Collaborative Research: Tropical waves and their effects on circulation from 3D GPS radio occultation sampling from stratospheric balloons in Strateole-2

热带对流层上层和平流层下层是各种波运动的家园，这些波运动在天气、气候和大气环流中起着关键作用。由大面积热带对流产生的宽波（水平波长跨越几个纬度）但浅波（垂直波长约1至4公里）是本研究的主题。这些波之所以引起人们的兴趣，有三个原因：首先，这些波可以在热带对流层顶层（TTL）中诱发卷云的形成，TTL是对流层和平流层之间的过渡区，从大约14公里延伸到18.5公里。TTL卷云可以形成与上升的空气运动相关的波降低空气温度，并导致水蒸气冻结成卷云冰晶。由此产生的云可能太薄，无法从地面或卫星上看到，但它们具有重要的气候效应，因为它们捕获了向外发射的红外辐射，从而使大气变暖。这种云的流行程度很难量化，与深积云冰粒子流出形成卷云相比，波浪运动作为TTL卷云来源的相对重要性尚不清楚。其次，由波浪引起的温度下降导致的水蒸气冻结可能是限制进入平流层的水蒸气量的重要因素。TTL有时被称为“平流层的门户”，因为整个地球平流层中的大部分水蒸气都是通过TTL进入的。与对流层相比，平流层极其干燥，但平流层的水蒸气仍然很重要，因为它具有相对较强的温室效应，并可导致极地平流层云的形成，而极地平流层云是形成臭氧空洞的关键。第三，波浪可以将动量从对流层输送到平流层，而波动量输送是平流层准两年振荡（QBO）的主要驱动因素，这是一种发生在全球热带地区的东风和西风交替，循环时间超过两年。虽然QBO仅限于低纬度平流层，但它可以通过对北大西洋涛动等主要气候变率模态的影响影响全球天气和气候。虽然波浪动量输运理论已经建立，但不同波型在驱动QBO中的相对重要性仍然存在不确定性，目前的天气和气候模式难以模拟QBO。该项目旨在通过建造和发射一个气球载仪器来提高对TTL波的理解，该仪器接收来自全球导航卫星系统（GNSS，其中包括美国发射的GPS卫星）卫星的定位信号。全球导航卫星系统信号在穿过大气时发生折射，折射量可用于推断对流层上层的气温。由于这些剖面是相对于接收机的GNSS卫星的上升和下降或掩星进行检索的，因此气球载仪器的首字母缩写为ROC，即无线电掩星。ROC被开发用于气球流动，作为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stratole -2野外运动的一部分。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。ROC定位于从气球飞行路径两侧的GNSS卫星检索信号，观测在8公里至约20公里的飞行高度之间，垂直分辨率在200米至250米之间。观测的几何结构是这样的，在较低的高度上的观测距离气球更远，因此在18,15和12km高度上的观测对应于气球两侧大约100,200和300km的距离。感兴趣的波周期从几小时到几天不等，ROC每小时可以记录两到三次掩星。因此，当气球沿着其轨迹前进时，ROC测量捕获了波的三维结构。ROC伴随着另外两个提供补充观察的仪器。一个是气球载云超调观测激光雷达（BeCOOL），由法国大气、环境、空间观测实验室（LATMOS，皮埃尔·西蒙·拉普拉斯研究所的一个实验室）与法国国家空间研究中心合作提供。激光雷达提供了卷云的测量数据，这些数据可以与ROC观测相结合，以检查波浪运动在生成卷云中的作用。另一个是温度传感器（TSEN），这是LMD的一种仪器，用于记录贡多拉的大气温度和压力。贡多拉的位移是由ROC精确确定的，而TSEN的观测结果被用来计算贡多拉相对于环境波动的运动（这些是在恒定密度水平上飞行的超压气球）。然后利用位移数据估计飞行水平上与ROC观测到的大尺度波相关的波动量通量。这项工作具有更广泛的科学影响，因为观测值解决了关于波动对TTL云、平流层湿度和QBO的影响的各种问题。在这个项目中收集的观测结果将通过科罗拉多大学大气和空间物理实验室的服务器提供给研究社区，以便研究社区可以自由地检查它们。该项目还通过加州大学圣地亚哥分校、亚利桑那大学（UA）和墨西哥自治大学（UNAM）同时提供的研究课程吸引本科生，学生们在该课程中设计一个基于ROC试飞的研究项目。接下来是在加州大学圣地亚哥分校的本科生研究实习，UNAVCO（致力于将GNSS技术应用于地球科学的大学NAVSTAR联盟）的学生固体地球科学研究经验（RESESS）计划，以及大学大气研究公司的大气研究与科学重大机遇（SOARS）计划。除了这些更广泛的影响，该项目还支持了两名研究生。

The tropical upper troposphere and lower stratosphere are home to a variety of wave motions which play key roles in weather, climate, and atmospheric circulation. Waves which are broad (horizontal wavelengths spanning several degrees latitude) but shallow (vertical wavelengths of about one to four kilometers), generated by large areas of tropical convection, are the subject of this investigation. These waves are of interest for three reasons: first, the waves can induce the formation of cirrus clouds in the tropical tropopause layer (TTL), the transition zone between the troposphere and stratosphere that extends from about 14km to 18.5km. TTL cirrus can form as rising air motions associated with the waves depress air temperatures and cause water vapor to freeze out as cirrus ice crystals. The resulting clouds may be too thin to see from the ground or from satellites, yet they have an important climatic effect as they trap outgoing infrared radiation and thus warm the atmosphere. The prevalence of such clouds is difficult to quantify, and the relative importance of wave motions as a source of TTL cirrus, in comparison to cirrus formation due to outflow of ice particles from deep cumulus clouds, is not known.Second, the freezing out of water vapor by wave-induced temperature depression could be an important constraint on the amount of water vapor entering the stratosphere. The TTL is sometimes referred to as the "gateway to the stratosphere", as most of the water vapor in the stratosphere over the entire globe enters through the TTL. The stratosphere is extremely dry compared to the troposphere, but stratospheric water vapor is nevertheless important as it has a relatively strong greenhouse effect and can lead to the formation of the polar stratospheric clouds which are key to the formation of the ozone hole.Third, waves can transport momentum from the troposphere to the stratosphere, and wave momentum transport is the primary driver of the stratospheric Quasi-Biennial Oscillation (QBO), an alternation between easterly and westerly winds occurring over the global tropics with a cycling time in excess of two years. While the QBO is narrowly confined to the low-latitude stratosphere, it can influence weather and climate worldwide through its effects on prominent modes of climate variability such as the North Atlantic Oscillation. While the theory of wave momentum transport is well established, uncertainties remain as to the relative importance of different wave types in driving the QBO, and current weather and climate models have difficulty in simulating it.This project seeks to improve understanding of waves in the TTL by building and launching a balloon-borne instrument which receives positioning signals from satellites of the Global Navigation Satellite System (GNSS, which includes the GPS satellites launched by the US). The GNSS signals are refracted as they pass through the atmosphere, and the amount of refraction can be used to infer air temperature in the upper troposphere. Because the profiles are retrieved from the rising and setting, or occultation, of the GNSS satellites relative to the receiver, the balloon-borne instrument has the acronym ROC, for Radio OCcultation.ROC is developed for use on balloons flow 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. ROC is oriented to retrieve signals from GNSS satellites on either side of the balloon flight path, with observations taken between 8km and the flight level of about 20km and a vertical resolution between 200m and 250m. The observing geometry is such that observations at lower levels are farther away from the balloon, so that observations at 18, 15, and 12km altitude correspond to distances of roughly 100, 200, and 300km on either side of the balloon. The waves of interest have periods from hours to days and ROC can record two to three occultations per hour. Thus the three-dimensional structure of the waves is captured by the ROC measurements as the balloon advances along its trajectory.ROC is accompanied by two other instruments which provide complementary observations. One is the Balloonborne Cloud Overshoot Observation Lidar (BeCOOL), provided by the Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS, a laboratory of the Institut Pierre Simon Laplace) in collaboration with CNES. The lidar provides measurements of cirrus clouds which can be combined with ROC observations to examine the role of wave motions in generating cirrus clouds. The other is the Temperature SENsor (TSEN), an instrument from LMD which records atmospheric temperature and pressure at the gondola. Gondola displacements are precisely determined by ROC, and TSEN observations are used to factor out gondola movement relative to the ambient wave motion (these are super-pressure balloons which fly at a level of constant density). The displacement data are then used to estimate the wave momentum flux at flight level associated with the large-scale waves observed by ROC.The work has scientific broader impacts due to the value of the observations for addressing a variety of questions regarding the effect of wave motions on TTL clouds, stratospheric humidity, and the QBO. Observations collected in this project will be made available to the research community from servers at the Laboratory for Atmospheric and Space Physics at the University of Colorado so that they can be freely examined by the research community. The project also engages undergraduate students through a research class, offered simultaneously at the University of California San Diego, the University of Arizona (UA), and the Autonomous University of Mexico (UNAM), in which students design a research project based on a test flight of ROC. The class is followed by undergraduate research internships at UCSD, UA, the Research Experiences in Solid Earth Sciences for Students (RESESS) program at UNAVCO (the University NAVSTAR Consortium, dedicated to applying GNSS technology to earth science), and the Significant Opportunities in Atmospheric Research and Science (SOARS) program of the University Corporation for Atmospheric Research. Beyond these broader impacts, the project supports two graduate students.