Collaborative Research: Improving Constraints on Tropical Climate Feedbacks with Inverse Modeling of the Stable Isotopic Composition of Atmospheric Water Vapor

亚热带空气的相对湿度（RH），即赤道两侧沉降气流带的相对湿度，是决定亚热带云的行为及其对气候影响的重要因素。副热带的空气一般在赤道附近辐合带的深层对流云中上升后进入对流层上层。空气的相对湿度在很大程度上取决于它在云内上升过程中遇到的最冷温度，因为冷凝将空气干燥到其温度给出的饱和水分值（冷空气的水分含量较低）。但其他因素也会影响下沉的亚热带空气的相对湿度，特别是空气可以通过与靠近地表的更潮湿水平的空气混合而变得湿润。相同的相对湿度可以通过相对温暖的最后饱和温度和少量混合来实现，也可以通过相对寒冷的最后饱和温度和更多混合来实现。在变暖的气候中，达到相同相对湿度的两种途径可能对云反馈和相对湿度变化产生不同的影响。然而，这两种途径可以通过检查水蒸气的较重同位素的相对浓度来区分，水蒸气中的一个氢原子被氘取代，或者氧16原子被氧18取代。粗略地说，较重形式的水蒸气比普通的水蒸发得更慢，凝结得更容易，这取决于蒸发或凝结发生的温度。因此，重同位素为理解副热带RH的形成过程以及副热带云及其气候反馈提供了重要线索。基于这一动机，pi利用卫星和地面观测以及气候模式模拟来研究亚热带对流层中水汽的同位素浓度。地面观测包括由首席PI在智利Chajnantor高原进行的测量（见AGS-1158582）。大部分工作都是使用逆建模技术进行的，其中使用模仿自然选择的机器学习算法确定一组最佳参数（包括最后饱和温度和垂直混合等）。逆技术的优势在于它计算成本低，并且可以同样好地应用于观测和模型输出。这项工作具有社会意义，因为它可以使人们更好地理解亚热带云在气候变化中的作用，这是预测温室气体增加引起的变暖程度的一个核心问题。此外，该项目还包括通过阿尔伯克基的新墨西哥自然历史和科学博物馆开展广泛的教育和推广工作。该博物馆为大量西班牙裔和美洲原住民提供服务，包括内城和农村社区。该项目通过夏令营项目吸引小学生，通过“初级讲解员”暑期项目吸引初高中学生，通过专业发展研讨会吸引初高中教师。此外，该项目为研究生提供支持和培训，从而为该研究领域的未来劳动力提供支持。

The relative humidity (RH) of air in the subtropics, meaning RH in the belts of subsiding air found on either side of the equator, is an important factor in determining the behavior of subtropical clouds and their effects on climate. Air in the subtropics generally enters the region in the upper troposphere after ascending in the deep convective clouds found in the convergence zones near the equator. The RH of the air is largely determined by the coldest temperature it encounters during in-cloud ascent, as condensation dries the air to the saturation moisture value given by its temperature (lower moisture content for colder air). But other factors also influence the RH of the subsiding subtropical air, in particular the air can be moistened by mixing with air from more humid levels closer to the surface. The same RH can be achieved either by a relatively warm last saturation temperature with little mixing or a relatively cold last saturation temperature followed by greater mixing, The two pathways to the same RH can have different implications for cloud feedbacks and RH change in a warming climate. However, these two pathways can be distinguished by examining the relative concentrations of heavier isotopes of water vapor, water vapor in which one of the hydrogen atoms is replace by deuterium or the oxygen 16 atom is replace by oxygen 18. Roughly speaking, the heavier forms of water vapor evaporate more sluggishly and condense more readily than ordinary H2O, an effect which depends on the temperature at which the evaporation or condensation takes place. Thus, heavy isotopes contain important clues to understanding the processes which set subtropical RH and relate it to subtropical clouds and their climate feedbacks. With this motivation the PIs examine the isotopic concentration of water vapor in the subtropical mid-troposphere using satellite and ground-based observations as well as climate model simulations. Ground-based observations include measurements taken by the lead PI on the Chajnantor Plateau in Chile (see AGS-1158582). Much of the work is performed using an inverse modeling technique in which an optimal set of parameters (including last saturation temperature and vertical mixing, among others) is determined using a machine learning algorithm that mimics natural selection. The inverse technique is advantageous in that it is computationally inexpensive and can be applied equally well to both observations and model output. The work has societal relevance as it can lead to a better understanding of the role of subtropical clouds in climate change, a central issue in efforts to anticipate the amount of warming caused by greenhouse gas increases. In addition, the project includes an extensive education and outreach effort through the New Mexico Museum of Natural History and Science in Albuquerque. The museum serves a large Hispanic and Native American population including both inner city and rural communities. The effort engages elementary school students through a summer camp program, middle and high school students through a "junior docent" summer program, and middle and high school teachers through a professional development workshop. In addition, the project provides support and training for a graduate student, thereby providing for the future workforce in this research area.