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Abstract The Pico-STRAT Bi Gaz spectrometer provides in situ mixing ratio measurements of water (H₂O) and methane (CH₄) [or carbon dioxide (CO₂)] under balloon. The instrument was flown in the tropical upper troposphere and lower stratosphere in 2019/20 and 2021/22 during the Strateole 2 campaigns for a total of five flights of 20–80 days between 18- and 20-km altitude. In this frame, in situ measurements of water vapor and methane were performed every 4–12 min in the equatorial tropopause layer. On several occasions, water vapor measurements of Pico-STRAT Bi Gaz have been compared with localized measurements from the Fluorescence Lyman-Alpha Stratospheric Hygrometer for Balloon (FLASH-B) Lyman-αhygrometer and vertical profiles of the NOAA Global Monitoring Laboratory (GML) frost point hygrometer over Hilo, Hawaii. Pico-STRAT Bi Gaz measurements agreed with the FLASH-B hygrometer to within 2.2% ± 5.3% between 18.2 and 18.7 km in 2021 and to within 1.3% ± 5.3% near 19 km in December 2019. Pico-STRAT Bi Gaz agreed with NOAA’s frost point hygrometer (FPH) hygrometer to within 1.2% ± 4.1% between 18 and 19 km on four occasions during the two campaigns. These are within both instruments’ uncertainties. Methane measurements from Pico-STRAT Bi Gaz have been compared with in situ measurements from the whole air sampler (WAS) instrument, flown aboard the NASA WB-57 aircraft during the Asian Summer Monsoon Chemical and Climate Impact Project (ACCLIP) 2022 campaign over South Korea, 8 months after the Pico-STRAT Bi Gaz overpass. The relative difference between both instruments is found to be −0.1% ± 0.9% within the altitude range from 17 to 19 km and within the Pico-STRAT measurement uncertainty. 

Abstract. A novel fiber-optic distributed temperature sensing instrument, the Fiber-optic Laser Operated Atmospheric Temperature Sensor (FLOATS), was developed for continuous in situ profiling of the atmosphere up to 2 km below constant-altitude scientific balloons. The temperature-sensingsystem uses a suspended fiber-optic cable and temperature-dependent scattering of pulsed laser light in the Raman regime to retrieve continuous3 m vertical-resolution profiles at a minimum sampling period of 20 s.FLOATS was designed for operation aboard drifting super-pressure balloons inthe tropical tropopause layer at altitudes around 18 km as part of theStratéole 2 campaign. A short test flight of the system was conductedfrom Laramie, Wyoming, in January 2021 to check the optical, electrical, andmechanical systems at altitude and to validate a four-reference temperaturecalibration procedure with a fiber-optic deployment length of 1170 m. During the 4 h flight aboard a vented balloon, FLOATS retrieved temperatureprofiles during ascent and while at a float altitude of about 19 km. TheFLOATS retrievals provided differences of less than 1.0 ∘Ccompared to a commercial radiosonde aboard the flight payload during ascent.At float altitude, a comparison of optical length and GPS position at thebottom of the fiber-optic revealed little to no curvature in the fiber-opticcable, suggesting that the position of any distributed temperaturemeasurement can be effectively modeled. Comparisons of the distributed temperature retrievals to the reference temperature sensors show strongagreement with root-mean-square-error values less than 0.4 ∘C. Theinstrument also demonstrated good agreement with nearby meteorologicalobservations and COSMIC-2 satellite profiles. Observations of temperatureand wind perturbations compared to the nearby radiosounding profiles provide evidence of inertial gravity wave activity during the test flight. Spectral analysis of the observed temperature perturbations shows that FLOATS is an effective and pioneering tool for the investigation of small-scale gravity waves in the upper troposphere and lower stratosphere. 

Abstract. The tropical tropopause layer (TTL; 14–18.5 km) is the gateway formost air entering the stratosphere, and therefore processes within thislayer have an outsized influence in determining global stratospheric ozoneand water vapor concentrations. Despite the importance of this layer thereare few in situ measurements with the necessary detail to resolve the fine-scale processes within this region. Here, we introduce a novel platform forhigh-resolution in situ profiling that lowers and retracts a suspendedinstrument package beneath drifting long-duration balloons in the tropics.During a 100 d circumtropical flight, the instrument collected over a hundred 2 km profiles of temperature, water vapor, and aerosol at 1 m resolution, yielding unprecedented geographic sampling and verticalresolution. The instrument system integrates proven sensors for water vapor,temperature, pressure, and cloud and aerosol particles with an innovativemechanical reeling and control system. A technical evaluation of the systemperformance demonstrated the feasibility of this new measurement platformfor future missions with minor modifications. Six instruments planned fortwo upcoming field campaigns are expected to provide over 4000 profilesthrough the TTL, quadrupling the number of high-resolution aircraft andballoon profiles collected to date. These and future measurements willprovide the necessary resolution to diagnose the importance of competingmechanisms for the transport of water vapor across the TTL.