Exploring the Tropical Atmosphere with Advanced Radio Occultation: Processing, Serving, and Analyzing Radio Occultation Data to Advance Atmospheric Science

全球导航卫星系统（GNSS）是一组在世界范围内发射的卫星星座，为各种形式的导航提供定位信息。该系统包括欧盟发射的伽利略星座、俄罗斯的格洛纳斯卫星、中国的北斗和美国的全球定位系统（GPS）。GPS在美国是众所周知的，因为我们依赖它来指导驾驶方向，以及不断扩展的手机应用程序。但是，为GNSS定位传输的无线电波对观测大气也有巨大的价值：GNSS无线电波被大气折射的程度取决于低层大气的温度和水蒸气含量以及电离层中的电荷密度。当电波到达接收器时，折射会导致相位延迟，如果接收器被放置在轨道卫星上，相位延迟可以用来测量电离层电荷、大气温度和水蒸气（尽管水蒸气需要额外的信息）。由于测量是在GNSS发射卫星相对于接收卫星的上升和下降或掩星期间进行的，因此该技术被称为GNSS无线电掩星（GNSSRO）。GNSSRO已被证明是一种非常有效的观测方法，因为测量可以在所有天气条件下进行，不受云和气溶胶的影响，并且通过支持GNSS发射机的原子钟进行绝对校准（SI可追踪）。它们还具有相对较高的垂直分辨率，提供了大气特性的剖面图，可用于识别对流层顶和大气重力波引起的温度波动等尖锐特征。该奖项支持在大学大气研究公司（UCAR）开展GNSSRSO测量的开发和科学使用工作。UCAR从20世纪90年代的GPS/MET概念验证卫星任务开始，在GNSSRO技术中发挥了基础作用，他们领导了气象、电离层和气候星座观测系统（COSMIC，也称为COSMIC-1或C1）的开发，该星座由6颗GNSSRO接收卫星组成，于2006年发射。他们还管理了COSMIC-2 （C2，由美国国家海洋和大气管理局领导的一项任务）的开发，这是一个由六颗卫星组成的后续星座，在热带和亚热带（南纬30度至北纬30度）上空运行。UCAR小组，正式名称为UCAR COSMIC计划，也一直在收集和处理来自其他几个卫星任务的GNSSRO数据，例如SAC-C（来自阿根廷），GRACE （NASA任务），Metop-A和-B （EUMETSAT任务），以及PAZ（来自西班牙）。包括所有的卫星任务在内，GNSSRO现在有20年的连续观测，这一记录足够长，对气候趋势的研究有价值。这里的工作重点是低纬度地区，这是由于C2提供的高密度和高质量的低纬度观测。需要解决的一个问题是，基于ro的观测在多大程度上可以更好地了解飓风和其他热带气旋（TC），例如，在发展中的TC附近的深层水汽积累在多大程度上促进了风暴的发展。另一个问题是观测到的热带对流层变暖廓线与响应温室气体增加而模拟的廓线之间的差异是否是由于气候模式的不足或观测中的偏差造成的。RO测深的高分辨率和绝对校准使它们成为解决这一问题的理想选择。其他需要研究的问题包括更好地描述驱动平流层准两年一次振荡的热带波，以及使用RO来评估常用再分析数据集中的水蒸气代表。该项目还包括对低纬度电离层的研究，利用部署在C2卫星上的离子速度计（IVM）作为次要有效载荷，以及最近发射的两个NASA任务，全球尺度的边缘和磁盘观测（GOLD）和电离层连接探索者（ICON）。利用C2、ICON和GOLD观测套件研究了赤道等离子体气泡和赤道发电机对电离层的影响。进一步的工作旨在改进GNSSRO观测资料对电离层模型的同化。除了这些研究应用之外，该项目还包括推进GNSSRO理论和算法的研究，以最大限度地发挥RO在大气研究中的价值。其中一个关切是随机折射率波动对RO测量的影响，因为这种波动被认为会导致通常在地表附近发现的RO衍生大气湿度的负偏倚。另一个值得关注的问题是超折射的探测，在这种情况下，折射变得足够强，可以在地表附近捕获GNSS无线电波，并阻止它们用于确定温度和水蒸气。必须检测到超折射，以使RO剖面的导管部分的数据不被使用。另外的工作是发展电离层的反渗透方法，包括更好的探测局部电荷不规则性的方法和射线追踪技术，这些技术对GNSS卫星发射的两个波长使用单独的射线路径。由于使用GNSSRO进行天气预报和气候监测，这项工作具有社会和科学价值。大多数天气预报中心都使用GNSSRO，根据该合同开发的RO算法的改进可直接应用于天气预报。鉴于缺乏可用于初始飓风预报的观测资料，对GNSSRO作为飓风观测技术的研究尤为重要。电离层的工作也直接适用于预测空间天气的努力。在气候监测领域，该项目开发了多个卫星任务的RO观测数据集，以一致的方式进行处理，以便用于评估温度和湿度的长期趋势。该奖项包括支持托管、存档、服务和支持研究社区对数据集的访问。为了促进研究界使用全球导航卫星系统RO数据，若干教育和外联工作扩大了数据集和数据服务。其中之一是一个博士后研究项目，该项目支持职业生涯早期的科学家从事反卫星观测工作。该项目还与UCAR大气研究与科学重大机遇（SOARS）项目合作，通过该项目为来自代表性不足群体的学生提供指导。计划在2022年举行为期两周的夏季研讨会，汇集GNSSRO专家，为早期职业研究人员提供熟悉GNSSRO及其研究应用的机会。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The Global Navigation Satellite System (GNSS) is the set of satellite constellations launched worldwide to provide positioning information for all forms of navigation. The system includes the Galileo constellation launched by the European Union, the GLONASS satellites from Russia, Beidou from China, and the US Global Positioning System (GPS). GPS is well known in the US as we rely on it for driving directions and an ever-expanding array of cell phone apps. But the radio waves transmitted for GNSS positioning also have tremendous value for looking at the atmosphere: GNSS radio waves are refracted by the atmosphere to an extent which depends on the temperature and water vapor content of the lower atmosphere and the density of electric charge in the ionosphere. The refraction causes a delay in the phase of the waves when they reach a receiver, and if the reciever is placed on an orbiting satellite the phase delay can be used to measure ionospheric charge and atmospheric temperature and water vapor (although water vapor requires additional information). Because the measurements are made during the rising and setting, or occultation, of a GNSS transmitter satellite relative to a receiver satellite, the technology is referred to as GNSS Radio Occultation (GNSSRO). GNSSRO has proven to be a highly effective observing method as the measurements can be made under all weather conditions, are unaffected by clouds and aerosols, and are absolutely calibrated (SI traceable) through the atomic clocks that support the GNSS transmitters. They also have relatively high vertical resolution, providing a profile of atmospheric properties which can be used to identify sharp features such as the tropopause and fluctuations in temperature caused by atmospheric gravity waves.This award supports work on the development and scientific use of GNSSRSO measurements at the University Corporation for Atmospheric Research (UCAR). UCAR has played a foundational role in GNSSRO technology starting with the GPS/MET proof-of-concept satellite mission in the 1990s, and they led the development of the Constellation Observing System for Meteorology, the Ionosphere, and Climate (COSMIC, also called COSMIC-1 or C1), a constellation of six GNSSRO receiver satellites launched in 2006. They also managed the development of COSMIC-2 (C2, a mission led by the US National Oceanic and Atmospheric Administration), a follow-on constellation of six satellites orbiting over the tropics and subtropics (30 degrees south to 30 north) launched in 2019. The UCAR group, formally known as the UCAR COSMIC Program, has also been collecting and processing GNSSRO data from several other satellite missions, for example SAC-C (from Argentina), GRACE (a NASA mission), Metop-A and -B (EUMETSAT missions), and PAZ (from Spain). Including all satellite misions there are now two decades of continuous GNSSRO observations, a long enough record to be valuable for research on climate trends. Work here emphasizes the low latitudes, motivated by the high density and quality of low-latitude observations available from C2. One question to be addressed is the extent to which RO-based observations can lead to better understanding of hurricanes and other tropical cyclones (TCs), for instance the extent to which accumulation of water vapor in a deep layer near the developing TC promotes storm development. Another is whether the discrepancy between the observed profile of tropical tropospheric warming and the profile simulated in response to greenhouse gas increases is due to inadequacies of climate models or biases in observations. The high resolution and absolute calibration of RO soundings makes them ideal address this question. Other issues to be pursued include better characterization of the tropical waves which drive the stratospheric Quasi-Biennial Oscillation and the use of RO to assess the representation of water vapor in commonly used reanalysis datasets.The project also includes research on the low-latitude ionosphere, taking advantage of the Ion Velocity Meter (IVM) deployed as a secondary payload on the C2 satellites as well as two recently launched NASA missions, the Global-Scale Observations of the Limb and Disk (GOLD) and Ionospheric Connection Explorer (ICON). The suite of C2, ICON, and GOLD observations is used to study equatorial plasma bubbles and the effect of the equatorial dynamo on the ionosphere. Further work seeks to improve the assimilation of GNSSRO observations into ionospheric models.In addition to these research applications the project includes research to advance GNSSRO theory and algorithms so as to maximize the value of RO for atmospheric research. One concern is the effects of random refractivity fluctuations on RO measurements, as such fluctuations are thought to contribute to the negative bias in RO-derived atmospheric moisture commonly found near the surface. A further concern is the detection of super-refraction, in which refraction becomes strong enough to trap GNSS radio waves near the surface and prevent their use for determining temperature and water vapor. Super-refraction must be detected so that data from the ducted portion of the RO profiles is not used. Additional work develops RO methods for the ionosphere, including better methods for detecting localized charge irregularities and ray-tracing techniques which use separate ray paths for the two wavelengths transmitted by GNSS satellites.The work has societal as well as scientific value due to the use of GNSSRO for weather prediction and climate monitoring. GNSSRO is used in most weather forecasting centers and improvements in RO algorithms developed under this award have direct application in weather prediction. The examination of GNSSRO as an observational technique for looking at hurricanes is particularly relevant given the lack of observations that can be used to initialize hurricane forecasts. The ionospheric work is also directly applicable to efforts to predict space weather. In the area of climate monitoring, the project develops datasets from RO observations over multiple satellite missions, processed in a consistent manner so that they can be used to assess long-term trends in temperature and moisture. The award includes support for hosting, archiving, serving, and supporting access to datasets by the research community.The datasets and data services are augmented by several education and outreach efforts seeking to facilitate use of GNSS RO data by the research community. One of these is a postdoctoral research program which supports early-career scientists seeking to work with RO observations. The project also works with the UCAR Significant Opportunities in Atmospheric Research and Science (SOARS) program, through which it provides mentorship to students from underrepresented groups. A two-week summer colloquium is planned for 2022 to bring together experts in GNSSRO and give early career researchers an opportunity to become familiar with GNSSRO and its research applications.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.