Relating new theories of extratropical cyclone development to the present and future atmosphere

飓风对中纬度国家的人民和经济有重大影响。破坏性影响包括与强降雨相关的严重大风和洪水。这些天气系统对温度、云量和降雨量的影响不那么剧烈，但却至关重要。例如，2000年秋季，英国降雨量达到了前所未有的程度和持续时间，引发了人们对其与气候变化关系的猜测。作为回应，DEFRA委托了一份报告（FD2304）来审查这一联系。降雨与异常持续和反复通过的锋面低压有关，而不是特别极端的气旋。哈德利中心气候模式在本世纪末的模拟中发现，这种季节性极端降雨更为频繁，尽管自然变率如此之大，以至于2000年秋季的降雨不能归因于人类对气候的影响。气候模式对地球系统的描述存在许多不确定性，特别是考虑到风暴的行为。然而，从理论角度来看，风暴频率和强度变化背后的物理机制还没有得到充分的理解，无法确定模拟结果在物理上是否合理。英国的天气预报很少不涉及正在或即将到来的气旋及其相关锋面。数值天气预报模式通过在“确定尺度”上数值求解运动和热力学的“原始方程”，以及参数化较小尺度的影响，通常可以很好地预报气旋的发生，预报时间可达5天以上。虽然大尺度动力学理论涉及由称为位涡度的单个量的演变所控制的平衡流，但这种平衡模型不够精确，无法用于业务预测。此外，关于气旋发展的理论假设它们是从一个“背景状态”发展起来的，这个状态不会在纬度圈周围变化，但是大气从来不会穿过这样的状态。因此，在理论能力和预报模式对大气的描述之间存在不匹配。尽管如此，该理论确实为理解预测的演变及其误差提供了一个框架，并且还有改进理论的余地，可以从数量上区分潜在的增长机制。理论将在未来风暴行为变化的有力论证中发挥更大的作用。例如，政策制定者和社会的许多其他部门想知道，由于温室气体的强迫作用，在一个变暖的世界里，“极端风暴”是否会变得更加频繁——如果这些变化确实发生，它们将产生重大的经济影响。该项目的目的是将一些有关中纬度气旋的最新理论应用于实际大气的分析，试图确定气旋特性（如地面风力强度）与背景状态特性（如极地到赤道的温度梯度）之间依赖关系的可靠预测。通过建立背景状态（气候）和风暴行为之间的定量关系，该项目将产生一个框架，用于分析不同气候情景下极端风暴的变化。如果气候模型模拟的风暴变化符合基于理论的合理物理推理的预期，那么就有可能对气候预测提供更大的信心。

Cyclones have a major impact on people and the economies of countries in the mid-latitudes. Damaging impacts include severe gales and flooding associated with heavy rainfall. These weather systems also have less dramatic but nevertheless crucial impacts on temperature, cloudiness and rainfall. For example, autumn 2000 brought UK rainfall of unprecedented extent and duration, and prompted speculation about its relationship with climate change. In response, DEFRA commissioned a report (FD2304) to examine the link. The rainfall was associated with the unusually persistent and repeated passage of frontal depressions, rather than especially extreme cyclones. Such seasonal rainfall extremes were found to be more frequent in simulations of the end of the century by the Hadley Centre climate model, although the natural variability is so large that the autumn 2000 rainfall cannot be attributed to human influence on climate. There are many uncertainties associated with the representation of the Earth system by climate models, especially considering the behaviour of storms. However, the physical mechanisms behind changes in storm frequency and intensity are not sufficiently well understood from a theoretical perspective to determine whether the results of the simulations are physically reasonable. A weather forecast for the UK is hardly ever made without reference to a cyclone and its associated fronts that are with us or due to arrive soon. Numerical weather prediction models generally provide good forecasts of cyclone occurrence out to more than 5 days ahead by solving numerically the 'primitive equations' of motion and thermodynamics on 'resolved scales' and parameterising the effects of smaller scales. Although the theories of large-scale dynamics involve balanced flow governed by the evolution of single quantity called potential vorticity, such balanced models are not sufficiently accurate to be used in an operational forecasting context. Furthermore, theories of cyclone development assume that they grow from a 'background state' that does not vary around latitude circles, but the atmosphere never passes through such a state. Therefore, there is a mismatch between the capabilities of theory and the description of the atmosphere by forecast models. Nevertheless, the theory does provide a framework for understanding the evolution of forecasts and their errors and there is scope for improving theory to the point where it can distinguish quantitatively between potential mechanisms of growth. Theory has a greater role to play in robust arguments for changes in storm behaviour in the future. For example, policy makers and many other sectors of society would like to know whether 'extreme storms' will become more frequent in a warmer world resulting from greenhouse gas forcing - if these changes do occur they will have major economic impacts. The aim of this project is to bring some of the latest theories of mid-latitude cyclones to bear on analyses of the actual atmosphere in an attempt to identify robust predictions for the dependence of cyclone properties, such as surface wind strength, on background state properties, such as the pole to equator temperature gradient. By establishing quantitative relationships between background states (climate) and storm behaviour, this project will produce a framework for analysing changes to extreme storms under different climate scenarios. If the changes to storms simulated by a climate model accord with the expectations of sound physical reasoning, grounded in theory, it would be possible to place greater confidence in the climate projections.