CO2M level-2 GHG science support

The service provides support on topics including methods for retrieval of CO2 column-averaged dry-air mole fraction (XCO2), scientific algorithm data flows and functional requirements, as well as calibration, product validation and monitoring requirements.

Three state-of-the-art retrieval methods are being further developed for the mission:

• Fast atmOspheric traCe gAs retrievaL (FOCAL), developed and maintained by the Institute of Environmental Physics (IUP) of the University of Bremen (IUP-UB).
• Fusional-P UOL-FP full physics algorithm, developed and maintained by the Institute of Environmental Physics (IUP) of the University of Bremen (IUP-UB).
• RemoTAP, developed and maintained by the Netherlands Institute for Space Research (SRON).

In order to achieve the highest quality retrievals, a particular aim of the study is to take advantage of measurements from all three instruments on the CO2M platform and to support the identification of synergies between the three algorithms (referred to as ‘companion algorithms’).

Objectives

Study objectives include:

• Scientific and software development to improve product quality from each method, documented in evolving Algorithm Theoretical Basis Documents (ATBDs).
• Developing proposals/strategies for the use of data from the additional instruments on the CO2M platform (the Cloud Imager (CLIM) and the Multi Angle Polarimeter (MAP)) in the level-2 processing.
• Testing the results of product quality improvements using OCO-2 and/or GOSAT data.
• Exploiting and evolving the benefits of using multiple GHG retrieval algorithms.
• Installing and executing the retrieval code on EUMETSAT’s Technical Computing Environment (TCE) using specific test data and evaluating performance.
• Producing input/output data flow models for each algorithm.
• Establishing lists of input and output geophysical parameters and their associated characteristics.
• Supporting EUMETSAT on issues related to generation of level-1 test datasets.
• Supporting EUMETSAT in the specification of level-2 product formats.
• Making proposals for bias reduction and monitoring including, where applicable, using results from companion algorithms.
• Supporting planning for routine Cal/Val and quality monitoring.

Overview

The scientific service support focusses on the retrieval of XCO2 and other climate relevant parameters (e.g. methane and Solar Induced Fluorescence (SIF)) using the following three methods for greenhouse gas retrievals from shortwave infrared, high spectral resolution spectrometers.

FOCAL

The fast atmospheric trace gas retrieval (FOCAL) algorithm has been initially developed for OCO-2 XCO2 retrieval, where XCO2 is the column-average dry-air mole fraction of atmospheric CO2. In recent years FOCAL has also been applied to the retrieval of XCO2, XCH4 and other parameters from GOSAT and GOSAT-2.

FOCAL includes a radiative transfer model, which has been developed to approximate light scattering effects by multiple scattering at an optically thin scattering layer. This reduces the computational costs by several orders of magnitude, which is important especially for future sensors such as CO2M.

In order to find out if FOCAL meets the demanding accuracy and precision requirements as formulated for CO2M XCO2 and XCH4 retrievals, simulated CO2M radiance simulations have been carried out. These simulations have been generated with the radiative transfer model SCIATRAN taking into account realistic scenarios for aerosols, clouds, surface properties, etc. By application of the FOCAL retrieval to these simulated data, confidence is gained that the FOCAL method is able to fulfil the challenging requirements for systematic errors for the CO2M mission (spatio-temporal bias < 0.5 ppm for XCO2 and < 5 ppb for XCH4).

CO2 retrievals from simulated CO2M data via FOCAL are shown in Figure 1.

Fusional-P UOL-FP

The Fusional-P UOL-FP full physics retrieval algorithm is a sophisticated algorithm for the retrieval of greenhouse gas columns based on the Bayesian optimal estimation method.

The retrieval utilises full multiple scattering radiative transfer making use of the Low-Stream-Interpolation method to accelerate the radiative transfer. Aerosols are described with two aerosol types representing a large and small mode of aerosols, with scene-dependent optical properties that are either informed by the L2 product of the Multi-Angle Polarimeter onboard CO2M or by aerosol data from the Copernicus Atmosphere Monitoring Service (CAMS).

The Fusional-P UOL-FP code has its heritage in the NASA OCO mission and the predecessor of the code has been applied to several satellite missions including GOSAT, Tansat and OCO-2.

To test the retrieval algorithm, realistic CO2M spectra have been simulated for six orbits based on atmospheric model and surface datasets that differ to those used in the retrieval algorithm so that independence of the simulations from the retrieval assumptions can be assured.

RemoTAP

To make use of the added value of the MAP instrument aboard the CO2M mission, the Remote sensing of Trace gas and Aerosol Product (RemoTAP) algorithm is developed to perform simultaneous retrieval of trace gas and aerosol properties from both MAP and CO₂ imager (CO2I) measurements. At the same time, it has the capability to perform the retrieval of trace gas from only CO2I measurements, or aerosol properties from MAP-only measurements.

The RemoTAP algorithm combines heritage from RemoTeC, an operational algorithm used to infer CO2 and CH4 column abundances from spectrometer measurements of e.g. GOSAT, GOSAT-2, OCO-2, Sentinel-5 (and its precursor mission S5-P), and the SRON-MAP algorithm used to infer aerosol properties from PARASOL and the SPEXone multiangle polarimeter of NASA’s PACE mission.

To test the RemoTAP algorithm, comprehensive sets of synthetic measurements has been created using a complex atmospheric model to challenge the assumptions made in the retrieval. The synthetic measurements comprise 3 days per month for a full year. From RemoTAP retrievals on the synthetic measurements, we can clearly see the added value of joint CO2I-MAP retrievals compared to CO2I-only retrievals (see Fig. 3).

Methods for retrieval of CO2

Given the strategic nature of the European integrated system, and its contribution towards assessing the effectiveness of CO2 emission strategies, the service is driven by a stringent set of requirements for the geophysical products, with rigorous targets for accuracy and uncertainty. Within this context, this activity aims to further consolidate the performance and accuracy of the algorithms to date and advance their role within the multi-algorithm GHG retrieval framework for the operational CO2M level-1b/c to level-2 processor, accompanied by retrievals of aerosol amount, cloud detection, and NO2 surface and emission plume concentration.

All data flows available from CO2M level-1b/c (including MAP and CLIM data and visible channels) are being specified, as well as the required and available static and dynamic auxiliary information (including spectroscopy and numerical weather prediction (NWP) and climate modelling (CM)). Opportunities for improvements are also being explored through the use of companion retrieval results and assessed with respect to overall mission requirements.

The proposed algorithm improvements are demonstrated and their performance evaluated with both synthetic retrievals and, where possible, real satellite data (e.g. OCO-2/3 and/or GOSAT). This includes a special focus on improvements in the error analysis and reduction of the overall uncertainties, also through use of the output of the companion algorithm results.

Scientific algorithm data flows and functional requirements

The service addresses the requirements definition and specification of all aspects of science data flows for the three algorithms, as needed for the future operational processor implementation in the Payload Data Processing (PDP) element of the Mission Data Processing Subsegment (MDPS). This includes the flow of ancillary static, dynamic and third-party data, the specification of all datasets, interface analysis and the specification of the processing needs of the scientific methods applied. The interaction between the three GHG algorithms, as well as the aerosol, cloud and NO2 level-2 algorithms within the overall functional processing chain is a particular focus.

Calibration, product validation and monitoring requirements

The service also contributes to EUMETSAT’s definition of requirements for continuous instrument and product cal/val, and to the continuous CO2M level-2 product quality monitoring and product evolution, thus informing the specific methods to be implemented operationally and the objectives and procedures forming the CO2M cal/val and monitoring plan. This includes topics such as:

• Methods for algorithm validation with respect to accuracy, as well as possible systematic and structural random errors.
• Validation datasets, including in situ observations and/or comparable products from other missions.
• The data flows required for continuous calibration, validation and monitoring, including a description of all required (user) interfaces and a proposal for a set of reporting tasks. The development of a basic description of continuous monitoring methods to evaluate data-quality online is also included.