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

尽管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被很好地理解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增湿为这些风暴系统提供了燃料。然而，值得注意的是，大平原夏季的雷雨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。这项提议试图理解为什么大平原夏季的大型雷暴复合体往往在夜间出现。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，以及该地区上空的风暴系统是否通过与下午晚些时候不同的过程维持。考虑到用于预测天气的数值模式很难描述这些夜间风暴，理解这些夜间风暴的框架是很重要的。考虑到夜间雷暴可以遏制大风、冰雹和暴雨，对这些事件的准确预测对社会来说很重要。具体地说，这项拟议的工作试图通过提高对夜间低空急流(NLLJ)的时空变化与维持夜间中尺度对流系统(NMCSs)的动力过程之间相互作用的了解，来解释夏季大平原上有组织的雷暴活动的夜间最大值。这项建议将利用多机构山核桃(夜间平原高空对流)和国际H2O项目(IHOP_2002)实地活动的观测以及数值模拟和理论。这项工作的一个方面是在NLLJ的新框架内研究该区域热力学和风的时空变化，该框架表明，该急流不是简单的由惯性振荡驱动的南风极大值，而是一种更复杂的现象，最大西风风高于最大南风风，并有可能缓慢、持续地上升。这一框架意味着，NLLJ将通过辐射过程来影响对流不稳定，这些辐射过程控制着稳定的夜间边界层的形成和该地区倾斜地形上的加热和冷却，以及局地混合、差动平流和中尺度上升等动力过程。调查的一个关键组成部分将是开发一个框架，解释NLLJ如何影响水平风、对流可用势能(CAPE)、对流抑制(CIN)和自由对流水平(LFC)的垂直剖面。这项工作的另一个方面是侧重于钻孔和其他重力波现象在NLLJ环境中启动和维持对流的作用。最近的研究表明，钻孔和重力波往往是大平原上空NMCSs的固有组成部分。研究小组将调查这些垂直剖面上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。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.