Clouds with sun rays

CO2M CLIM Level-2 product requirements

Clouds with sun rays
Clouds with sun rays

This study provides scientific support to EUMETSAT in establishing the requirements for the cloud products derived from the Cloud Imager (CLIM) on board the Copernicus CO2M mission.

Last Updated

16 June 2021

Published on

10 May 2021

This is a sub-study within the Copernicus Sentinel-3 Synergy Cloud Mask Development project.

The CO2M mission is the space component of the European integrated observation and inversion modelling system dedicated to the monitoring of anthropogenic CO2 emissions.

This Monitoring and Verification System of systems (MVS) is proposed as a future Copernicus programme component by the European Commission and will be implemented jointly by ESA, EUMETSAT and ECMWF.

The European integrated system has the following objectives:

  • Atmospheric CO2 measurements obtained from dedicated space-borne sensors, complemented by in-situ networks and ancillary observations.
  • The operational provision of bottom-up anthropogenic CO2 emissions, with high spatial and temporal resolution and short time updates.
  • An operational data-assimilation system, which will integrate atmospheric measurements with bottom-up information into consistent and accurate estimates of anthropogenic CO2 emissions and their trends.
  • A decision support tool.

The CO2M mission will contribute to the European MVS, for which EUMETSAT, and its partners, will provide the Payload Data Ground Segment (PDGS) and its operational products, under Objective 1.

This study will provide scientific support in establishing the requirements and algorithm for cloud products derived from the CLIM instrument on the CO2M mission. These products will support the retrievals of the main XCO2, XCH4 and NO2 products by identifying the presence of cloud and quantifying the obstruction by clouds of the observed scene.


The objectives of this study are:

  • Initial development of the CLIM cloud mask and cloud obstruction algorithm.
  • Subsequent optimisation of the algorithm.


This scientific service is supporting the development of the CLIM cloud detection algorithm and the provision of the corresponding Algorithm Theoretical Baseline Document (ATBD). The work is based on the re-use of information collected in the framework of the EUMETSAT study Sentinel-3 Synergy Cloud Mask Development (S3-SYN-CM) , to set up the CLIM cloud detection algorithm in a fast and efficient way.

This approach can be adopted because CLIM is equipped with a set of channels that represent a sub-sample of the channels available in the S3 OLCI/SLSTR synergy dataset. Moreover, the S3-SYN-CM study has provided a flexible cloud masking approach suitable for both clear-sky conservative (e.g. SST and ocean colour retrievals) and cloudy-sky applications (e.g. cloud properties retrievals). Therefore, information collected in the context of the S3-SYN-CM study contains all necessary elements to set up a cloud masking approach for CLIM and the developed methods will be directly applicable to CLIM, although in a simplified form.

The study will develop a cloud detection algorithm for CLIM taking into account:

  • The specific requirements of cloud detection in support of the CO2 mission.
  • The CLIM spectral and geometric characteristics, i.e. the cloud mask product has initially to be derived at the CLIM native spatial resolution.
  • The user requirements for the format and content of the CLIM cloud mask product.