Influence of atmospheric waves and deep convection on water vapour in the equatorial lower stratosphere seen from long-duration balloon measurements

Abstract. The STRATEOLE 2 project consists of 3 campaigns of stratospheric superpressure balloons released from the Seychelles and intended to fly over the equatorial belt transported by winds during 3 to 4 months. During the two campaigns which have already been carried out, (2019/2020 and 2021/2022) five Pico-STRAT Bi Gaz spectrometers have been released in order to measure in situ water vapour, methane (CH₄) and carbon dioxide (CO₂) around 18.5 km and 20.5 km. In this paper, we have developed a methodology based on the calculation of in situ water vapour and temperature anomalies to estimate the modulation of water vapour due to atmospheric waves and deep convection. The calculation of Pearson correlation coefficient is performed between averaged ERA5 reanalysis temperatures and in situ water vapour anomalies. In case of a monotonic vertical gradient of water vapour, the absolute value of the Pearson’s r is high (typically 0.65) when atmospheric waves are a predominant factor of modulation. This is the case for the flight C0_05_TTL2. In case of other flights, we notice a decrease of the Pearson’s r absolute value which can be explained by the change in time of the vertical gradient of water vapour, and large convective systems with turrets overshooting the tropopause. This is the case for the flight C1_15_TTL4 which flew over the Rai typhoon (Pearson’s r of 0,31 due to both contributions).