Collaborative Research: Dynamics of Subtropical Humidity

水汽反馈是放大气候对人为温室气体增加响应的最重要机制。副热带自由对流层的干燥区域在确定这种反馈的强度方面起着特别重要的作用。干燥的亚热带地区的湿度取决于在某一给定时间终止于亚热带地区的轨迹系综上沿着遥远位置的气团最后遇到饱和时的温度。水汽反馈的变化取决于这些最后饱和度的统计数据如何随着气候变暖而变化。发展了一种基于示踪剂的全球气候模式（GCMs）和再分析资料中末次饱和统计量诊断技术。这项工作的研究目标是使用这些诊断技术，以更好地了解在现代和未来的气候中控制亚热带湿度的过程。智力优点：这项工作的完成将导致（1）根据再分析数据和大气环流模式模拟的记录，发展现代亚热带气候中最后饱和度的统计数据，重点是季节和年际变化以及热带外涡旋和Hadley和步行者环流在控制亚热带湿度方面的作用;（2）进一步理解在全球变暖情景下最后的饱和度统计数据可能如何变化;（3）使用理想化的GCM在更基本的、面向过程的水平上检验控制亚热带湿度的机制。 更广泛的影响：这项研究将为两名研究生提供热带湿度变化诊断分析以及对当前和未来全球气候影响的培训。研究结果将有助于更广泛的气候变化评估工作，并可能有利于社会和政策考虑。

The water vapor feedback is the most important mechanism for amplifying the climatic response to increased anthropogenic greenhouse gases. The dry regions of the subtropical free troposphere play an especially important role in determining the strength of this feedback. The humidity in the dry subtropics depends on the temperature at which saturation was last encountered by air parcels at remote locations along the ensemble of trajectories that terminate in the subtropics at a given time. Changes in the water vapor feedback depend on how these statistics of last saturation may change as the climate warms. A tracer-based technique is developed for diagnosing the statistics of last saturation in Global Climate Models (GCMs) and reanalysis data. The research objective of this work is to use these diagnostic techniques to better understand the processes that control the humidity of the subtropics in modern and future climates. Intellectual merit: Completion of this work will result in (1) development of the statistics of last saturation within the modern climate of the subtropics, as recorded in reanalysis data and in GCM simulations, with a focus on seasonal and interannual variability and on the roles of extratropical eddies and the Hadley and Walker circulations in controlling subtropical humidity; (2) further understanding of how the last saturation statistics may change under global warming scenarios; (3) examination of the mechanisms controlling subtropical humidity at a more basic, process-oriented level using idealized GCMs. Broader impacts: The research will provide training to two graduate students in diagnostic analysis of tropical moisture variability and impacts on present and future global climates. The results will aid the broader climate change assessment efforts and will be of potential benefit for societal and policy considerations.