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

平流层突然变暖(SSW)是整个大气中最戏剧性的事件之一。在短短几天内，高空急流停止，有时甚至逆转，极地平流层温度可以飙升高达50摄氏度。它们的影响向上传播，调制电离层的近地空间环境，向下传播，在欧洲和北美等人口稠密的地区造成极端冬季天气。SSW平均每三个冬天发生两次，但可能在一个冬天发生几次，然后几年都不会发生。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月发生在欧洲的“来自东方的野兽”和2014年1月发生在美国东部的“极地涡旋冬季”直接归因于前几天发生的SSW，但许多SSW的发生几乎对地面天气没有影响。在飞马座，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，以及它们如何和为什么影响地面天气和空间天气。我们将(I)测试最近的一种理论，该理论改变了我们对SSW发生的方式和原因的理解，(Ii)调查SSW如何、何时和在哪里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，这对GPS和无线电通信有影响。(I)传统上，我们认为SSW是由穿过大气的极大和异常强烈的‘行星波’引发的。这些巨浪严重扰乱急流，使其坍塌并触发SSW。然而，最近的工作表明，这个概念模型不能很好地解释观测到的SSW记录。相反，一种新的理论从根本上挑战了这一模型。这一新的理论是，在SSW之前的几周里，较小尺度的“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SSW的开始。这些重力波在空间、时间和强度上的精确分布也可能影响SSW表面效应的严重程度。因此，有必要对这一新理论进行检验。帕伽索斯会这么做的。我们将使用先进的卫星测量大行星波和小得多的重力波的新方法来研究过去16年来每一次SSW的发展。我们还将研究理想化的数学模型(即剔除不必要细节的模型)，以了解SSW如何演变和发展的潜在物理和数学。这将为新理论提供强有力和关键的测试。(Ii)这种观测和理论洞察的结合将使我们能够测试和评估40个领先的气候模型再现SSW的情况。我们将使用这些信息来选择最佳的此类模型，并根据(I)中的观测和理论进行测试。然后，我们将仔细研究这些模式，以了解SSW的哪些特征会影响地面和高层大气，目的是更好地预测未来SSW的发展和地面效应。特别是，我们将根据SSW对急流的影响，密切研究两种不同类型SSW的地面效应之间的差异，这些效应被称为“分裂”和“位移”。(Iii)最后，我们将量化SSW对全球全球定位系统信号和无线电通信的影响，使我们不仅了解SSW的地面天气影响，还了解它们的空间天气影响。这将使用从一极到另一极的五个最先进的雷达链，以及来自卫星测量的高层大气组成的全球测量。

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.