The Fast and Slow Response of the Hydrological Cycle

水文循环除了负责形成云和产生降雨外，还是大气能量循环的一个关键组成部分，它将通过蒸发进行的表面冷却与通过大气中潜热释放进行的大气加热联系起来。由于水文循环的变化而引起的大气水蒸气和云的变化也通过改变对入射太阳能的吸收和反射以及红外辐射的发射而改变地表-大气的交换。在过去的世纪，人们观察到降雨量和暴雨事件频率的变化。这些变化在一定程度上是人类排放温室气体的结果。 二氧化碳、气溶胶和其他温室气体水平的增加导致地表和大气变暖，这反过来又导致更多的（海洋）表面蒸发，大气中的水分和云的数量增加，以及更多的降雨。随着水循环的加强，从地表转移出去的额外热量使大气进一步变暖。这与大气中更多的水分导致的更强的温室效应一起导致向地表发射更强的红外辐射，这补偿了由于更强的蒸发导致的地表变暖减少。 这些变化可以在缓慢（几年到几十年）和快速（几天到几个月）的时间尺度上发生。正是这些能量流动的变化，而不是水分的增加，导致全球降雨量增加，以应对温室气体的增加。因此，为了充分了解和预测未来降雨量的变化，需要更好地了解地表和大气之间的能量转移在温暖的气候中将如何变化。这项研究使用了一种创新的方法来计算温室气体和气溶胶的增加以及更温暖和更潮湿的气候对地表和大气之间能量流动的影响。通过这种称为辐射核的新方法，可以分离出不同过程的单独贡献，并量化它们对降水变化的贡献。在这样做的过程中，这项研究将提供一个更好的理解的物理机制，基础模型预测未来降水量的变化。特别是，这项研究努力更好地了解不同模型预测的降雨量变化之间的差异的原因，并使用这些信息来改进模型，并最终提供更可靠的预测未来的变化，因为降雨量在确定淡水的可用性和农业粮食生产的作用，其数量，分布或强度的变化对人类和环境产生重大影响。此外，强降雨事件发生的变化改变了洪水的频率，洪水造成基础设施的破坏、财产损失和生命损失。因此，预测气候变化导致的未来降雨量变化是一项具有重要社会效益的关键科学挑战。从这项研究中获得的知识将最终导致更好地了解控制降雨变化的基本物理过程，并改善对未来降雨变化的预测。通过提高降雨量变化长期预测的可靠性，该项目将帮助决策者制定适当的适应和缓解战略，从而减少未来气候变化的成本和后果。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The hydrological cycle, besides being responsible for the formation of clouds and producing rainfall, is a key component of the atmospheric energy cycle that links surface cooling via evaporation to atmospheric heating via latent heating release in the atmosphere. Changes in atmospheric water vapor and clouds due to changes in the hydrological cycle also alter the surface-atmosphere exchanges via changes in absorption and reflection of the incoming solar energy and in emission of infrared radiation. Changes in the amount of rainfall and the frequency of heavy rain events have been observed over the last century. These changes are, in part, a result of human emission of greenhouse gases. Increasing levels of carbon dioxide, aerosols, and other greenhouse gases causes warming of the surface and atmosphere, which in turns leads to more evaporation from (ocean) surfaces and increases of the amount of moisture and clouds in the atmosphere, as well as more rainfall. The extra heat transferred away from the surface associated with the strengthening of hydrological cycle warms the atmosphere further. This together with the stronger greenhouse effect due to more moisture in the atmosphere leads to a stronger emission of infrared radiation to the surface, which compensates the surface warming reduction due to stronger evaporation. These changes can occur on both slow (years to decades) and fast (days to months) timescales. It is these changes in the flow of energy, not the increase in moisture, that causes rainfall to increase globally in response to increasing greenhouse gases. Thus, in order to fully understand and predict future changes in rainfall requires a better understanding how the transfer of energy between the surface and atmosphere will change in a warmer climate. This research uses an innovative method of computing the effects of an increase in greenhouse gases and aerosols, and a warmer and more humid climate, on the flow of energy between the surface and atmosphere. Through this new method, termed radiative kernels, the individual contributions of different processes can be isolated and their contributions to the changes in precipitation quantified. In doing so, this research will provide a better understanding of the physical mechanisms that underlie model projections of future precipitation change. In particular, this research strives to better understand the cause of disparities between different model projections of rainfall change and use this information to improve models and ultimately provide more reliable projections of future changes.Because of the role of rainfall in determining the availability of fresh water and agricultural food production, changes in its amount, distribution or intensity have significant impacts on people and the environment. In addition, changes in the occurrence of heavy rainfall events alters the frequency of floods, which cause damage to infrastructure, loss of property, and loss of life. Predicting future changes in rainfall that stem from changes in climate therefore represents a key scientific challenge with important societal benefits. The knowledge gained from this research will ultimately lead to both a better understanding of the underlying physical processes that govern rainfall change and improved predictions of future rainfall change. By increasing the reliability of long term projections of changes in rainfall, this project will help decision makers plan for adequate adaptation and mitigation strategies that, in turn, lessen the cost and consequences of future changes in climate.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.