Collaborative Research: Investigating Thermal Structure, Dynamics, and Dehydration in the Tropical Tropopause Layer with Fiber Optic Temperature Profiling from 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。该项目旨在通过开发和部署温度分析器来提高对TTL的理解，这是长期气球运动的一部分。该剖面仪的首字母缩写为FLOATS（光纤激光操作大气温度传感器），它沿着一根长2公里的光纤电缆测量温度，该电缆悬挂在气球吊舱下方。FLOATS的工作原理是将激光注入光纤电缆并感应拉曼背散射光。它可以以0.3K的精度测量温度，垂直分辨率为3米，每两到五分钟生成一个剖面。一个概念验证原型在EAGER合同下开发，编号AGS-1419932。FLOATS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。浮标的测量用于解决三个科学目标，所有这些目标都使用沿光纤电缆的温度变化来检查重力波的行为（空气中的波浪运动，其中浮力提供恢复力）。首先是通过垂直传播的重力波降低对流层顶冷点（CPT）温度，这是决定有多少水蒸气通过TTL进入平流层的关键因素。如果在相对湿度达到100%时发生冷凝或升华，则在CPT温度下进入平流层的水蒸气量不应超过100%相对湿度对应的量。其中一个pi和其他人的早期工作表明，波浪引起的CPT温度下降可能是显著的，波浪活动的增加可能是导致CPT温度和平流层水汽下降的主要原因，据信发生在21世纪的第一个十年。第二个科学目标是了解重力波在TTL中产生卷云的作用。这个问题与温度下降造成的平流层脱水是互补的，因为更低的温度阻止水蒸气进入平流层，而是冻结形成卷云冰粒。第三个目标是测量与波浪有关的垂直动量通量。这些波对平流层准两年一次振荡（QBO）至关重要，它可以影响长期天气预报的技巧。动量通量将通过结合浮子测量（给出波浪振幅、垂直波长和相速度）和气球贡多拉上的附加测量（包括从GPS定位推断的波浪引起的方向位移）来估计。由于观测值对解决有关TTL过程和平流层QBO的各种问题具有价值，因此该工作具有更广泛的科学影响。特别是，波动量通量数据对于发展重力波参数化以改进用于天气预报的数值模式具有价值。在科罗拉多大学博尔德分校的大气和空间物理实验室的服务器上，浮标的观测结果将提供给研究团体，以便研究团体可以自由地对它们进行检查。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.Work under this project seeks to improve understanding of the TTL by developing and deploying a temperature profiler as part of a long-duration ballooning campaign. The profiler, with the acronym FLOATS (Fiber-optic Laser Operated Atmospheric Temperature Sensor), measures temperature along a fiber optic cable 2km long which is suspended below a balloon gondola. FLOATS works by injecting laser light into a fiber optic cable and sensing Raman backscattered light. It can measure temperature with 0.3K precision and a vertical resolution of 3m, generating a profile every two to five minutes. A proof-of-concept prototype was developed under an EAGER award, AGS-1419932. FLOATS is deployed as 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.The FLOATS measurements are used to address three scientific objectives, all of which use temperature variations along the fiber optic cable to examine the behavior of gravity waves (wave motions in the air in which buoyancy provides the restoring force). The first is the depression of the cold point tropopause (CPT) temperature by vertically propagating gravity waves, which is a key factor in determining how much water vapor enters the stratosphere through the TTL. If condensation or sublimation occurs when relative humidity reaches 100%, then the amount of water vapor entering the stratosphere should not exceed the amount corresponding to 100% relative humidity at the CPT temperature. Earlier work by one of the PIs and others suggests that wave-induced CPT temperature depression could be significant, and an increase in wave activity could be responsible for a substantial portion of the decrease in CPT temperature and stratospheric water vapor believed to have occurred in the first decade of the 21st century. The second science objective is to understand the role of gravity waves in producing cirrus clouds in the TTL. The problem is complementary to stratospheric dehydration produced by temperature depression, as water vapor prevented from entering the stratosphere by colder temperatures instead freezes out to form cirrus ice particles.The third objective is to measure the vertical momentum flux associated with the waves. These waves are critical for the stratospheric quasi-biennial oscillation (QBO), which can influence the skill of long-range weather forecasts. Momentum flux would be estimated by combining FLOATS measurements (which give wave amplitude, vertical wavelength, and phase speed) with additional measurement taken on the balloon gondola including wave-induced directional displacement inferred from GPS positioning.The work has scientific broader impacts due to the value of the observations for addressing a variety of questions regarding TTL processes and the stratospheric QBO. In particular, wave momentum flux data is of value for developing gravity wave parameterizations for use in improving numerical models used for weather forecasting. FLOATS observations will be made available to the research community from servers at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder, so that they 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.