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

Abstract. Most atmospheric species enter the stratosphere through the tropical tropopause layer (TTL), a place of interplay between many processes of different scales. Water vapour (H2Ovap) is a key compound in this layer and its entry into the tropical stratosphere is crucial for stratospheric chemistry and climate. We present a methodology based on the calculation of in situ H2Ovap and temperature anomalies to estimate the modulation of H2Ovap due to atmospheric waves and deep convection. H2Ovap data were obtained from in situ measurements of five Pico-Strat Bi Gaz spectrometers that were flown under long-duration balloons during the Strateole 2 campaigns. The calculation of Pearson's correlation coefficients is performed between averaged ERA5 reanalysis temperatures and in situ H2Ovap anomalies. In the case of a monotonic vertical gradient of H2Ovap, the absolute value of the correlation coefficient is high (typically 0.65). For the other flights we highlight lower correlations, due to changes in time of the vertical gradient of stratospheric H2Ovap, and large convective systems overshooting the tropopause. This is the case for one of the flights, which flew over the Raï typhoon (correlation coefficient of 0.31 due to both contributions). Depending on the flights, we also show that for 47 % up to 70 % of the probed nights, H2Ovap anomalies can be explained by atmospheric waves, which highlights the major role played by waves on H2Ovap in the TTL. We also show that long-duration balloon measurements are important in highlighting the overshooting signature of H2Ovap in the upper TTL.