Continuing Measurements of Water Vapor, Clouds, Aerosol, and Waves Above, and Across, the Tropical Tropopause Layer with in Situ Instruments on Circum-Tropical Isopycnic Balloons

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

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 Laboratory for Dynamic Meteorology (LMD) 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 (near 18km) or in the lower stratosphere at an altitude close to 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 funds for the PIs' participation in these deployments were provided through AGS-1643022. Funds provided here support participation in the third campaign, scheduled to begin in October 2025. The primary task under this award is the production of balloon-borne instruments to observe temperature, moisture, aerosols, and cirrus cloud ice particles. One instrument is the LASP Particle Counter (LPC), named for the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder. The LPC flies on the balloon gondola and detects aerosols and ice particles using laser backscatter. It can determine the concentration of particles of sizes ranging from 0.3 to 30 microns in 32 size bins. The LPC is accompanied by a moisture and temperature sensor developed for use in weather balloons, the Vaisala RS41, which can detect moisture at the low humidities found in the TTL and lower stratosphere. Five LPCs will be flown in the 2025 deployment, three at 18km and two at 20km. A second instrument is the Reeldown Aerosol, Cloud, Humidity and Temperature Sensor (RACHuTS), a 2kg package which is reeled up and down on a cord to generate soundings extending 2km below the balloon. RACHuTS integrates the Thermodynamic SENsor (TSEN, developed by a French team) with a smaller version of the LPC called the ROPC (RACHuTS Optical Particle Counter, with 8 size bins from 0.3 to 10 microns), along with an RS41 and the Tunable Diode Laser (TDL) moisture sensor developed under AGS-2233136. TDL is an improvement over the FLASH-B moisture sensor used in the previous deployments since it is not sensitive to ambient light and can thus be used during the day, doubling the opportunities for profiling. RACHuTS makes profiles with one meter vertical resolution, with one profile as it descends and another as it returns to the gondola. Three RACHuTS profilers will be flown in the 2025 deployment, all at 18km. As in the earlier deployments the measurements taken using LPC and RACHuTS will be used to address three issues in TTL science. The first is the extent to which supersaturation is an important factor in determining humidity in the stratosphere. Stratospheric humidity is thought to be limited by the saturation vapor pressure at the coldest temperature air encounters as it rises through the TTL to enter the stratosphere, under the assumption that water vapor condensation or deposition occurs when relative humidity reaches 100%. But data from previous deployments shows relative humidities above 150%, suggesting that the cold point saturation vapor pressure may not be as strict a limit as previously supposed. A related issue is the role of atmospheric waves in the formation of cirrus clouds in the TTL, as previous RACHuTS deployments and other evidence suggests that the majority of ice particle layers within the TTL are generated by cooling associated with rising motions in waves. The layers can be quite thin, perhaps only 10 meters, thus the one meter resolution of RACHuTS profiles is necessary to observe the layers. The third issue to be addressed in the project is the finding of layers of large particles, in the size range from 2 to 20 microns, at relative humidities too low to allow ice formation. The nature of these particles has yet to be determined. One strategy for studying them is to fly the LPC on two balloons at the 20km level, which is above the TTL and typically too warm and dry to support ice clouds. The detection of super-micron particles at that level would be a strong argument that the particles are not composed of ordinary ice. The work has scientific broader impacts due to the unique dataset to be collected, in particular the long sampling time, high vertical resolution, and high quality of the observations. RACHuTS and LPC observations will be made freely available to the research community so that they can be used in ways not anticipated here. In addition, the project develops novel technologies for meteorological observations that 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. The project develops a substantial international collaboration and also provides support and training to a research scientist and an undergraduate student at the University of Colorado. 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.