Synoptic-Scale Deformation and Tropical Cloud Bands

Howard B. Bluestein Department of Meteorology, Massachusetts Institute of Technology, Cambridge 02139

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Abstract

Satellite-observed tropical and extratropical cloud bands often look alike. It is hypothesized that synoptic-scale deformation and tropical cloud band orientation are related, since the axis of dilatation associated with the former is related to extratropical frontal cloud band orientation.

This study finds the following evidence for the hypothesis:

  1. The orientations of climatological bands of cloudiness in tropical oceanic areas are for the most part well-correlated with the respective climatological, gradient-level, axis-of-dilatation orientations.

  2. Significant correlations were found between the cloud-band and axis-of-dilatation orientations of the objectively analyzed nondivergent wind at 950, 850 and 700 mb during BOMEX Phase IV. The objective analyses made use of synchronous and asynchronous data. The significance decreased with height, and was very small at 450 mb. On days when cloud bands were zonally oriented, the average axis-dilatation orientation was zonal; when cloud bands were nonzonally oriented, the average axis-of-dilatation orientation was nonzonal.

  3. The orientation of a spectacular cloud band during GATE was well-correlated with the axis of dilatation of the subjectively analyzed 850 mb wind.

Abstract

Satellite-observed tropical and extratropical cloud bands often look alike. It is hypothesized that synoptic-scale deformation and tropical cloud band orientation are related, since the axis of dilatation associated with the former is related to extratropical frontal cloud band orientation.

This study finds the following evidence for the hypothesis:

  1. The orientations of climatological bands of cloudiness in tropical oceanic areas are for the most part well-correlated with the respective climatological, gradient-level, axis-of-dilatation orientations.

  2. Significant correlations were found between the cloud-band and axis-of-dilatation orientations of the objectively analyzed nondivergent wind at 950, 850 and 700 mb during BOMEX Phase IV. The objective analyses made use of synchronous and asynchronous data. The significance decreased with height, and was very small at 450 mb. On days when cloud bands were zonally oriented, the average axis-dilatation orientation was zonal; when cloud bands were nonzonally oriented, the average axis-of-dilatation orientation was nonzonal.

  3. The orientation of a spectacular cloud band during GATE was well-correlated with the axis of dilatation of the subjectively analyzed 850 mb wind.

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