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，强降水最常发生在午夜和清晨之间。这种夜间强降水占每日对流降水的近80%，严重影响了该地区的水资源供应、经济和社区安全。这种夜间强降水是违反直觉的，在这个意义上，在大平原上的强对流活动发生在深夜时，对流层低层是稳定的层结，对流有效位能低。这种矛盾的情况挑战了传统的理解发展的严重对流，但也提供了一个机会，探索新的机制，严重的夜间对流和降雨在美国大平原。本研究的目的是研究一种机制，启动夜间对流的前或对流复合体的通道的情况下，在对流层低层的对流可用的势能低。研究结果的意义在于探索了大平原暖季强对流和凌晨降水的新机制。新机制的验证将填补在缺乏锋边界和对流复合体的情况下夜间对流启动知识的差距，并将扩展夜间对流的知识，超越传统的思维。这些发现将有助于提高模型预测美国大平原严重对流和降雨的时间和强度的能力。这项研究的更广泛影响还包括它有可能改善美国大平原社区的安全和福祉，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，并欣赏地球科学在改善人们生活方面的影响。中心假设是，夜间低气压的加剧，大平原的水平南风受到科里奥利偏转的影响，这又会导致一个沿沿着指向北方的水平轴旋转的涡旋（北方涡旋）的发展。北方涡旋与落基山脉吹来的风相互作用，产生一个升力，可以将对流层下部的空气提升到足够高的高度，从而引发对流，这是大平原午夜至清晨几个小时内强降水的原因。主要研究任务是：（1）识别导致北方低涡发展的过程，（2）量化产生低涡升力的条件，（3）量化大气湿度在产生升力中的作用，（4）量化大气湿度对低涡升力的影响。和4）的方法识别丢失或错误描述的过程，这些过程使当前模型在再现这种升高的对流时失败。该奖项反映了NSF的法定使命，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.