The Fast and Slow Response of the Hydrological Cycle

水循环除了负责云的形成和降雨外，还是大气能量循环的关键组成部分，大气能量循环通过蒸发将地表冷却与通过大气中的潜热释放将大气加热联系起来。由于水文循环的变化而引起的大气水汽和云的变化，也通过改变进入的太阳能的吸收和反射以及红外辐射的发射来改变地面-大气交换。在过去的一个世纪里，人们观察到了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会导致地表和大气变暖，进而导致(海洋)表面蒸发增加，大气中的水分和云量增加，以及更多的降雨量。随着水文循环的加强，从地表转移的额外热量使大气进一步变暖。再加上大气中更多的水分导致更强的温室效应，导致向地表发射更强的红外辐射，这抵消了由于更强的蒸发而导致的地表变暖减少。这些变化既可以发生在缓慢(几年到几十年)的时间尺度上，也可以发生在快速(几天到数月)的时间尺度上。正是这些能量流动的变化，而不是湿度的增加，导致全球降雨量增加，以应对日益增加的温室气体。因此，为了充分了解和预测未来降雨量的变化，需要更好地了解在气候变暖的情况下地表和大气之间的能量转移将如何变化。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个体贡献，并量化它们对降水变化的贡献。通过这样做，这项研究将更好地理解未来降水变化模型预测所依据的物理机制。特别是，这项研究致力于更好地理解不同降雨变化模型预测之间差异的原因，并利用这些信息来改进模型，最终提供更可靠的未来变化预测。由于降雨在决定淡水可获得性和农业粮食生产方面的作用，其数量、分布或强度的变化对人类和环境具有重大影响。此外，暴雨事件发生的变化改变了洪水的发生频率，从而导致基础设施受损、财产损失和生命损失。因此，预测未来由气候变化引起的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最终将使人们更好地理解支配降雨变化的潜在物理过程，并改进对未来降雨变化的预测。通过提高降雨变化的长期预测的可靠性，该项目将帮助决策者规划适当的适应和缓解战略，进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智力优势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.