ABSTRACT

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.

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