Rainbands and Symmetric Instability

Wein-Yih Sun Department of Geosciences, Purdue University, West Lafayette, IN 47907

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Abstract

A linearized two-dimensional model was used to study the formation and evolution of the rainbands which produce an intense precipitation along the Baiu front over Eastern Asia during spring and summer. This model is also applied to squall lines over the eastern part of the United States during 3–4 April 1974.

Condensation is included in this nonhydrostatic model, in which the total condensation within a vertical cloud column is proportional to the convergence of moisture at cloud base, but there is no cooling in the descending branches of the circulation. The horizontal temperature gradient is introduced in order to maintain a vertical wind shear according to the thermal wind relationship.

The results of this study show that rainbands propagate toward the warm side. The small, weak bands are dissipated by the subsidence warming associated with the downward motion branches of their stronger surroundings; therefore, they merge into a few large rainbands with a horizontal wavelength of a few hundred kilometers in the mature stage. These results are consistent with observations near the Baiu front, and, the squall lines of 3–4 April 1974 in the United States. It is also found that subsidence warming is very important compared with condensation heating inside the cloud, especially for selection of the preferred horizontal wavelength.

This study also shows that the observational rainbands or the squall lines cannot be simulated by symmetric instability without including diabatic heating.

Abstract

A linearized two-dimensional model was used to study the formation and evolution of the rainbands which produce an intense precipitation along the Baiu front over Eastern Asia during spring and summer. This model is also applied to squall lines over the eastern part of the United States during 3–4 April 1974.

Condensation is included in this nonhydrostatic model, in which the total condensation within a vertical cloud column is proportional to the convergence of moisture at cloud base, but there is no cooling in the descending branches of the circulation. The horizontal temperature gradient is introduced in order to maintain a vertical wind shear according to the thermal wind relationship.

The results of this study show that rainbands propagate toward the warm side. The small, weak bands are dissipated by the subsidence warming associated with the downward motion branches of their stronger surroundings; therefore, they merge into a few large rainbands with a horizontal wavelength of a few hundred kilometers in the mature stage. These results are consistent with observations near the Baiu front, and, the squall lines of 3–4 April 1974 in the United States. It is also found that subsidence warming is very important compared with condensation heating inside the cloud, especially for selection of the preferred horizontal wavelength.

This study also shows that the observational rainbands or the squall lines cannot be simulated by symmetric instability without including diabatic heating.

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