Collaborative Research: Four-Dimensional (4D) Investigation of Tropical Waves Using High-Resolution GNSS Radio Occultation from Strateole2 Balloons

该奖项支持首席调查人员继续参加由法国航天局(法国国家空间研究中心)和巴黎萨克雷大学动力气象实验室组织的Strateole-2实地活动。该活动利用气球观测热带对流层顶层(TTL)，即热带对流层和平流层之间大约14公里到18公里的大气层，气球设计用于长达3个月的飞行，在恒定高度漂浮。气球从塞舌尔群岛发射升空，围绕赤道漂浮在TTL顶部(18公里)或平流层下部(20公里)。Strateole-2计划为三次部署，一次是8次气球飞行的初步工程部署，然后是两次每次20次飞行的科学部署。前两次部署发生在2019年和2021年，私人投资机构使用AGS-1642650和AGS-1642644的资金参与了这些部署。这里支持PI参与计划于2025年10月开始的第三次部署。PI在Strateole-2中的角色是建造和飞行一个名为ROC的无线电掩星接收器，该接收器探测全球导航卫星系统(GNSS)卫星发射的无线电波的折射，其中包括美国发射的GPS卫星。折射的强度可以用来推断中华民国和发射卫星之间视线上的大气温度，因此中华民国可以通过跟踪GNSS卫星下降到地平线或从地平线下升起来创建温度分布。这笔资金用于建造六个ROC接收器，管理它们的现场部署，并收集和分析它们产生的数据。ROC的温度分布很有趣，因为它们显示了与大范围热带对流产生的TTL中的波动有关的温度波动。这些波之所以令人感兴趣，一个原因是它们推动了准两年振荡(QBO)，这是赤道平流层东风和西风之间的交替，从平流层上层开始，在大约两年的时间里下降到对流层顶。QBO仅限于热带地区，但它会影响世界各地的天气和气候。众所周知，QBO是由从TTL向上传播的波的垂直动量通量驱动的，但目前还不清楚哪种类型的波，特别是波长和频率，对驱动QBO最重要。另一个令人感兴趣的原因是，它们的上下运动与周围空气的冷却和变暖有关，而由上升运动引起的冷却可以导致水蒸气冻结成冰粒(这一过程被称为沉积)。结冰之所以重要，是因为它在空气进入平流层时使空气脱水，从而调节平流层的湿度，而且因为冰粒形成卷云，通过捕获发出的红外辐射来影响地球气候。QBO的波浪驱动工作重点放在周期为三到四天的波上，这在以前的部署中被发现是突出的。PI试图确定波的三维结构和它们的固有频率，这些因素共同决定了它们的波动量通量，从而它们对QBO驱动的潜在重要性。PI已经开发了探测波结构的技术，利用RO剖面从气球上侧视的事实，并随着距离气球吊车的距离在连续较低的高度测量温度。因此，通过组合气球飞行路径上连续的RO剖面，可以重建波的三维结构。至于卷云的形成，六个ROC接收器中的四个将搭载一种名为BeCOOL的向下指向的激光雷达，即由法国团队开发的气球搭载的云超调观测激光雷达(BeCOOL)。BeCOOL对卷云的观测可以与对波浪引起的温度波动的ROC观测相结合，以确定TTL中卷云在波浪的冷相中出现的程度。这项工作通过与天气预报的联系具有社会价值。卫星上的无线电掩星接收器是业务天气预报中使用的重要观测源，这里所做的工作包括将ROC观测同化到天气模式中并检验其预报价值。数据同化和预测工作涉及与两个业务中心的合作。此外，天气模式很难模拟QBO及其对全球的影响，因此，更好地了解QBO的波浪驱动有助于更好地预报模式。这项运动的所有数据都免费提供给全球研究界，并可用于超出运动目标的各种方式。该项目还通过支持两名研究生和为包括两名来自塞舌尔的本科生提供实习机会来建设科学队伍。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This award supports the continued participation of the Principal Investigators (PIs) in the Strateole-2 field campaign, organized by the French space agency (CNES, for Centre National d'Etudes Spatiales) and the Dynamic Meteorology Laboratory at the University of Paris-Saclay. The campaign makes observations of the tropical tropopause layer (TTL), the layer of the atmosphere from roughly 14km to 18km between the tropical troposphere and stratosphere, using balloons designed to float at a constant altitude for flights of up to 3 months. The balloons are launched from the Seychelles and float around the equator at the top of the TTL (18km) or in the lower stratosphere (20km). Strateole-2 was planned as a set of three deployments, a preliminary engineering deployment with 8 balloon flights followed by two science deployments with 20 flights each. The first two deployments took place in 2019 and 2021 and the PIs participated in these deployments using funds from AGS-1642650 and AGS-1642644. The PIs' participation in the third deployment, scheduled to begin in October 2025, is supported here.The PIs' role in Strateole-2 is to build and fly a Radio OCcultation receiver called ROC, which detects the refraction of radio waves transmitted by satellites from the Global Navigation Satellite System (GNSS, which includes the GPS satellites launched by the US). The strength of the refraction can be used to infer atmospheric temperature along the line of sight between ROC and a transmitter satellite, thus ROC can create temperature profiles by tracking a GNSS satellite as it descends to the horizon or rises from below it. Funds from this award are used to build six ROC receivers, manage their field deployment, and collect and analyze the data they generate.The temperature profiles from ROC are of interest because they show temperature fluctuations associated with wave motions in the TTL generated by large areas of tropical convection. One reason these waves are of interest is that they drive the quasi-biennial oscillation (QBO), an alternation between eastward and westward winds in the equatorial stratosphere which begins in the upper stratosphere and descends to the tropopause over the course of roughly two years. The QBO is confined to the tropics but it affects weather and climate around the world. It is well known that the QBO is driven by vertical momentum flux from waves that propagate upward from the TTL, but it is not clear what types of waves, particularly in terms of wavelengths and frequencies, are most important for driving the QBO. Another reason the waves are of interest is that their up-and-down motions are associated with cooling and warming of the ambient air, and cooling induced by rising motions can cause water vapor to freeze into ice particles (a process called deposition). Ice formation matters because it dehydrates air as it enters the stratosphere, thereby regulating the humidity of the stratosphere, and because ice particles form cirrus clouds which affect Earth's climate by trapping outgoing infrared radiation.Work on the wave driving of the QBO focuses on waves with periods of three or four days which were found to be prominent in the previous deployments. The PIs seek to determine the three-dimensional structure of the waves and their intrinsic frequencies, factors which together determine their wave momentum flux and thus their potential importance for QBO driving. The PIs have developed techniques for probing wave structure using the fact that the RO profiles are side-looking from the balloon and measure temperature at successively lower heights with distance from the balloon gondola. The three-dimensional structure of the waves can thus be reconstructed by combining consecutive RO profiles along the balloon flight path.As for cirrus cloud formation, four of the six ROC receivers will be flown with a downward-pointing lidar called BeCOOL, the Balloon-borne Cloud Overshoot Observation Lidar (BeCOOL), developed by a French team. BeCOOL observations of cirrus clouds can be combined with ROC observations of wave-induced temperature fluctuations to determine the extent to which cirrus clouds occur in the cold phases of waves in the TTL.The work has societal value through its connections to weather forecasting. Radio occultation receivers on satellites are an important source of observations used in operational weather prediction and work performed here includes an effort to assimilate ROC observations into weather models and test their value for prediction. The data assimilation and prediction effort involves collaborations with two operational centers. In addition, weather models have difficulty simulating the QBO and its global impacts, thus better understanding of the wave driving of the QBO can contribute to better forecast models. All data from the campaign are made freely available to the global research community and can be used in a variety of ways that go beyond the goals of the campaign. The project also builds the scientific workforce by supporting two graduate students and providing internship opportunities for undergraduates including two students from the Seychelles.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.