Extratropical Persistent Anomalies on a Warmer Earth: Connections to Extratropical Storms and Storm Tracks

这是一项关于大气环流中持续异常（PAs）的研究，它被定义为在超过几天的时间内保持近似固定的大气流动的不寻常状态。在中纬度地区，当天气系统在某一特定地点的正常东进进程被中断时，这里讨论的PAs就会发生。它们通常与急流的分裂或经向移动有关，大致等同于大气阻塞，尽管阻塞有一个更严格的定义。对PAs的兴趣主要来自于它们与高影响天气事件的关联，包括冷空气爆发、干旱、暴雨和热浪。该项目旨在了解决定持续异常（PAs）的频率、强度和持续时间的基本动力学，以及由于温室气体增加导致的变暖而可能发生的PA行为变化。在空间尺度上，PAs比统称为天气系统的天气锋和气旋范围更广。对阻滞的研究表明，天气系统在阻滞的发生和维持中起着重要作用，这一发现也适用于PAs。PIs假设，PAs的产生和维持取决于天气系统内部发生的较小尺度过程，包括将含雨云（冷凝加热发生的地方）组织成中尺度特征（例如雨带和飑线）。这一假设的部分原因是，研究表明，尽管在低分辨率模型中，与网格间距相比，区块的大小较大，但具有较高分辨率的全球天气和气候模型在模拟区块方面做得更好。pi提出更高的分辨率更好，因为它允许更准确地表示冷凝加热发生的小得多的中尺度特征。冷凝加热的更好表现导致更有活力的系统，这可以更有效地将低涡度空气从热带的急流转移到高纬度地区，这是PA形成所需要的。这一假设的一个含义是，更有利于高强度天气系统的模式或气候状态将产生更强的pa。这一论点表明，随着气候变暖，PAs可能会变得更强或更突出，因为温暖空气中更大的水分含量允许更强的降雨和冷凝加热。该项目通过确定一系列案例，并使用全球大气模式（即天气研究与预报模式，或跨尺度预测模式，如果可用的话）对这些案例进行“后播”模拟，来检查当前气候中的PAs。将确定对这些病例产生最佳模拟的模型配置，并进行一些诊断，以确定导致PA启动和持续的关键因素。为了评估这些特征在PA形成中的作用，使用一种技术对模拟的初始大气状态进行了修改，以去除特定的天气特征。气候变化对PAs的影响是通过重复后验来研究的，但对背景状态进行了修改，以接近气候模式模拟的温室变暖效应。长模型集成用于从统计意义上考虑PA行为，简单的单层模型用于进一步研究PA的基本动态。鉴于PAs与影响人类活动的高影响天气事件密切相关，PAs的行为既是一个实用的话题，也是一个科学的话题。这项工作解决了与提高天气模式预测PA相关极端天气的能力以及气候模式预测PA行为未来变化的能力直接相关的问题。此外，该项目还开设了两门强调合作研究的新课程，一门是数据分析，另一门是数值模拟，为本科生提供了研究机会。该项目每年夏天还支持本科生实习，并在少数民族服务机构发布招聘广告。此外，该项目将雇用和培训两名研究生和一名博士后，从而为该研究领域的未来劳动力发展提供支持。

This is a study of persistent anomalies (PAs) in atmospheric circulation, defined as unusual states of the atmospheric flow that remain approximately fixed over periods longer than a few days. PAs addressed here occur in middle latitudes when the normal eastward progression of weather systems over a given location is disrupted. They are generally associated with splitting or meridional shifting of jet streams and are loosely synonymous with atmospheric blocking, although blocking has a more restrictive definition. Interest in PAs comes largely from their association with high-impact weather events including cold air outbreaks, droughts, pluvials, and heat waves. The project seeks to understand the fundamental dynamics which determine the frequency, intensity, and duration of persistent anomalies (PAs), as well as the changes in PA behavior that are likely to occur as a result of warming due to greenhouse gas increases.PAs are broader in spatial scale than the weather fronts and cyclones that are collectively referred to as synoptic systems. Studies of blocking show that synoptic systems play a large role in the onset and maintenance of blocks, a finding that holds true for PAs as well. The PIs hypothesize that the initiation and maintenance of PAs depends on smaller-scale processes that occur inside the synoptic systems, including the organization of rain-bearing clouds (where condensational heating occurs) into mesoscale features (rain bands and squall lines, for example). The hypothesis is motivated in part by studies showing that global weather and climate models with higher resolution do a better job in simulating blocks, despite the large size of blocks compared to grid spacing even in low resolution models. The PIs propose that higher resolution is better because it allows a more accurate representation of the much smaller mesoscale features where the condensational heating takes place. Better representation of condensational heating leads to more vigorous systems, which can more effectively move low vorticity air across the jet stream from the tropics to high latitudes as required for PA formation. One implication of this hypothesis is that models or climate states which are more conducive to high intensity synoptic systems will produce more strong PAs. This argument suggests that PAs could become stronger or more prominent as climate warms, as the greater moisture content of warmer air allows for more intense rainfall and condensational heating.The project examines PAs in present-day climate by identifying a set of cases and performing "hindcast" simulations of those cases using a global atmospheric model (the Weather Research and Forecasting model, or the Model for Prediction Across Scales if it becomes available). Model configurations which produce the best simulations of these cases will be identified, and a number of diagnostics will be performed to identify the key ingredients which lead to PA initiation and persistence. Additional experiments are performed using a technique in which the initial atmospheric state for the simulation is modified to remove particular synoptic features, in order to assess the role of these features in PA formation. The impact of climate change on PAs is studied by repeating the hindcasts but with the background state modified to approximate the effects of greenhouse warming as simulated by climate models. Long model integrations are used to consider PA behavior in a statistical sense, and simple single-layer models are used to further examine the basic dynamics of PAs.The behavior of PAs is a topic of practical as well as scientific interest, given the close association of PAs with high-impact weather events that affect human activities. The work addresses questions which are directly relevant to improving the ability of weather models to forecast PA-related extreme weather, and the ability of climate models to anticipate future changes in PA behavior. In addition, the project provides undergraduate research opportunities through two new courses emphasizing collaborative research, one on data analysis and the other on numerical simulations. An undergraduate internship is also supported in each summer of the project, and the position is advertised at minority-serving institutions. In addition, the project will employ and train two graduate students and a postdoctoral fellow, thereby providing future workforce development in this area of research.