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，以应对天气扰动，如锋面和热带气旋，但地球上大部分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。这些雷暴的一般时间是很好理解的，因为太阳每天加热地球表面，以及相关的低层大气的变暖和增湿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不遵循这一总体趋势。相反，大平原上雷暴活动的最大值发生在深夜和清晨。这项建议旨在了解为什么大雷雨复合体往往发生在夜间在大平原在夏季。拟议的工作重点是确定大平原上空的大气是否具有导致夜间最大值的相对独特的特征，以及确定该地区的风暴系统是否通过与下午晚些时候的风暴系统不同的过程来维持。考虑到用于预测天气的数值模型很难代表这些夜间风暴，理解这些夜间风暴的框架很重要。这些事件的准确预测对社会很重要，因为夜间雷暴可能包含大风，冰雹和暴雨。具体而言，这项拟议的努力旨在解释夜间最大的有组织的雷暴活动在大平原在夏季通过推进夜间低空急流（NLLJ）的时空变化和维持夜间中尺度对流系统（NMCSs）的动力学过程之间的相互作用的知识。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）现场活动的观测结果沿着数值模拟和理论。这项工作的一个方面是调查的空间和时间的变化，在热力学和风在这一地区的背景下，新的框架内的NLLJ表明，射流不只是一个南风最大驱动的惯性振荡，但更复杂的现象与西风最大的峰值南风和一个缓慢的，持续上升的可能性。这个框架意味着NLLJ将影响对流不稳定，通过辐射过程控制形成稳定的夜间边界层和加热和冷却的斜坡地形的区域沿着动力过程，如本地混合，差分平流，中尺度上升。调查的一个关键组成部分将是制定一个框架，解释NLLJ如何影响水平风的垂直廓线，对流可用势能（CAPE），对流抑制（CIN）和自由对流水平（LFC）。这项工作的另一个方面是集中在孔和其他重力波现象在启动和维持对流的NLLJ环境中的作用。最近的研究表明，钻孔和重力波往往是固有的组成部分，NMCS在大平原。该研究小组将调查这些垂直剖面的空间和时间变化如何影响NMCSs的结构，运动和演变，以及如何在NLLJ环境中产生钻孔和重力波，以影响NMCSs.This奖项的几代人反映NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.