Multiscale interaction with topography and extreme rainfall events in the northeast Indian region
Multiscale interaction with topography and extreme rainfall events in the northeast Indian region
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DOI：
10.1029/2009jd012275
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发表时间：
2010-06-22
影响因子：
4.4
通讯作者：
Goswami, B. N.
中科院分区：
文献类型：
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作者：
Goswami, Bidyut Bikash;Mukhopadhyay, P.;Goswami, B. N.
与极端降雨事件有关的山洪是印度东北部地区的一项重大水文灾害，因为该地区独特的地形特征以及此类事件发生频率的增加。因此，了解这些事件在该区域的时空分布并了解造成这些事件的因素将对适当备灾极为有用。利用该地区15个测站近32年(1975-2006年)的逐日降水资料表明，这些事件的发生频率不是在季风前的雷暴季节，而是在季风高峰期(6-7-8月)。这一事实，再加上这些事件大多发生在长降雨期间，表明东北地区的极端事件主要与季风天气事件有关，而不是与孤立的雷暴有关。我们还发现，与印度中部最近发现的极端降雨事件增加趋势相反，该地区极端降雨事件的总数有显著减少的趋势(Goswami等人，2006年)。极端事件的减少趋势与观测到的该地区对流有效势能的减少和对流抑制能的增加趋势是一致的。对与该区域极端降雨事件有关的对流结构的研究表明，它们是通过环流与当地地形的多尺度相互作用而发生的。结果发现，在所有台站，这些事件都与嵌入到更大尺度对流组织中的中尺度对流结构有关。我们认为这一大尺度组织是热带辐合带某些阶段的表现，这些阶段与夏季风季节内振荡向北传播的活跃-间歇期有关。进一步研究表明，中尺度环流与局地地形相互作用，产生具有日周期的南向传播重力波。与中尺度组织内的重力波相关的强上升气流导致了非常深的对流事件和极端降雨。我们的研究提供的见解在设计改进极端事件预测的模型时将是有用的。
Flash floods associated with extreme rain events are a major hydrological disaster in the northeast Indian (NEI) region because of the unique topographic features of the region as well as increased frequency of occurrence of such events. Knowledge of the spatiotemporal distribution of these events in the region and an understanding of the factors responsible for them, therefore, would be immensely useful for appropriate disaster preparedness. Using daily rainfall data from 15 stations over the region for 32 years (1975-2006), it is shown that the frequency of occurrence of these events is largest not during the premonsoon thunderstorm season but during the peak monsoon months (June-July-August). This fact together with the fact that most of these events occur during long rainy spells indicate that the extreme events in the NEI region largely occur in association with the monsoon synoptic events rather than isolated thunderstorms. We also find that the aggregate of extreme rain events over the region has a significant decreasing trend in contrast to a recent finding of an increasing trend of such events in central India (Goswami et al., 2006). This decreasing trend of extreme events is consistent with observed decreasing trend in convective available potential energy and increasing convective inhibition energy over the region for the mentioned period. Examination of the structure of convection associated with the extreme rain events in the region indicates that they occur through a multiscale interaction of circulation with the local topography. It is found that at all the stations, the events are associated with a mesoscale structure of convection that is embedded in a much larger scale convective organization. We identify that this large-scale organization is a manifestation of certain phases of the tropical convergence zone associated with the northward propagating active-break phases of the summer monsoon intraseasonal oscillation. Further, it is shown that the mesoscale circulation interacting with the local topography generates southward propagating gravity waves with diurnal period. The strong updrafts associated with the gravity waves within the mesoscale organization leads to very deep convective events and the extreme rainfall. The insights provided by our study would be useful when designing models to improve the prediction of extreme events.