Water Vapor, Clouds, and Aerosol in the Tropical Tropopause Layer with in situ and Profiling Measurements from Long Duration Strateole-2 Balloons

热带对流层顶层（TTL）是对流层（云层、降水和天气系统的领域）和平流层（平流层上方包含臭氧层的寒冷干燥区域）之间的过渡区。TTL在热带上空从14公里延伸到18.5公里，除了最高的对流云之外，它在所有云的顶部之上。它通常包含薄卷云，虽然很难看到，但在捕获辐射从而使对流层变暖方面发挥着巨大的作用。它有时被称为“通往平流层的门户”，因为平流层中的大部分水蒸气和许多其他平流层成分都是通过TTL向上移动进入平流层的。与对流层相比，平流层极其干燥，但平流层的水蒸气引起了人们的兴趣，因为它具有重要的温室效应，可以导致极地平流层云的形成，而平流层云在臭氧空洞的形成中起着重要作用。该项目旨在通过在TTL顶部建造和发射几个气球上的仪器来提高对TTL过程的理解。这些气球是由法国国家空间研究中心（CNES）、法国航天局和巴黎萨克雷大学气象动力学实验室（LMD）组织的“战略-2”野外活动的一部分。战略-2是一个为期五年的项目，2018年进行小规模的验证部署，2020-2021年和2022-2023年进行全面的科学部署。气球从塞舌尔发射（在印度洋大约5秒），预计每个气球将环绕地球长达90天，并在20S至15N之间观察TTL。该奖项支持美国参与验证活动和首次全面的科学部署，以及活动后的分析。这是美国PIs参与战略-2的三个合同之一，全套合同是AGS-1643022、AGS-1642277/1642246和AGS-1642650/1653644。其中一个要建造和部署的仪器是一个光学粒子探测器，被称为LASP粒子计数器（LPC，以科罗拉多大学大气和空间物理实验室命名）。LPC使用散射激光探测大小在0.3至10微米之间的气溶胶颗粒，并用于估计通过TTL顶部进入平流层的气溶胶通量。除了LPC之外，pi正在开发一种名为RACHuTS的仪器包，用于卷取气溶胶云湿度和温度传感器。RACHuTS包装重量不到2公斤，悬挂在尼龙电缆上，尼龙电缆被卷绕到气球贡多拉下方2公里处，然后再次卷绕起来，每次都能在15分钟内产生TTL条件的垂直剖面。当气球在高空飞行时，RACHuTS每晚下降和上升5次（光学组件在白天不起作用）。纳入rachut的仪器是在其他地方开发的。其中包括紧凑型光学后向散射气溶胶探测器（COBALD，由苏黎世联邦理工学院开发）、气球荧光高级平流层湿度计（FLASH-B，由莫斯科中央气象台开发）和热力学传感器（TSEN，由LMD开发）。商用无线电探空仪也用于记录大气压力和GPS位置。在这个项目中需要解决的一个问题是，在确定有多少水蒸气通过TTL进入平流层时，过饱和是一个重要的考虑因素。进入平流层的水汽量受到对流层顶最低温度的限制，假设该水平以上的水汽量不能超过该水平100%相对湿度所对应的水汽量。但是，在冰粒浓度足够低且上升速度足够快的情况下，相对湿度可能超过100%，从而使水分在升华发生之前到达平流层。RACHuTS仪器非常适合研究过饱和效应，因为温度、湿度和冰粒浓度可以在TTL深度的很大一部分上一起观察。使用LPC解决的另一个问题是初级气溶胶进入平流层的运输。pi试图限制这些气溶胶的大小分布，并解决与分布的稳定性、地理变异性、与对流超调的关系以及先前对低数量浓度的大颗粒的观测有关的问题。由于对解决有关TTL过程的各种问题的观察值，该工作具有更广泛的科学影响。rachut和LPC的观察结果将从LASP的服务器上提供给研究界，以便研究界可以自由地对其进行检查。此外，该项目是一项气象观测新技术的示范，它可能具有广泛的科学应用。除此之外，该项目还支持一名博士后研究助理，从而在该研究领域发展未来的科学劳动力。

The tropical tropopause layer (TTL) is transition zone between the troposphere (the domain of clouds, precipitation, and weather systems) and the stratosphere (the cold and dry region above it which contains the ozone layer). The TTL extends from about 14 to 18.5km over the tropics and is above the tops of all but the tallest convective clouds. It often contains thin cirrus clouds which, though difficult to see, play an outsized role in trapping outgoing radiation and thus warming the troposphere. It is sometimes called the "gateway to the stratosphere" because most of the water vapor in the stratosphere, and many other stratospheric constituents, enter the stratosphere by moving upward through the TTL. The stratosphere is extremely dry compared to the troposphere, but stratospheric water vapor is of interest because it has an important greenhouse effect and can lead to the formation of polar stratospheric clouds which play a role in ozone hole formation.This project seeks to improve understanding of TTL processes by building and launching several instruments on balloons intended to fly at the top of the TTL. The balloons are part of the Strateole-2 field campaign organized by the Centre National d'Etudes Spatiales (CNES), the French Space Agency, and the Laboratoire de Meteorologie Dynamic (LMD) of the University of Paris-Saclay. Strateole-2 is a five-year campaign, with a small validation deployment in 2018 and full science deployments in 2020-2021 and 2022-2023. Balloons are launched from the Seychelles (about 5S in the Indian Ocean), with the expectation that each balloon will circle the earth for up to 90 days and observe the TTL between 20S and 15N. This award supports US participation in the validation campaign and the first full science deployment, along with post-campaign analysis. It is one of three awards made to US PIs for participation in Strateole-2, the full set being AGS-1643022, AGS-1642277/1642246, and AGS-1642650/1653644.One instrument to be built and deployed is an optical particle detector referred to as the LASP Particle Counter (LPC, named after the Laboratory for Atmospheric and Space Physics at the University of Colorado). The LPC uses scattered laser light to detect aerosol particles with sizes ranging from 0.3 to 10 microns and is used to estimate the flux of aerosols entering the stratosphere through the top of the TTL. In addition to the LPC the PIs are developing an instrument package called RACHuTS, for Reel-down Aerosol Cloud Humidity and Temperature Sensor. The RACHuTS package weighs less than 2kg and is suspended from a nylon cable which is reeled down to 2km below the balloon gondola and then reeled back up again, each time generating a vertical profile of TTL conditions over 15 minutes. RACHuTS is lowered and raised 5 times each night while the balloon is aloft (the optical components do not function in daylight). The instruments integrated into RACHuTS are developed elsewhere. These include the Compact Optical Backscatter Aerosol Detector (COBALD, developed at ETH Zurich), the Fluorescence Advanced Stratospheric Hygrometer for Balloon (FLASH-B, developed at the Central Aerological Observatory in Moscow), and the Thermodynamic SENsor (TSEN, developed at LMD). A commercial radiosonde package is also used to record atmospheric pressure and GPS position.One issue to be addressed in the project is the extent to which supersaturation is an important consideration in determining how much water vapor passes through the TTL and enters the stratosphere. The amount of water vapor entering the stratosphere is limited by the temperature minimum at the tropopause, under the assumption that the amount of water vapor above that level cannot exceed the amount corresponding to 100% relative humidity at that level. But it is possibly for relative humidity to exceed 100% in cases where ice particle concentrations are low enough and ascent is fast enough to allow moisture to reach the stratosphere before sublimation can occur. The RACHuTS instruments are well suited to the study of supersaturation effects as temperature, humidity, and ice particle concentrations are observed together over a substantial portion of the depth of the TTL. A further issue, to be addressed using the LPC, is the transport of primary aerosols into the stratosphere. The PIs seek to constrain the size distribution of these areosols and address questions related to the stability of the distribution, its geographical variability, its relationship to convective overshooting, and previous observations of large particles with low number concentrations.The work has scientific broader impacts due to the value of the observations for addressing a variety of questions regarding TTL processes. RACHuTS and LPC observations will be made available to the research community from servers at LASP so that it can be freely examined by the research community. In addition, the project constitutes a demonstration of a novel technology for meteorological observations which could have a wide array of scientific applications. Beyond these considerations, the project supports a postdoctoral research associate, thereby developing the future scientific workforce in this research area.