The cause of fluctuations of the stratospheric quasi-biennial oscillation (QBO) period has been debated since its discovery. Explanations for variations in the QBO period are of great interest given the relevance of the QBO as one of the major modes of atmospheric interannual variability that affects not only the tropics, but also on the surface weather and climate in the extra-tropics. This study analyses the latest versions of atmospheric reanalyses, which resolve a part of the gravity wave spectrum, to show that the period fluctuations can be explained by seasonal variations in the forcing by small-scale (gravity) waves in the equatorial region. In doing so, the study improves our understanding of one of the fundamental modes of stratospheric variability and suggests that improvements in our ability to capture multi-scale interactions in the models can improve the accuracy of seasonal forecasts and the reliability of future climate projections.

The paper presents the first quantification of the freezing behaviour of siver iodide (AgI) containg seed particles derived from glaciogenic cloud seeding experiments in the real atmosphere. The study uses a unique approach by releasing the seeding particles from an unmanned airborne vehicle in supercooled low stratus clouds and measuring the formed ice crystals by a tethered balloon placed downwind. Based on their unique observation data the authors propose a novel freezing mechanism based on the hygroscopic growth of the particles in to solution droplets, subsequent freezing of droplets and further growth of ice crystals while ruling out competing freezing mechanisms like contact freezing.