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A DIAGNOSTIC STUDY OF MID-TROPOSPHERIC DEVELOPMENT

GEORGE P. CRESSMANJoint Numerical Weather Prediction Unit, U.S. Weather Bureau, Suitland, Md.

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Abstract

An equation for vertical velocity is applied to data from 850, 500, and 200 mb. for calculations of vertical velocity and divergence in special cases characterized by pronounced failures of barotropic forecasts. The results of the calculations show that a non-development situation was adequately described by the equivalent-barotropic picture of a single quasi-horizontal surface of non-divergence. The onset of mid-tropospheric development, as shown by the appearance of large errors of barotropic forecast, was characterized by the appearance of a double surface of non-divergence, with a deep mid-tropospheric convergence layer in the vicinity of the trough line. This picture of development is confirmed by a second case study.

The appearance of the mid-tropospheric convergence layer is related to the low-level cold push, the tilt of the flow patterns with height, and the high-level jet stream, confirming synoptic studies by J. J. George, H. Riehl, and others.

Abstract

An equation for vertical velocity is applied to data from 850, 500, and 200 mb. for calculations of vertical velocity and divergence in special cases characterized by pronounced failures of barotropic forecasts. The results of the calculations show that a non-development situation was adequately described by the equivalent-barotropic picture of a single quasi-horizontal surface of non-divergence. The onset of mid-tropospheric development, as shown by the appearance of large errors of barotropic forecast, was characterized by the appearance of a double surface of non-divergence, with a deep mid-tropospheric convergence layer in the vicinity of the trough line. This picture of development is confirmed by a second case study.

The appearance of the mid-tropospheric convergence layer is related to the low-level cold push, the tilt of the flow patterns with height, and the high-level jet stream, confirming synoptic studies by J. J. George, H. Riehl, and others.

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