The Environments of Convective Storms: Challenging Conventional Wisdom

大雷暴是最具破坏性的天气现象之一。冰雹会摧毁庄稼，山洪会淹没城镇和房屋，闪电会威胁到人们并引发火灾，强风会破坏交通和基础设施。对流风暴及相关现象每年在整个欧洲造成50 - 80亿欧元的损失。这种风暴有可能被预测，公众也会提前得到警告，但随着预测时间的延长，预测变得越来越困难。在短期内，观测和高分辨率的计算机模拟可以对即将到来的风暴提供足够的警告，但在雷暴爆发前超过三天的时间里，即使对大范围气流的预测很好，也必须从计算机预报中间接推断雷暴的爆发。该项目的目的是提高我们对雷暴（也称为对流风暴）与大气中更大尺度环境之间关系的理解，以便更好地了解负责帮助预报员解释模式预测的物理过程。对流风暴需要三个要素：足够的水分来凝结并为风暴提供燃料，不稳定性或温度随高度下降的速度（温度随高度下降得更快更好），以及提升空气以释放不稳定性的东西。在本建议中，我们将重点放在不稳定性因素上。在美国，具有较大不稳定性的环境被认为是由于美国西部高架地形的加热，导致高架混合层（EML）的产生。在欧洲，EMLs被认为是由于经过西班牙中部的高地，导致西班牙羽流。像落基山脉或西班牙高原这样的高热源对对流层低层空气（海平面以上3-5公里）的明显加热是EML急剧递减率的自然解释。升温对不稳定性的形成有多大的贡献？与落基山脉相比，西班牙高地的规模较小，很难想象西班牙高地会造成如此大的不稳定。关于陡递减率起源的一个假设是撒哈拉沙漠，在那里一个混合良好的边界层形成陡递减率，可以从北非平流（称为撒哈拉空气层）。然而，这一假设还没有得到验证，无论是对西班牙羽流还是其他高热地形下游地区。另一个解释不稳定现象的因素是高空低温或低湿度空气的不同输送。虽然这样的解释已经在文献中使用，但其他研究质疑这一因素的适用性。我们提出的研究是问什么过程在全球范围内产生中纬度对流风暴的环境。什么样的环境有利于不稳定，这在全球有何不同？造成不稳定的物理过程是什么？不稳定——在整个欧洲，尤其是英国——是由于气温升高（如美国中部的EML），还是由于远程运输？尽管传统观点认为，升高的混合层是造成高地形下游不稳定的原因，但这种观点尚未得到验证。我们的目的是为了更好地理解中纬度对流风暴的高地形、大尺度过程和不稳定性之间的关系。这些问题激发了一个多方面的研究项目来回答这些问题。造成不稳定性的物理过程是什么？Q2：地形如何为深层潮湿对流创造有利的环境？问题3：不同的温度和水分平流对造成不稳定有多重要？

Large thunderstorms are one of the most damaging of weather phenomena. Hail can devastate crops, flash flooding can inundate towns and homes, lightning can threaten people and ignite fires, and strong gusts can damage transport and infrastructure. Convective storms and associated phenomena cause 5-8 billion euro per year in damage across Europe. Such storms have the potential to be forecast and the public warned beforehand, but forecasting becomes increasingly difficult as the length of a forecast increases. In the near-term, observations and high-resolution computer modelling can provide adequate warning of impending storms, but for periods longer than three days ahead the outbreak of thunderstorms has to be deduced indirectly from the computer forecast even if the large-scale flow is well forecasted. The aim of this project is to improve our understanding of the relationship between thunderstorms (also called convective storms) and the larger-scale environment in the atmosphere, to provide better understanding of the physical processes responsible to aid forecasters in interpreting the model predictions.Convective storms require three ingredients: sufficient moisture to condense and fuel the storm, instability or the rate at which temperature decreases with height (temperature dropping quickly with height is better), and something to lift air to release the instability. In this proposal, we focus on the instability ingredient.In the United States, environments with large instability are believed to occur because of heating over the elevated terrain of the western United States, resulting in the elevated mixed-layer (EML). In Europe, EMLs are attributed to passage over the elevated terrain of central Spain, resulting in the Spanish plume. Such sensible heating of lower-tropospheric air (3-5 km above sea level) by an elevated heat source such as the Rockies or Spanish plateau is a natural explanation for the steep lapse rates in the EML.How much of a contribution is the elevated heating to the formation of instability? The smaller scale of the Spanish high terrain compared to the Rocky Mountains makes it difficult to imagine that the Spanish high terrain creates such large instability. One hypothesis for the origin of the steep lapse rates is the Sahara Desert, where a well-mixed boundary layer forms steep lapse rates that can be advected away from northern Africa (known as the Saharan Air Layer). Yet, this hypothesis has not been tested, either for the Spanish plume or other regions downstream of high heated terrain. A different factor said to explain the occurrence of instability is the differential transport of air with low temperature or low moisture aloft. Although such explanations have been used in the literature, other studies have questioned the applicability of this factor. Our proposed research asks what processes produce the environment for midlatitude convective storms around the globe. What environments are favourable for instability, and how does this differ around the globe? What are the physical processes that create instability? Is instability - in Europe generally and the UK specifically - attributed to elevated heating, as in the EML of the central United States or by long-range transport? Despite conventional wisdom stating that the elevated mixed layer is responsible for creating the instability downstream of high terrain, it remains untested. Our aim in this proposal is to develop a better understanding of the relationship between high terrain, large-scale processes, and instability for midlatitude convective storms. These concerns motivate a multifaceted research project to answer these questions. Q1: What are the physical processes responsible for creating instability?Q2: How does topography create a favourable environment for deep moist convection?Q3: How important is differential temperature and moisture advection to creating instability?