A Study of the Mesoscale Convective System under Vertical Shear Flow in the Latently Unstable Atmosphere with North-South Asymmetry
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概要
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In order to understand the structure and mechanism of the mesoscale convective system (MCS) under a vertical shear flow, numerical experiments are performed by use of a cumulus-convection-resolving model. The horizontal grid size is taken to be 1km. This study was motivated to understand the orientation of a rainband which caused Fukui heavy rainfall in the Baiu season in 2004. Although the flow having strong low-level vertical shear was westerly, the rainband was not oriented in the west-east, but in the westnorthwest-eastsoutheast. One of the bases for understanding this problem is that rainbands tend to take a carrot shape when the envi-ronmental flow has unidirectional shear without any jet. This study discusses the importance of the north-south asymmetry such that the atmosphere is more latently unstable in the southern portion than in the northern portion, as observed in the Baiu frontal area. In this situation, convection is more enhanced in the southern portion of the rainband. As a result, such orientation of the rainband (MCS) as mentioned above is realized under the westerly flow. In the MCS treated in this study, the effect of rainwater evaporation is important, and the major convection in the western portion of the MCS tilts on the downshear side. Under such a situation, the so-called back-building takes place and plays an important role in the maintenance and slow movement of the MCS. The environmental wind near the surface is easterly relative to the moving MCS. The reason why the back-building takes place in spite of this situation is that convective activity induces strong vertical circulation and resulting low-level westerly wind which is stronger than the environmetal easterly wind. In addition, the low-level flow with pronounced southerly component of the wind is caused by convective activity. This flow as well as the northerly flow associated with downdrafts plays an important role to enhance mesoscale convection which constitutes the MCS. When the low-level air having the southerly component of the wind asecnds, most of the air maintain the southerly component. That is, the southerly component is dominant not only in the lower layer (below 2-km height) to the south of the MCS, but also in the middle and upper layers (above a height of 2-3km) in the MCS area and to its north.
- 社団法人日本気象学会の論文
- 2009-04-25
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