Explaining the Nocturnal Maximum in Convection over the Great Plains Through Understanding the Interactions Between the Nocturnal Low-level Jet and Mesoscale Convective Systems

虽然雷暴可以在一天中的任何时间发生，以应对天气干扰，如锋面和热带气旋，但地球上大多数陆地上的雷暴活动往往最频繁地发生在下午晚些时候和傍晚早些时候。这些雷暴的一般时间是很容易理解的，因为太阳每天对地球表面的加热以及与之相关的低层大气的变暖和湿润为这些风暴系统提供了燃料。然而，值得注意的是，夏季大平原上的雷暴并没有遵循这种普遍趋势。相反，大平原地区雷暴活动的最大值发生在深夜和凌晨。这个提议试图理解为什么夏季大雷暴复合体往往发生在大平原的夜间。这项工作的重点是确定大平原上空的大气是否具有相对独特的特征，从而导致这种夜间最大值，并确定该地区的风暴系统是否通过不同的过程维持，而不是通过下午晚些时候的风暴系统维持。考虑到用于预测天气的数值模式难以代表这些夜间风暴，理解这些夜间风暴的框架是很重要的。鉴于夜间雷暴可能包含大风、冰雹和强降雨，对这些事件的准确预测对社会很重要。具体来说，本研究旨在通过进一步了解夜间低空急流（NLLJ）的时空变化与维持夜间中尺度对流系统（NMCSs）的动力过程之间的相互作用，来解释夏季大平原地区有组织雷暴活动的夜间最大值。该提案将利用多机构PECAN（夜间平原高架对流）和国际H2O项目（IHOP_2002）野外活动的观测结果以及数值模拟和理论。这项工作的一个方面是在NLLJ的新框架背景下研究该地区热力学和风的时空变化，该框架表明喷气机不仅仅是由惯性振荡驱动的南风最大值，而是一个更复杂的现象，西风最大值高于南风峰值，并且可能缓慢持续上升。这一框架意味着NLLJ将通过控制夜间稳定边界层形成的辐射过程，以及局部混合、差动平流和中尺度上升等动力过程，影响该地区倾斜地形的加热和冷却，从而影响对流不稳定性。研究的一个关键组成部分将是建立一个框架，解释NLLJ如何影响水平风的垂直剖面、对流有效势能（CAPE）、对流抑制（CIN）和自由对流（LFC）水平。这项工作的另一个方面是关注井眼和其他重力波现象在NLLJ环境中启动和维持对流中的作用。最近的研究表明，钻孔和重力波往往是大平原上nmcs的固有组成部分。研究小组将研究这些垂直剖面的时空变化如何影响nmcs的结构、运动和演化，以及NLLJ环境中如何产生钻孔和重力波来影响nmcs的世代。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。

Although thunderstorms can occur at any time of the day in response to weather disturbances, such as fronts and tropical cyclones, thunderstorm activity over most of the planet's land masses tends to most frequently occur in the late afternoon and early evening. The general timing of these thunderstorm is well understood as the daily heating of the earth's surface by the sun and the associated warming and moistening of the lower atmosphere provides the fuel for these storm systems. Remarkably, however, thunderstorms over the Great Plains during the summer do not follow this general tendency. Instead, the maximum in thunderstorm activity over the Great Plains takes place late in the night and into the early morning hours. This proposal seeks to understand why large thunderstorm complexes tend to occur at night over the Great Plains during the summer. The proposed effort focuses on determining whether the atmosphere over the Great Plains has relatively unique characteristics that lead to this nighttime maximum and on determining whether storm systems over this region are maintained through different processes than their late afternoon counterparts. A framework for understanding these nocturnal storms is important given that the numerical models utilized to predict weather have difficulty in representing these nocturnal storms. Accurate predictions of these events are important to society given that the nighttime thunderstorms can contain high winds, hail, and heavy rainfall. Specifically, this proposed effort seeks to explain the nocturnal maximum in organized thunderstorm activity over the Great Plains during the summer through advancing knowledge of the interaction between the spatial and temporal variations of the nocturnal low-level jet (NLLJ) and the dynamical processes maintaining nocturnal mesoscale convective systems (NMCSs). This proposal will utilize observations from the multi-agency PECAN (Plains Elevated Convection at Night) and the International H2O Project (IHOP_2002) field campaigns along with numerical simulations and theory. One aspect of this effort is to investigate the spatial and temporal variations in the thermodynamics and winds over this region within the context of the new framework for the NLLJ that shows that the jet is not simply a southerly wind maximum driven by an inertial oscillation, but a more complicated phenomenon with a westerly wind maximum above the peak southerly winds and the possibility of a slow, persistent ascent. This framework means that the NLLJ will impact convective instability through radiative processes controlling the formation of the stable nocturnal boundary layer and the heating and cooling on the sloping terrain of the region along with dynamical processes, such as local mixing, differential advection, and mesoscale ascent. A key component of the investigation will be to develop a framework that explains how the NLLJ impacts the vertical profiles of horizontal winds, Convective Available Potential Energy (CAPE), Convective Inhibition (CIN), and the Level of Free Convection (LFC). Another aspect of this effort is to focus on the role of bores and other gravity wave phenomena in initiating and maintaining convection in the NLLJ environment. Recent research has shown that bores and gravity waves are often inherent component of NMCSs over the Great Plains. The research team will investigate how the spatial and temporal variations in these vertical profiles impact the structure, movement, and evolution of NMCSs and how bores and gravity waves are generated in the NLLJ environment to impact generations of NMCSs.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.