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Analysis and Diagnosis of the Composition of Ageostrophic Circulations in Jet-Front Systems

Jean-Pierre CammasObservatoire de Physique du Globe de Clermont-Ferrand, Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Ferrand, France

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Daniel RamondObservatoire de Physique du Globe de Clermont-Ferrand, Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Ferrand, France

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Abstract

Diagnostic case studies of ageostrophic circulations in upper-tropospheric jet-front systems are presented, using the numerical analyses of the ECMWF model as data sources. The impact of the ECMWF analysis procedure on the retrieved ageostrophic fields is first discussed. Upper-level fields of the ageostrophic wind and its divergence are computed in the natural coordinates system in order to analyze the composition of the transverse (cross-stream) and the alongstream ageostrophic components that are induced by alongstream wind variations and curvature effects respectively. The analysis shows that the curvature effects can contribute in a predominant way to the upper-tropospheric divergence field in entrance or exit regions of jet streaks, so as to sometimes inhibit the expected transverse circulations. Several transverse indirect circulations are displayed in the exit regions of jet streaks. A case of lateral shift towards the anticyclonic side of the jet axis of an indirect circulation is discussed with respect to the conceptual approach based on the frontogenetic forcing in highly idealized two-dimensional flow configurations.

Abstract

Diagnostic case studies of ageostrophic circulations in upper-tropospheric jet-front systems are presented, using the numerical analyses of the ECMWF model as data sources. The impact of the ECMWF analysis procedure on the retrieved ageostrophic fields is first discussed. Upper-level fields of the ageostrophic wind and its divergence are computed in the natural coordinates system in order to analyze the composition of the transverse (cross-stream) and the alongstream ageostrophic components that are induced by alongstream wind variations and curvature effects respectively. The analysis shows that the curvature effects can contribute in a predominant way to the upper-tropospheric divergence field in entrance or exit regions of jet streaks, so as to sometimes inhibit the expected transverse circulations. Several transverse indirect circulations are displayed in the exit regions of jet streaks. A case of lateral shift towards the anticyclonic side of the jet axis of an indirect circulation is discussed with respect to the conceptual approach based on the frontogenetic forcing in highly idealized two-dimensional flow configurations.

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