a reasonable fit to theory, except for the absence of a correlation between individual speeds and intensities within the binary systems, and a reversal of the inverse rotationseparation relationship at the range of 1400–1800 km.
This study is the first attempt to describe the midlatitude binary systems using potential vorticity concepts (PV thinking), which implies that a binary interaction takes place between the 3D flow patterns induced by upperPV or surfacethermal anomalies rather than by the surface cyclones alone. It is argued that the upperanomalies dominate the rotation process, and hence the rotational speeds of the interacting surface cyclones are more closely correlated with the relative intensities of their corresponding upperlevel anomalies rather than with their own intensities, as reflected in weather charts. Data analysis indicates that midlatitude binary cyclones are normally associated with at least one upperPV anomaly. This explains the absence of a correlation between the rotation speed and the intensity of the surface cyclones there.
A unique type of a midlatitude binary system is identified, in which one cyclone coincides with an upper major PVanomaly and the other moves along the periphery of the former. Such a binary system is entitled here the ‘Contact Binary System’ (CBS), in contrast with remote interacting systems implied by the point vortex theory.
Analytical considerations yield an increase in the rotation rate with separation for CBSs of separation smaller than ∼1000–1500 km, in contrast to the normal decrease with R ^{2}. The contribution of CBSs is suggested here to explain the abnormal increase in rotation rate at 1400–1900 km range.
 Title
 Rotation of midlatitude binary cyclones: a potential vorticity approach
 Journal

Theoretical and Applied Climatology
Volume 76, Issue 34 , pp 189202
 Cover Date
 200312
 DOI
 10.1007/s007040030011x
 Print ISSN
 0177798X
 Online ISSN
 14344483
 Publisher
 SpringerVerlag
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