Multi‐scale description of a Sahelian synoptic weather system representative of the West African monsoon
Multi‐scale description of a Sahelian synoptic weather system representative of the West African monsoon
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代表西非季风的萨赫勒天气系统的多尺度描述
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发表时间：
2002
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通讯作者：
J. Lafore
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作者：
J. Redelsperger;A. Diongue;A. Diedhiou;J. Ceron;M. Diop;J. Guérémy;J. Lafore
1992年8月在水文大气试点试验（HAPEX - SAHEL）期间观测到的一个萨赫勒天气系统的参考案例，从季节以及天气和对流系统的角度进行了描述。结果表明，案例研究能够代表所有这些尺度的气候学，并呈现出许多相互作用的尺度和物理过程。在季节内尺度上，季风发生的特征是降水的突变以及非洲东风急流（AEJ）和对流的纬度迁移。在月和日尺度上，对流活动发生在热带偏东急流的明显纬向断裂中。1992年8月天气活动强烈。涡度场以偏北（干）和偏南（湿）分量为主，分别位于AEJ两侧850 hPa处。它们在20至40天的季节内调制导致天气活动的活跃和中断阶段。8月21日左右，700 hPa涡度场表现为典型的东风波传播，先是向西传播气旋环流，后是反气旋环流。对流活动主要发生在700 hPa涡度最大值之前，并在Aïr山脉形成飑线，以15 m s - 1向西南方向传播。对流系统的传播速度大约是涡旋核心的两倍，而欧洲中期天气预报中心再分析中的对流则与涡度保持一致。飑线对应于1992年8月经过HAPEX - SAHEL地区的系统的最大贡献者；介绍了它的环境条件及其对大气的影响，包括表面参数。版权所有©2002英国皇家气象学会
A reference case of a Sahelian weather system observed during the Hydrological Atmospheric Pilot Experiment, HAPEX‐SAHEL, in August 1992, is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints. It is shown that the case‐study is representative of the climatology at all these scales and presents many interacting scales and physical processes. At intraseasonal scale, the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the African easterly jet (AEJ) and convection. At the month and day scales, the convective activity occurs in an apparent zonal break of the tropical easterly jet. The month of August 1992 exhibits intense synoptic activity. The vorticity field is characterized by northerly (dry) and southerly (wet) components located at 850 hPa on each side of the AEJ. Their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity. Around 21 August, the 700 hPa vorticity field features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation. Convective activity occurs mainly ahead of the 700 hPa vorticity maximum with the formation of a squall line on Aïr mountains propagating south‐westward at 15 m s−1. The convective system propagates about twice as fast as the vortex core, in contrast with the convection in the European Centre for Medium‐Range Weather Forecasts re‐analysis which stays in phase with the vorticity. The squall line corresponds to the largest contributor to the systems passing in August 1992 over the HAPEX‐SAHEL region; its environmental conditions and its effects on the atmosphere including the surface parameters are presented. Copyright © 2002 Royal Meteorological Society