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A Numerical Study of a Low-level Jet and Its Accompanying Secondary Circulation in a Mei-Yu System

Wu-Ron HsuDepartment of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

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Wen-Yih SunDepartment of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana

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Abstract

A primitive equation numerical model has been used to study the Mei-Yu system that occurred in the middle of May 1987. Although cumulus heating was not included in one of the experiments, all of the major features of a Mei-Yu system other than deep convection are reproduced in that experiment. These features are a cloud band along the coastline in southern China; a weak temperature gradient, a strong moisture gradient, and a strong wind shear across the cloud band; and a supergeostrophic low-level jet (LLJ) with an accompanying secondary circulation.

When the moist air coming from the south encounters the migrating high pressure system in the north, it turns to the east-northeast and becomes saturated. The resulting stratiform cloud covers all of southern China and the pressure trough at the 850-hPa level deepens by 1.5 hPa due to the latent heat released. It induces convergent motion in the lower level and generates a direct, cross-frontal secondary circulation that helps keep the low-level wind supergeostrophic. Without such large-scale latent heating, the low-level wind would be weaker; the amount of moisture that can be transported to the frontal zone would also be drastically reduced. Once the LLJ forms. it may trigger heavy precipitation in the later stages of the Mei-Yu system. Our study suggests that cumulus heating may not have played a critical role in the formation of the LLJ, at least for the particular Mei-Yu system under consideration.

Abstract

A primitive equation numerical model has been used to study the Mei-Yu system that occurred in the middle of May 1987. Although cumulus heating was not included in one of the experiments, all of the major features of a Mei-Yu system other than deep convection are reproduced in that experiment. These features are a cloud band along the coastline in southern China; a weak temperature gradient, a strong moisture gradient, and a strong wind shear across the cloud band; and a supergeostrophic low-level jet (LLJ) with an accompanying secondary circulation.

When the moist air coming from the south encounters the migrating high pressure system in the north, it turns to the east-northeast and becomes saturated. The resulting stratiform cloud covers all of southern China and the pressure trough at the 850-hPa level deepens by 1.5 hPa due to the latent heat released. It induces convergent motion in the lower level and generates a direct, cross-frontal secondary circulation that helps keep the low-level wind supergeostrophic. Without such large-scale latent heating, the low-level wind would be weaker; the amount of moisture that can be transported to the frontal zone would also be drastically reduced. Once the LLJ forms. it may trigger heavy precipitation in the later stages of the Mei-Yu system. Our study suggests that cumulus heating may not have played a critical role in the formation of the LLJ, at least for the particular Mei-Yu system under consideration.

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