Planetary and Gravity Waves as Drivers of Sudden Stratospheric Warmings (PEGASUS)

平流层突然变暖（SSWs）是整个大气中最引人注目的事件之一。在短短几天内，高空急流停止，有时甚至逆转，极地平流层的温度可以飙升至50摄氏度。它们的影响向上传播，在那里它们会调节电离层的近地空间环境，向下传播，在那里它们会在欧洲和北美等人口稠密的地区造成极端的冬季天气。ssw平均每三个冬天发生两次，但可能在一个冬天发生几次，然后几年都不发生。提前几天预测它们是极具挑战性的。它们的影响也很难预测——虽然（例如）2018年2月在欧洲发生的“东方之兽”和2014年1月在美国东部发生的“极地涡旋冬季”可直接归因于几天前发生的ssw，但许多ssw的发生对地面天气几乎没有影响。在PEGASUS中，我们将使用新的卫星测量技术和先进的计算机模型来更好地了解ssw如何发展的物理原理，以及它们如何以及为什么影响地面天气和空间天气。我们将(i)检验最近的一个理论，该理论改变了我们对ssw如何以及为什么发生的理解，（ii）调查ssw如何、何时、何地影响地面天气的细节，以及（iii）测量ssw对全球高层大气的影响，以及对GPS和无线电通信的影响。(i)传统上，我们认为ssw是由穿越大气层的极其巨大和异常强烈的“行星波”引发的。这些巨大的波浪严重扰乱了急流，使其崩溃并引发了西南涡旋。然而，最近的研究表明，这一概念模型并不能很好地解释观测到的SSW记录。相反，一个新的理论从根本上挑战了这个模型。这个新理论认为，在南纬向风之前的几周内，较小规模的“重力波”将急流推入不那么强劲的状态，这种状态足够弱，以至于正常的冬季天气足以触发南纬向风的开始。这些引力波在空间、时间和强度上的精确分布也可能影响到SSW表面效应的严重程度。因此，检验这一新理论是非常必要的。PEGASUS将会这样做。我们将使用先进的新卫星方法来测量大的行星波和小得多的重力波，以研究过去16年来每一次SSW的发展。我们还将研究理想化的数学模型（即去掉不必要细节的模型），以了解ssw如何演变和发展的基础物理和数学。这将为新理论提供有力而关键的检验。（ii）这种观测和理论见解的结合将使我们能够测试和评估40种领先的气候模型如何很好地再现ssw。我们将使用这些信息来选择最佳的此类模型，并对(i)中的观测和理论进行测试。然后，我们将仔细研究这些选定的模式，以了解SSW的哪些特征导致它们影响地面和高层大气，以便更好地预测未来SSW的发展和地面效应。特别是，我们将密切研究两种不同类型的SSW（即所谓的“分裂”和“位移”）对地面影响的差异，这是基于它们对急流的影响。（iii）最后，我们将量化ssw如何影响全球GPS信号和无线电通信，使我们不仅了解ssw对地面天气的影响，还了解它们对空间天气的影响。这将使用一个由五个最先进的雷达组成的链，从极点到极点，并通过卫星测量对高层大气成分进行全球测量。

Sudden stratospheric warmings (SSWs) are some of the most dramatic events in the entire atmosphere. Over just a few days, the high-altitude jet stream stops and sometimes even reverses, and polar stratospheric temperatures can shoot up as much as 50 degrees Celsius. Their effects propagate upwards, where they modulate the near-Earth space environment of the ionosphere, and downwards, where they can cause extreme winter weather in densely-populated regions such as Europe and North America.SSWs occur on average twice every three winters, but may occur several times in one winter and then not at all for several years afterwards. Forecasting them more than a few days in advance is extremely challenging. Their effects are also very difficult to predict - while (for example) the 'Beast From The East' of February 2018 in Europe and the 'Polar Vortex Winter' of January 2014 in the eastern United States were directly attributable to SSWs which happened a few days earlier, many SSWs have occurred with almost no effect on surface weather.In PEGASUS, we will use new satellite measurement techniques and advanced computer models to better understand the physics of how SSWs develop, and of how and why they affect both surface weather and space weather. We will (i) test a recent theory that changes our understanding of how and why SSWs happen, (ii) investigate the details of how, when and where SSWs affect surface weather and (iii) measure the effects of SSWs on the global upper atmosphere, with implications for GPS and radio communications.(i) Traditionally, we thought that SSWs were triggered by extremely large and unusually intense 'planetary waves' travelling through the atmosphere. These large waves seriously disrupt the jet stream, making it collapse and triggering an SSW. However, recent work has shown that this conceptual model does not properly explain the observed SSW record. Instead, a new theory challenges this model at a fundamental level. This new theory is that smaller-scale 'gravity waves' over the weeks before the SSW nudge the jet stream into a less robust state, weak enough that normal winter weather can be enough to trigger the start of an SSW. The precise distribution of these gravity waves, in space, time and intensity, may also affect how severe the surface effects of the SSW are. There is thus an important need to test this new theory. PEGASUS will do so. We will use advanced new satellite methods of measuring both the large planetary waves and the much smaller gravity waves to study the development of every SSW in the last sixteen years. We will also study idealised mathematical models (i.e. models which strip away unnecessary details) to understand the underlying physics and mathematics of how SSWs evolve and develop. This will provide a robust and critical test of the new theory.(ii) This combination of observational and theoretical insight will let us test and assess how well forty leading climate models reproduce SSWs. We will use this information to select the best such models, tested against both observations and theory from (i). We will then study these selected models in close detail to understand what features of SSWs cause them to affect the surface and the upper atmosphere, with the aim of better predicting both SSW development and surface effects in future. In particular, we will closely study the differences between the surface effects of two different types of SSW, known as 'splits' and 'displacements' based on how they affect the jet stream.(iii) Finally, we will quantify how SSWs affect global GPS signals and radio communications, allowing us to understand not just the surface weather effects of SSWs but also their space-weather effects. This will use a chain of five state-of-the-art radars spanning from pole-to-pole, and global measurements of upper-atmospheric composition from satellite measurements.