Clouds with sun rays

Sentinel-3 synergy cloud mask development

Clouds with sun rays
Clouds with sun rays

This study is developing a novel algorithm for the generation of an atmospheric obstruction mask for Sentinel-3.

Last Updated

22 February 2021

Published on

12 February 2021

EUMETSAT is responsible for delivering best quality operational geo-bio-physical parameters to its international users, and to the Copernicus services, in support of a variety of applications (atmospheric, climate and marine). In particular, EUMETSAT is entrusted by the European Commission (EC), and its member states, to provide operational ocean colour, sea (ice) surface temperature and Near Real Time (NRT) atmospheric composition and meteorology products (e.g. water vapour content, aerosols, fires, clouds, and atmospheric motion vectors) from the Copernicus Sentinel-3 satellites. The quality of these data, and its usability in further processing, critically depends on the screening of the top of atmosphere signal for conditions unsuitable for processing. Optical obstruction of the target of interest can be due to clouds, aerosols or a, more or less, complex mixture of both, and the level of obstruction depends on concentration and type of aerosol and clouds.

Cloud Mask example
Figure 1: Example of a critical cloud masking issue identified using the SLSTR-A basic cloud mask product, 5 Feb 2019. The RGB image (left) shows the presence of dust and clouds, while the basic cloud mask (right) mixes these two types of obstruction and flags them as 'cloudy' in red. Consequently, a lot of dusty pixels are missing in the downstream L2 NRT AOD product. Coastlines are also systematically flagged as 'cloudy' cases.


This project is supported by EUMETSAT member states. Its main objective is to specify a novel algorithm (and develop the relative prototype software) for the generation of an atmospheric obstruction mask from Sentinel-3. This mask shall be able to efficiently guide a broad range of Level 2 users and applications of Sentinel-3 observations from the Ocean and Land Colour Instrument (OLCI) and the Sea and Land Surface Temperature Radiometer (SLSTR). Such a mask shall identify and characterise the nature of obstruction (clouds, aerosols or the mixture, when possible), and further exploit the spectral synergy of OLCI and SLSTR complementary bands to maximise the potential to distinguish clouds/aerosols.


The study consists of two phases:

  • Phase 1 is intended to specify the synergy Sentinel-3 atmospheric mask, relying on a thorough review of the cloud masking achievements by European and international scientists working with medium spectral resolution imagery sensors; the performance of the currently existing cloud masks from stand-alone OLCI and SLSTR sensors, and the needs of Sentinel-3 users for Level 2 land, marine and atmosphere applications.
  • Phase 2 is intended to consolidate the concept of the proposed synergy Sentinel-3 atmospheric mask and validate it on a statistically representative dataset. The prototype will be delivered to EUMETSAT at the end of the study and run in the in-house offline environment.

The concept proposed for the Sentinel-3 synergy atmospheric mask relies, not only, on the detection of atmospheric obstruction (and, when possible, the distinction of clouds and aerosols), but also on characterisation of the degree of obstruction. This degree of obstruction is estimated by simulating the Top Of the Atmosphere (TOA) radiance perturbation caused by the obstruction in a series of key OLCI/SLSTR channels. The perturbation is estimated based on retrieved physical quantities, such as Cloud Optical Thickness (COT) and Aerosol Optical Depth (AOD). The final product, containing both qualitative and quantitative obstruction information, shall be able to guide users with very diverse needs, spanning from clear-sky conservative applications with the need to remove any atmospheric obstruction, to more permissive applications, where the degree of obstruction can be tuned using thresholds based on the TOA perturbation metrics.

Before activating Phase 2, a group of six independent experts was invited to review the approach developed in Phase 1, identify potential weaknesses to be improved during Phase 2, and assess whether it would overcome the limitations of currently available OLCI and SLSTR single-sensor cloud masks. The overall outcome of the expert workshop was positive: it highlighted the benefits of the Sentinel-3 synergy atmospheric mask and provided important recommendations to implement during Phase 2 (Summary of outcome of the S3-SYN-CM Expert Meeting).

During the course of this study, the project team at Brockmann is actively communicating with experts at EUMETSAT, within the Remote Sensing and Products (RSP) Division, and with external experts (e.g. S3 Validation Team and Quality Working Groups). This continuous interaction allows integration of the most recent developments from the community into the study, in terms of sensor and algorithm developments.