Classification of large-scale environments that drive the formation of mesoscale convective systems over southern West Africa
Classification of large-scale environments that drive the formation of mesoscale convective systems over southern West Africa
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DOI：
10.5194/wcd-4-773-2023
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发表时间：
2023-09
影响因子：
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通讯作者：
F. Nkrumah;Cornelia Klein;K. A. Quagraine;Rebecca Berkoh-Oforiwaa;N. Klutse;Patrick Essien;G. M. Quenum;Hubert Azoda Koffi
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作者：
F. Nkrumah;Cornelia Klein;K. A. Quagraine;Rebecca Berkoh-Oforiwaa;N. Klutse;Patrick Essien;G. M. Quenum;Hubert Azoda Koffi
抽象的。中尺度对流系统(MCS)在西非南部(SWA)一年的大部分时间里经常被观测到。这些MCS事件是主要的降雨系统，贡献了西南亚州年降雨量的50%以上。然而，尚未确定西非季风季节周期的典型大尺度环境中的哪些变化可能有利于该区域MCS的发生。在这里，识别了9个不同的天气状态，并利用自组织映射(SOM)和再分析数据的输入进一步与干旱、过渡或季风季的天气环流类型有关。我们发现了一个明显的MCS数值的年度周期，其频率峰值分别在4月和10月，这可以分别与整个西南澳大利亚州主雨季的降雨开始和小雨季的最大降雨量有关。比较每天的MCS频率，MCSs最有可能在过渡条件下发展，其特征是向北移动的水汽异常(每天2.8MCSs)，这可能与加强的低层西风有关。考虑到这些过渡条件主要发生在季风前和季风后期间，这些模式在某些情况下可能代表季风爆发条件或延迟的季风消退。另一方面，在季风条件下，我们观察到在MCS日期间减弱的低层西南风，它减少了萨赫勒地区的水汽含量，但增加了沿海的水汽。最后，我们发现大多数MCS日天气状态都表现出正的纬向风切变异常。纬向风切变异常最强的季节与西南澳大利亚州北部最强的低层温度异常有关，突出表明较温暖的萨赫勒地区可以促进西南澳大利亚州有利于MCS的条件。总体而言，在西南澳大利亚州的MCS日，SOM确定的天气状态会聚为高湿度和高切变条件，这些条件发生的频率取决于天气状态。
Abstract. Mesoscale convective systems (MCSs) are frequently observed over southern West Africa (SWA) throughout most of the year. These MCS events are the dominant rain-bearing systems, contributing over 50 % of annual rainfall over SWA. However, it has not yet been identified what variations in typical large-scale environments of the seasonal cycle of the West African monsoon may favour MCS occurrence in this region. Here, nine distinct synoptic states are identified and are further associated with being a synoptic-circulation type of either a dry, transition, or monsoon season using self-organizing maps (SOMs) with inputs from reanalysis data. We identified a pronounced annual cycle of MCS numbers with frequency peaks in April and October that can be associated with the start of rainfall during the major rainy season and the maximum rainfall for the minor rainy season across SWA, respectively. Comparing daily MCS frequencies, MCSs are most likely to develop during transition conditions featuring a northward-displaced moisture anomaly (2.8 MCSs per day), which can be linked to strengthened low-level westerlies. Considering that these transition conditions occur predominantly during the pre- and post-monsoon period, these patterns may in some cases be representative of monsoon onset conditions or a delayed monsoon retreat. On the other hand, under monsoon conditions, we observe weakened low-level south-westerlies during MCS days, which reduce moisture content over the Sahel but introduce more moisture over the coast. Finally, we find a majority of MCS-day synoptic states exhibiting positive zonal wind shear anomalies. Seasons with the strongest zonal wind shear anomalies are associated with the strongest low-level temperature anomalies to the north of SWA, highlighting that a warmer Sahel can promote MCS-favourable conditions in SWA. Overall, the SOM-identified synoptic states converge towards high-moisture and high-shear conditions on MCS days in SWA, where the frequency at which these conditions occur depends on the synoptic state.