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)，即赤道两侧下沉空气带中的相对湿度，是决定副热带云的行为及其对气候影响的重要因素。亚热带地区的空气一般在赤道附近的辐合带中发现的深对流云上升后，进入对流层上部的区域。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低温度，因为冷凝将空气干燥到由其温度给出的饱和水湿值(较低的水分含量对于较冷的空气)。但其他因素也影响着下沉的副热带空气的相对湿度，特别是空气可以通过与更潮湿的空气混合而变湿，这些空气来自更潮湿的水平，更接近地面。相同的相对湿度可以通过较暖的最后饱和温度和较少的混合，也可以通过相对较低的最后饱和温度然后较大的混合来实现，这两条路径对气候变暖的云反馈和相对湿度变化可能有不同的含义。然而，这两个途径可以通过检查水蒸气中较重同位素的相对浓度来区分，水蒸气中的一个氢原子被重氢原子取代，或者氧16原子被氧原子18取代。粗略地说，较重形式的水蒸气蒸发更慢，更容易凝结，这一影响取决于蒸发或冷凝发生的温度。因此，重同位素包含了重要的线索，有助于理解决定副热带相对湿度的过程，并将其与副热带云及其气候反馈联系起来。出于这一动机，PIS利用卫星和地面观测以及气候模型模拟，检查了副热带对流层中层水蒸气的同位素浓度。地面观测包括由智利查南托高原的PI领队进行的测量(见AGS-1158582)。许多工作是使用反向建模技术进行的，其中，使用模仿自然选择的机器学习算法来确定最佳参数集(包括最后的饱和温度和垂直混合等)。逆技术是有利的，因为它在计算上是廉价的，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，因为它可以更好地理解副热带云在气候变化中的作用，这是预测温室气体增加导致的变暖数量的一个核心问题。此外，该项目还包括通过位于阿尔伯克基的新墨西哥州自然历史和科学博物馆开展广泛的教育和外联工作。该博物馆服务于大量西班牙裔和美国原住民，包括市中心和农村社区。这项工作通过夏令营项目吸引小学生，通过“初级讲解员”暑期项目吸引初中生，通过专业发展研讨会吸引初中和高中教师。此外，该项目还为研究生提供支持和培训，从而为这一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.