The Structure, Origin, and Long-Term Trends of the West Coast Thermal Trough

西海岸热槽（WCTT）在暖季（大约5月至10月）对北美西部的天气和气候起着至关重要的作用，在冷季也有很大的影响。WCTT表现为从美国西南部低气压区向太平洋西北部延伸的靠近地表的气压槽和暖空气舌，夏季热槽与热浪和空气质量差有关，并极大地影响可再生能源的生产和野火的生长。冬季热槽在许多北美西部沿海降雪事件中起着重要作用。该项目将首先完成一系列的案例研究，使用高分辨率的模型模拟和密集的区域观测，以确定北美西部热槽的详细结构演变，并确定其产生和维持的物理机制。将特别注意穿越区域地形障碍的热槽通道；这样的通道通常与风和温度的急剧变化有关，这可能导致野火爆发和风能的快速变化。这项工作将利用高分辨率网格模式再分析数据集，确定热槽的频率、强度和演变在过去半个世纪中是否发生了变化，然后将评估自然变率（如太平洋年代际涛动）或人为变暖是否可以解释这些变化。它还将评估下个世纪人为全球变暖对改变美国西部热槽特征的潜在影响。为此，将利用区域气候模式将各种温室气体情景下的全球气候模式动态缩小到高分辨率（12公里）。然后将这些场输入到客观算法中，以确定热槽频率和强度的趋势。将评估关于全球变暖对热槽强度和频率影响的具体假设。知识价值：在该奖项下完成的工作将提供对北美西部夏季天气特征的全面描述，并在世界各地观察到类似的现象。这项工作不仅将描述这些特征的详细结构演变，而且还将描述它们的基本物理机制。该研究将确定热槽发展的长期时间趋势，并评估这些趋势的起源。它将研究全球变暖是否会改变WCTTs的频率和强度，并解释这些变化背后的机制。更广泛的影响：加深对这一现象的了解将带来巨大的社会效益。热槽对区域温度、野火发展和控制、可再生能源、空气质量和其他关键的社会问题都有很大的影响。这一建议所产生的理解将有助于提高我们预测热槽发展和演变的能力，为与全球变暖相关的预期长期趋势提供见解，并为区域极端温度、可再生能源潜力和野火频率的预期变化提供建议。

The West Coast thermal trough (WCTT) plays a crucial role in western North American weather and climate during the warm season (approximately May through October) and has a substantial impact during the cold season. The WCTT is expressed as a pressure trough and tongue of warm air near the surface that extends into the Pacific Northwest from an area of low pressure over the southwestern U.S. Summer thermal troughs are associated with heat waves and poor air quality, and greatly impact renewable energy production and wildfire growth. Winter thermal troughs play a role in many western North America coastal snow events. The project will begin by completing a series of case studies, using both high-resolution model simulations and dense regional observations, to determine the detailed structural evolution of western North America thermal troughs, as well as to determine the physical mechanisms responsible for their generation and maintenance. Particular attention will be given to thermal trough passage across regional terrain barriers; such passages are often associated with sharp transitions in wind and temperature that can lead to wildfire blow-ups and rapid changes in wind energy. This work will determine whether thermal trough frequency, intensity, and evolution have changed over the past half-century using high-resolution gridded model reanalysis data sets, and then will evaluate whether natural variability (such as the Pacific Decadal Oscillation) or anthropogenic warming might explain such changes. It will also evaluate the potential of anthropogenic global warming for changing the characteristics of western U.S. thermal troughs over the next century. To do so, global climate models forced by a variety of greenhouse gas scenarios will be dynamically downscaled to high resolution (12 km) using a regional climate model. These fields will then be input into objective algorithms that will determine the trends in thermal trough frequency and intensity. Specific hypotheses regarding the effects of global warming on thermal trough intensity and frequency will be evaluated.Intellectual Merit :The work completed under this award will provide a comprehensive description of a feature that dominates the weather of western North America during the summer, with similar phenomena observed throughout the world. This work will not only describe the detailed structural evolution of such features, but their basic physical mechanisms as well. The research will determine the long-term temporal trends in thermal trough development and evaluate the origins of these trends. It will examine whether global warming will change the frequency and intensity of WCTTs, and will explain the mechanisms behind these changes.Broader Impacts :Improved understanding of this phenomenon will lead to substantial societal benefits. Thermal troughs have a large impact on regional temperatures, wildfire development and control, renewable energy, air quality, and other critical societal issues. The understanding generated by this proposal should help improve our ability to predict thermal trough development and evolution, provide insights into expected long-term trends associated with global warming, and suggest expected changes in regional temperature extremes, renewable energy potential, and wildfire frequency.