Polar Stratospheric Clouds in Aeolus optical products as gravity wave tracers in the early polar winters

Kurzfassung

The first Doppler Wind Lidar in space is the European Space Agency’s Earth Explorer Aeolus, launched in 2018: it has been measuring for more than 3 years wind profiles on a global scale, exceeding its expected lifetime. The mission successfully demonstrated that wind profiles obtained by a spaceborne DWL are of great benefit for Numerical Weather Prediction models improvement. Aeolus retrieves wind by measuring the Doppler shift in the laser beam reflected by a molecule or aerosol particle. The satellite is equipped with a single payload able to deliver near-realtime collocated profiles of wind and optical products. This thesis aims to highlight the unique opportunity that Aeolus products entail in the study of the dynamical aspects of cloud formation. Especially suited for this kind of study are Polar Stratospheric Clouds (PSCs), which have been observed from space as soon as the satellite era begun due to their important role in the ozone hole chemistry. Many studies show that Gravity Waves are an important trigger for PSC formation, especially in the early winter. In fact, in order to form, PSCs require very low temperatures, as low as −80◦ C, to condense the little water vapor present in the polar stratosphere. These temperatures occur every year over the South Pole, yet when these temperatures are just approach, GW-induced temperature perturbations become relevant as they may trigger PSC formation whereas synoptic scale temperatures would not allow it. Gravity waves are crucial in the climate system as they redistribute energy in the form of momentum, driving winds and weather patterns. Gravity waves are a small-scale perturbation due to vertical forcing of the main air flow. As such, they can arise from a variety of phenomena such as orographic lifting, tropospheric deep convection but also breaking of a planetary wave. There is a joint effort in building a Gravity Wave climatology, which would improve our ability to predict weather and climate patterns. This has mostly focused on well know hostspot of gravity wave activity. The objective of this thesis is to test weather PSCs can be characterized with a GW from a spaceborne simultaneous measure of cloud and wind profile. The focus of the present work is twofold: first to create a PSC climatology and validate it with previous studies, second to test whether the sensed PSCs can be characterized as GW- or non-GW-induced, by looking at the wind field perturbations. To achieve such goal I created a PSC Mask product, by handling the backscatter products with a filtering routine. The filtering is based on a backscatter threshold, geographical selection and altitude selection. This filtering tool produced a good agreement with an existing PSC Mask product developed for the CALIPSO satellite lidar. This validation was quite relevant since CALIPSO lidar is meant to be measuring clouds. A quantitative analysis of Aeolus PSC detection ability is carried out, based on a seasonal occurrence frequency.