Object-based characterization of Tropical Trade Cumulus Clouds During the EUREC4A/ATOMIC Field Campaign using Sentinel-2 Observations

In January and February 2020, the joint EUREC4A/ATOMIC field campaign took place in the Tropical Atlantic near Barbados, with the goal to advance our understanding of the interplay between clouds, convection and circulation, including their role in climate change. Within the scope of this campaign, several unique satellite-based datasets have been collected, including very high-spatial resolution multispectral images with 10x10m2 pixel size acquired by polar-orbiting Copernicus Sentinel-2 satellites. In this presentation, an analysis of these high-resolution observations focused on tropical trade cumuli is given. We will show that using the high-resolution Sentinel-2 observations, clouds and their shadows can be considered as individual objects with associated properties. A novel technique will be presented for deriving cloud height from Sentinel-2 observations, which exploits the geometric relationship between cloud objects and their shadows. Furthermore, the cloud fraction and cloud size distribution are calculated for various trade cumulus scenes. The uncertainties arising from decisions in our cloud and shadow detection scheme will be discussed.We show that a substantial fraction of clouds is unresolved at the spatial resolution of current meteorological satellite imagers. Consequently the cloud size distribution and domain-average cloud fraction from coarse resolution satellite imagers are shown to be biased and highly sensitive to pixel resolution. In addition, a large fraction of pixels identified as cloudy contains significant clear-sky contributions, and it is no longer possible to characterize clouds as objects. We will discuss how this affects the accuracy of cloud property determination and biases estimates of cloud radiative forcing.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)