Merging clouds retrieved from ALADIN/Aeolus and CALIOP/CALIPSO spaceborne lidars

Clouds play an important role for the energy budget of Earth. But, when it comes to predicting the climate's future, their behavior in response to climate change is a major source of uncertainty. To understand and accurately predict the Earth's energy budget and climate, it is necessary to have a thorough understanding of the cloud variability, including their vertical distribution and optical properties.

Satellite observations have been able to provide ongoing monitoring of clouds all around the globe. Among them, active sounders hold a special place thanks to their capability of measuring the vertical position of the cloud with an accuracy of about 100 meters and with a typical horizontal sampling on the order of hundreds of meters. However, clouds retrieved from two spaceborne lidars are different, because the instruments use different wavelengths, pulse energies, pulse repetition frequencies, telescopes, and detectors. In addition, they do not overpass the atmosphere at the same local time.

In this work, we discuss the approach to merging the clouds retrieved from the space-borne lidar ALADIN/Aeolus, which has been orbiting the Earth since August 2018 and operating at 355nm wavelength with the clouds measured since 2006 by CALIPSO lidar, which operates at 532nm.

We demonstrate how to compensate for the existing instrumental differences to get an almost comparable cloud dataset and we discuss the importance of the aforementioned differences between the instruments. The method developed in this study sets the path for adding future lidars (e.g. ATLID/EarthCare) to the global climate lidar cloud record.