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A Case Study of Saharan Cyclogenesis

C. D. ThorncroftDepartment of Meteorology, University of Reading, Reading, United Kingdom

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H. A. FlocasLaboratory of Meteorology, Department of Applied Physics, University of Athens, Athens, Greece

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Abstract

A case of Saharan cyclogenesis associated with the equatorward intrusion of a trough at the end of the Atlantic storm track is investigated. It is shown that a potential vorticity anomaly resulting from a baroclinic wave life cycle associated with the polar jet interacts with the low-level baroclinicity over subtropical Africa beneath the subtropical jet. It is suggested that low-latitude synoptic-scale cyclogenesis events of this type can be triggered only by upper-level potential vorticity anomalies if they have a sufficient depth scale and that the interaction may be aided by the presence of low static stability associated with a well-mixed boundary layer. A weak cold front also forms during the cyclogenesis event associated with the convergence of the baroclinicity of the polar trough front with the baroclinicity of the subtropical heat low.

Corresponding author address: Dr. Chris D. Thorncroft, Department of Meteorology, University of Reading, 2 Earley Gate, Whiteknights, P.O. Box 239, Reading RG6 2AU United Kingdom.

Abstract

A case of Saharan cyclogenesis associated with the equatorward intrusion of a trough at the end of the Atlantic storm track is investigated. It is shown that a potential vorticity anomaly resulting from a baroclinic wave life cycle associated with the polar jet interacts with the low-level baroclinicity over subtropical Africa beneath the subtropical jet. It is suggested that low-latitude synoptic-scale cyclogenesis events of this type can be triggered only by upper-level potential vorticity anomalies if they have a sufficient depth scale and that the interaction may be aided by the presence of low static stability associated with a well-mixed boundary layer. A weak cold front also forms during the cyclogenesis event associated with the convergence of the baroclinicity of the polar trough front with the baroclinicity of the subtropical heat low.

Corresponding author address: Dr. Chris D. Thorncroft, Department of Meteorology, University of Reading, 2 Earley Gate, Whiteknights, P.O. Box 239, Reading RG6 2AU United Kingdom.

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