Atmosphere Science Blog

CO2M MAP and CLIM Top-Of-Atmosphere simulations


Atmosphere Science Blog
Atmosphere Science Blog

This study will provide EUMETSAT with Top-Of-Atmosphere (TOA) radiance simulations for the Multi-Angle Polarimeter (MAP) and Cloud Imager (CLIM) instruments, for the evaluation of data processing for the forthcoming Copernicus CO2M mission.

Last Updated

27 January 2023

Published on

22 November 2021

This study aims to produce synthetic TOA test data for the MAP and CLIM instruments of the CO2M mission. The test data will be used for the evaluation of processing needs for the generation of CO2M products.

Orbits of CO2M considered for the CO2M test data studies
Figure 1: Orbits of CO2M considered for the CO2M test data studies. Three orbits over Europe and three orbits over Asia/Africa, with two orbits in nadir and one orbit in sunglint (pitch) mode. Each orbit will be modelled with and without realistic cloud fields, as provided in the scenario model data provided as input to the study.

Key elements of the work will be to provide full-orbit test data for MAP and CLIM observations, consistent with the design and purpose of the CO2M platform, and to support the simulation of cloud and aerosol retrievals from this test-data. The latter are in turn supporting the retrieval of greenhouse-gas products, as well as nitrogen dioxide, from the main spectrometer instrument on-board CO2M. Test data for the main CO2M spectrometer are provided through a separate study.


The objectives of the study are to:

  • provide TOA polarised radiance data for MAP and CLIM, based on observation geometries of six CO2M orbits for both instruments (see Figure 1);
  • establish a TOA polarised radiance simulation framework for the MAP and CLIM instrument which can be re-used for subsequent (revised) test-data simulations.


As input to the simulations, observation geometries, including parallax corrected geolocation calculations using terrain-height, are provided by EUMETSAT. Instrument specific design aspects like ISRF, spectral resolution and other relevant instrument performance figures will be taken from the Space Segment Requirements Document (SSRD). Atmospheric and surface modelling fields and other ancillary data will be input to this study and will be provided through the dedicated test-data study for the main spectrometer instrument on CO2M. This is to ensure maximum consistency of observation geometries and model atmosphere between all CO2M instrument test datasets. The provided atmospheric modelling fields will only be complemented by additional modelling or model parameters if absolutely required for realistic modelling of MAP and CLIM observations.

The TOA polarised radiance simulation framework for the MAP and CLIM instrument will be used for subsequent revised TOA test-data simulations by adopting different instrument settings, testing various instrument performance scenarios, or expanding on the number of orbits and satellite platforms simulated. The test-data processing framework will be configurable in such a way that the effort associated with the implementation of potential expansions or changes are minimised.

Data from this study will be delivered in the planned (level 1) format for TOA radiance data from the CO2M MAP and CLIM.

Figure 2: Spectral regions and spectral resolution of k-distribution set for CLIM and MAP spectral oversampling dataset

Test data will be provided corresponding to the instrument spectral response function (SRF) and also, for a smaller pixel sample, considering a spectral oversampling (see tables in Fig 2 for the spectral characteristics). This can be used for testing of the influence of SRF variability on L1 and L2 products.

Execution of the study will involve two main tasks/challenges:

  1. Setup of the radiative transfer model and input data: The computation of TOA radiance for MAP and CLIM is CPU demanding. The radiative transfer equation will be solved for each individual pixel of the 6 orbits. It also has to properly account for absorption/scattering coupling and for polarization. The optimization of an accuracy/CPU demand trade-off is then crucial. The test data must be produced for a scene definition consistent with the one used for the CO2M spectrometer test data. To set the ancillary data and the scene setting will be part of task 1.
  2. Production of the MAP and CLIM datasets: The production of the full test data represents a dedicated task requiring the optimisation of the parallelisation on the HPC cluster and a subsequent post-processing of the data.