Water Vapor, Clouds, and Aerosol in the Tropical Tropopause Layer with in situ and Profiling Measurements from Long Duration Strateole-2 Balloons

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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.This project seeks to improve understanding of TTL processes by building and launching several instruments on balloons intended to fly at the top of the TTL. The balloons are 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.One instrument to be built and deployed is an optical particle detector referred to as the LASP Particle Counter (LPC, named after the Laboratory for Atmospheric and Space Physics at the University of Colorado). The LPC uses scattered laser light to detect aerosol particles with sizes ranging from 0.3 to 10 microns and is used to estimate the flux of aerosols entering the stratosphere through the top of the TTL. In addition to the LPC the PIs are developing an instrument package called RACHuTS, for Reel-down Aerosol Cloud Humidity and Temperature Sensor. The RACHuTS package weighs less than 2kg and is suspended from a nylon cable which is reeled down to 2km below the balloon gondola and then reeled back up again, each time generating a vertical profile of TTL conditions over 15 minutes. RACHuTS is lowered and raised 5 times each night while the balloon is aloft (the optical components do not function in daylight). The instruments integrated into RACHuTS are developed elsewhere. These include the Compact Optical Backscatter Aerosol Detector (COBALD, developed at ETH Zurich), the Fluorescence Advanced Stratospheric Hygrometer for Balloon (FLASH-B, developed at the Central Aerological Observatory in Moscow), and the Thermodynamic SENsor (TSEN, developed at LMD). A commercial radiosonde package is also used to record atmospheric pressure and GPS position.One issue to be addressed in the project is the extent to which supersaturation is an important consideration in determining how much water vapor passes through the TTL and enters the stratosphere. The amount of water vapor entering the stratosphere is limited by the temperature minimum at the tropopause, under the assumption that the amount of water vapor above that level cannot exceed the amount corresponding to 100% relative humidity at that level. But it is possibly for relative humidity to exceed 100% in cases where ice particle concentrations are low enough and ascent is fast enough to allow moisture to reach the stratosphere before sublimation can occur. The RACHuTS instruments are well suited to the study of supersaturation effects as temperature, humidity, and ice particle concentrations are observed together over a substantial portion of the depth of the TTL. A further issue, to be addressed using the LPC, is the transport of primary aerosols into the stratosphere. The PIs seek to constrain the size distribution of these areosols and address questions related to the stability of the distribution, its geographical variability, its relationship to convective overshooting, and previous observations of large particles with low number concentrations.The work has scientific broader impacts due to the value of the observations for addressing a variety of questions regarding TTL processes. RACHuTS and LPC observations will be made available to the research community from servers at LASP so that it 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.