Collaborative Research: Understanding Downdrafts in Deep Convection

提高对雷暴的预测、其对全球气候模式的影响以及其产生恶劣天气的能力，依赖于对雷暴垂直运动过程的基本知识。最近，人们的注意力集中在风暴中的上升运动，即上升气流。风暴中的向下运动被称为下行气流，相对来说很少受到关注。本研究将解决上述与下沉气流相关的知识差距。该项目的目标是了解大气特性（如温度、湿度、风向和风速随高度的变化）对驱动下行气流的力量、下行气流产生的高度以及它们的大小的影响。这将通过理论分析和理想化的模拟来完成。从这项工作中获得的知识将有助于改进在天气和气候模式中如何表示下沉气流。因此，本研究将有利于科学界和公众提高对雷暴的基本认识，改善对强雷暴和降水灾害的预测，改善气候预测。在这项研究中纳入本科生，研究生和博士后学者也将对未来科学家的发展和培养产生直接影响。该项目将为改进下沉气流预报和参数化技术提供基础研究。该项目将以三种方式实现这一目标：首先，它将揭示目前尚不清楚的下沉气流的典型起源高度；第二，它将巩固下沉气流概念模型的物理基础，从而改进下沉气流加速度的预测；第三，它将展示我们改进的下降气流概念模型对积云参数化和全球气候模式性能的直接影响。这些目标将通过一系列理想化的大涡模拟来实现，这些模拟具有不同的、现实的基本状态热力学和运动学剖面，并使用评估下沉气流特性的新技术进行分析，包括分层和定向被动示踪剂以及已建立的轨迹分析技术。新的多重分析羽流（MAP）积云参数化将通过添加一个由理想模拟结果提供信息的下沉气流参数化来改进。该项目还将开发一种改进的探测衍生参数，用于预测下沉气流强度，该参数可以解释包裹理论的偏差，包括非流体静力垂直扰动、压力梯度、加速度和夹带。基础研究将为广泛使用的全球气候模式的敏感性实验提供信息，以了解下沉气流与大尺度气候状态模拟之间的联系。综合起来，所有的研究将有助于提高对对流现象的理解和对多个大气尺度的预报。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。

Improving the prediction of thunderstorms, their impacts on global climate patterns, and their ability to produce severe weather relies on fundamental knowledge of the processes that contribute to vertical motions in these storms. Recently, attention has been focused on upward motions in storms, known as updrafts. Downward motions in storms, which are called downdrafts, have received comparatively little attention. This research will address the aforementioned knowledge gap related to downdrafts. The project targets understanding the influence of atmospheric properties (e.g., temperature, moisture, and changes in the wind direction and speed with height) on the forces that drive downdrafts, the height at which downdrafts originate, and how large they become. This will be accomplished through theoretical analysis and idealized simulations. The knowledge gained from this work will allow for improvement in how downdrafts are represented in weather and climate models. Thus, this study will benefit scientific communities and the general public by improving the fundamental understanding of thunderstorms, improving forecasting of severe thunderstorm and precipitation hazards, and improving climate prediction. The inclusion of undergraduate students, graduate students, and a postdoctoral scholar in this research will also have a direct impact on the development and training of future scientists. This project will provide foundational research for improvements in downdraft forecasting and parameterization. The project will deliver on this front in three ways: first, it will uncover the typical origin heights of downdrafts, which are not well understood at present; second, it will solidify the physical basis of downdraft conceptual models, and consequently improve predictions of downdraft accelerations; third, it will demonstrate the direct impact of our improved conceptual models for downdrafts on cumulus parameterization and global climate model performance. These goals will be accomplished through a large suite of idealized large-eddy simulations with varying, realistic base-state thermodynamic and kinematic profiles, analyzed with novel techniques for assessing downdraft properties including layered and targeted passive tracers as well established trajectory analysis techniques. The new Multiple Analytic Plume (MAP) cumulus parameterization will be improved through the addition of a downdraft parameterization that is informed by the idealized simulation findings. The project will also develop an improved sounding-derived parameter for downdraft intensity forecasting that accounts for deviations from parcel theory, including nonhydrostatic vertical perturbation pressure gradient accelerations and entrainment. The foundational research will inform sensitivity experiments in a widely used global climate model to understand the connection between downdrafts and large-scale climate state simulations. Combined, all lines of research will facilitate improved understanding of convective phenomena and forecasts on multiple atmospheric scales.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.