EAGER: Lift in the Vertical Shear of Southerly Winds: A Mechanism of Elevated Convection at Night in the United States Great Plains

美国大平原暖季天气的一个独特特征是，强降水最频繁地发生在午夜到清晨之间。这种夜间强降水贡献了近80%的日对流降水，严重影响了该地区的水资源可用性、经济和社区安全。这种夜间强降水是违反直觉的，因为大平原上空强烈的对流活动发生在对流层下层稳定分层的深夜，对流有效势能较低。这种矛盾的情况挑战了对强对流发展的传统理解，但也为探索美国大平原夜间强对流和降雨的新机制提供了机会。本研究的目的是研究在对流层低层对流有效势能较低的情况下，在没有锋面或对流复合体通过的情况下启动夜间对流的机制。本研究结果的意义在于探索暖季强对流与大平原午夜至清晨降水的新机制。新机制的验证将填补在没有锋面边界和对流复合体的情况下夜间对流启动知识的空白，将夜间对流知识扩展到传统思维之外。这一发现将有助于提高模型预测美国大平原强对流和降雨的时间和强度的能力。这项研究的广泛影响还包括它有可能改善美国大平原地区社区的安全和福祉，并通过教育，扩大当地公立学校的科学教育计划，让学生学习流体动力学，并了解地球科学对改善人们生活的影响。核心假设是，夜间大平原低空南风的增强受到科里奥利偏转的影响，而科里奥利偏转又会导致沿水平轴向北旋转的涡旋（北涡旋）的发展。北部涡旋与来自落基山脉的风的相互作用产生了升力，可以将对流层低层的空气提升到足够高的高度，从而引发对流，导致大平原午夜到清晨之间的几个小时内出现强降水。研究的主要任务是：1)识别导致北方涡旋发展的过程；2)量化产生旋涡升力的条件；3)量化大气湿度在产生升力中的作用；4)识别缺失或错误描述的过程，这些过程使当前模型在再现这种高架对流时失败。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

One of the unique features of warm season weather in the U.S. Great Plains is that heavy precipitation takes place most frequently between midnight and early morning. Such nocturnal heavy precipitation contributes to nearly 80% of daily convective precipitation, critically affecting water resources availability, the economy, and the safety of the communities in the region. This nocturnal heavy precipitation is counterintuitive in the sense that the intensive convective activity over the Great Plains occurs at late night when the lower-troposphere is stably stratified, and the convective available potential energy is low. This paradoxical situation challenges the traditional understanding of development of severe convection yet also presents an opportunity for exploring new mechanisms for severe nocturnal convection and rainfall in the U.S. Great Plains. The goal of this study is to examine a mechanism that initiates nocturnal convection in the absence of passage of fronts or convective complex with a low level of convective available potential energy in the lower troposphere. The significance of outcomes of this study is the exploring of a new mechanism for warm season severe convection and rainfall in the Great Plains at mid-night to early morning hours. The validation of the new mechanism will fill the gap in the knowledge of initiation of nocturnal convection in the absence of frontal boundaries and convective complex and will extend the knowledge of nocturnal convection beyond the traditional thinking. The findings will help improve model capabilities to predict the timing and intensity of severe convection and rainfall in the U.S. Great Plains. The broader impacts of this study also include its potential to improve the safety and wellbeing of the communities across the U.S. Great Plains and, through education, expanding local public schools' science education programs by engaging students in learning fluid dynamics and appreciating the impacts of geosciences in improving people's lives.The central hypothesis is that the nocturnal intensification of the low-level southerly wind in the Great Plains is subject to Coriolis deflection, which would in turn lead to the development of a vortex rotating along the horizontal axis pointing to the north (the northern vortex). Interactions of the northern vortex with the wind blows from the Rockies generate a lift that can elevate air in the lower-troposphere high enough to initiate convection, responsible for heavy precipitation in the hours between mid-night and early morning in the Great Plains. The main research tasks are 1) identifying processes leading to development of the northern vortex; 2) quantifying the condition that creates the lift of the vortex; 3) quantifying the roles of the atmospheric moisture in generating the lift; and 4) identifying the missing or incorrectly described processes that have failed current models in reproducing the elevated convection of this kind.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.