Roundup of progress in first eighteen months of MetEOC-4
Marking a halfway reporting milestone, on 14 September 2022 EURAMET published its MetEOC-4 Publishable Summary that highlights significant recent progress made toward the long-term objective of the MetEOC series: development of an SI-traceable climate observation system.
As with preceding projects in this series, MetEOC-4 encompasses a diverse range of research activities, structured according to four themes matching a set of predefined objectives. It continues existing, and initiates new, lines of research, developing tools, methods, and infrastructure capable of assigning metrologically (SI) traceable uncertainties to data products derived from Earth Observations, with a focus on climate change.
More than half of the 54 atmosphere, land, and ocean Essential Climate Variables (ECVs) rely on measurements from space. These are physical, chemical, biological, or linked variables used to produce a full understanding of the climate system. Yet, in numerous instances, around a factor of ten improvements in measurement accuracy would be needed to reliably detect and discriminate targeted signals from naturally variable background signals. For that to happen, robust calibration and validation standards, and methods for pre- and post- launch calibrations of observation systems and networks, must be put into place. Measurements in the ‘field’ would be expected to detect changes of a few tenths of a per cent or per decade; levels of accuracy, right now, feasible mostly only in NMI laboratories.
The benefit of such ‘fit-for-purpose’ climate observation capabilities would be ‘trustable’ climate forecasts and confidence that climate adaptation and mitigation policies deliver intended outcomes.
Theme 1 — Remote sensing instrument Calibration
The first theme supports the development of calibration systems for satellite or airborne sensor systems of four example earth observation missions.
- Requirements of the ESA TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio-Studies) satellite mission have been analysed, to identify the challenges to be resolved for a pre-flight calibration facility. Nearly an order of magnitude improvement in the accuracy of response shape and effective wavelength of the imager compared to other missions is required. Design plans for the upgrade were submitted to the European Space Agency (ESA) for review. Slated for launch in early 2030, TRUTHS will carry a primary International System of Units (SI) reference system, capable of calibrating and validating data from other optical satellite instruments. In July, ESA announced that TRUTHS passed an important internal milestone: completion of its definition and preliminary design phase.
- The ESA Far-infrared-Outgoing-Radiation Understanding and Monitoring (FORUM) mission is being developed to measure radiation emitted from Earth across the far-infrared spectrum, in particular at 15–100 microns, to improve understanding of greenhouse effects. Two requirements of this mission were improved pre-flight radiometric calibration facilities and (reference) ‘black bodies’. These, in turn, required the selection and characterisation of angular reflectance properties (BRDF) of potential surface coatings. To achieve this, requirements for a Far Infrared gonio-reflectometer (an instrument that measures bidirectional reflectance distribution function, BRDF) were established and a first design study was performed. In parallel, reflective properties of several candidate ‘surfaces’ were modelled and the best of these (machined by 3D printing) prototyped, and modelled angular reflective properties verified by measurement.
- GLORIA, standing for Gimballed Limb Observer for Radiance Imaging of the Atmosphere, is a joint project of Research Center Juelich and Karlsruhe Institute of Technology, Germany. This type of atmospheric remote sensing instrument combines a classical Fourier transform spectrometer (FTS) with a 2-D detector array. It is designed to improve spatial sampling of atmosphere composition by up to an order of magnitude compared to existing limb scanning instruments and to operate on high-altitude research platforms such as aircraft and stratospheric balloons. So far in MetEOC-4, mid-IR range reflectance measurements for black coatings (for targets) and white coatings (for thermal management) were evaluated to use in a reference black body for a GLORIA balloon experiment as well as its surface topology. A prototype balloon black body and previously built black bodies for an aircraft version of GLORIA were calibrated, with both found to offer adequate performance and long-term stability.
- Picking up where MetEOC-3 left off, an updated calibration system was built for the AtmoCUBE A1 satellite sensor. Developed by University of Wuppertal and Research Center Juelich, the AtmoCUBE A1 sensor was designed to measure the temperature of the mesosphere, a part of the atmosphere particularly sensitive to climate change and an indicator of warming. The concept is based on a spatial heterodyne spectrometer (a Fourier transform spectrometer without moving parts) and thus can be made compact enough to be flown on cubesats. Tilted diffraction gratings in the instrument diffract light waves producing a wavefront tilt dependent on the wavelength, that by applying a numerical Fourier transformation to the resulting interference pattern can be used to determine the wavelength spectrum. The nature of this wavefront was measured in Meteoc-4 using a large shearing interferometer combined with specially developed software.
Theme 2 — Post-launch sensor performance Calibration, validation, and interoperability
This theme provides metrological support for techniques used to evaluate performance and to improve interoperability of satellites following launch, using vicarious (not on board the spacecraft) methods.
- Surface sites provide suitable references for evaluating satellite performance, but specific surface and observational characteristics and effects on satellite observations needed further evaluation. Software to evaluate effects of spatial uniformity, angular view, and spectral differences between sensors viewing such test sites were developed and tested to assess uncertainties achievable for different surface types, such as deserts, snow, water, and vegetation). Also, for a necessary sample size, to average out inhomogeneity, for a particular uncertainty level to be realised. An agreement was made with the Italian space agency to allow data from the PRISMA hyper-spectral sensor to be used in these evaluations.
- The inherent stability of the surface reflectance of the moon makes it useful as a calibration reference for radiometric values, providing corrections for the lunar cycle are determined and absolute values assigned. A software model derived from surface-based observations from another ESA project to characterise the lunar reflectance is progressing well. Additionally, collaborations were formalised to facilitate a comparison with the NASA Air-LUSI project, which provides similar observations from aircraft.
- Comparing satellite observations with ground test sites requires corrections for losses in transmission through the atmosphere. Eradiate is a software solution designed and developed by the Brussels-based company Rayference, with funding from the Copernicus programme following an initial pilot study in MetEOC-3. It applies an open-source 3D radiative transfer model based on physical principles and uses the Monte Carlo ray tracing technique to provide an independent assessment of performance and SI-traceability of models and code through a comparison of model predictions and measurements made of an artificial target. A sensitivity analysis using Eradiate RT code was performed to assess optimum characteristics of the target to allow performance to be evaluated. In parallel, a suitable target material was chosen and a prototype design was developed.
Theme 3 — Validating carbon stocktake: CHG Emissions and biosphere
For this theme, multiple partners are contributing to the development of metrologically-robust methods to evaluate and validate satellite-derived biophysical variables that impact the carbon cycle, as well as satellite-based emissions monitoring. The parameters being measured are used to support the UNFCC Paris Agreement, such as ECVs relevant to GHG emissions and sinks, such as forest biomass and ocean phytoplankton. An improved understanding of the dynamics of these natural ‘sinks’ and emission sources is needed for these effects to be properly accounted for in overall carbon ‘stocktaking’.
A review of satellite GHG retrieval algorithms was completed and comparisons were made with likely capabilities of next-generation satellite sensors to assess potential measurement needs.
Oceans are one of the two main natural ‘sinks’ of greenhouse gases, but the potency of the process varies year to year. Ocean colour, caused by biological activity and photosynthesis, might be an important factor as these processes absorb carbon dioxide. Clear and consistent guidelines for how to assess the measurement uncertainties of ocean colour measurements would be a valuable contribution — and these are being developed.
The variance between satellite observations and in-situ observations of ocean colour by reference networks such as NASA’s AERONET – Ocean Color (AERONET-OC) network were assessed, with early results showing consistency within uncertainty budgets — as detailed in these five peer-reviewed published papers:
- Assessment of OLCI-A and OLCI-B radiometric data products across European seas, April 2022
- Sensitivity of Ocean Color Atmospheric Correction to Uncertainties in Ancillary Data: A Global Analysis With SeaWiFS Data, 9 February 2022
- Ancillary Data Uncertainties within the SeaDAS Uncertainty Budget for Ocean Colour Retrievals, 21 January 2022
- From Validation Statistics to Uncertainty Estimates: Application to VIIRS Ocean Color Radiometric Products at European Coastal Locations, 23 December 2021
- Uncertainty Estimate of Satellite-Derived Normalized Water-Leaving Radiance, 10 December 2021
The other main sink for carbon is land-based vegetation and biomass.
Delivering as realistic as possible computer-based models is the objective, to evaluate interactions between solar radiation and vegetation in a ‘virtual test bed’. This is because the more accurate input data and digital parameterisation can be made, the more accurate these simulations might become.
COVID-19 lockdowns delayed some fieldwork designed to refine the use of optical drone-based observations for quantifying vegetation-related essential climate variables. Nonetheless, software to digitise the University of Oxford’s Wytham Woods test site from measurements was further developed.
In addition, testing of a drone capable of making synthetic aperture radar (SAR) measurements showed it capable of retrieving fine-grained detail of the 30 m high tree canopy.
Observations of solar-induced chlorophyll fluorescence (SIF) from space have emerged as a promising way to evaluate the carbon uptake of land ecosystems. Estimation via space-borne spectrometers requires both high spectral resolution and advanced retrieval schemes. In this project, sources of uncertainty were identified for calibrating laboratory measurements of passive SIF, that could be applied to validating instruments on board the ESA’s FLEX mission, as described in the paper:
- Spectroradiometer spectral calibration, ISRF shapes, and related uncertainties (Applied Optics, 16 June 2021).
Theme 4 — Surface-based ‘radiation’ networks
This theme is developing instruments and procedures to offer traceability for surface-based remote sensing networks. Highlights to date include:
- The underpinning primary scale, the standard to which broadband infrared irradiance radiometers (pyrgeometers) are compared with, is the World Infrared Standard Group (WISG), maintained at the World Radiation Center in Davos, Switzerland. These types of measurements are a key element in assessing radiation emitted by the sky and, thus, the radiance imbalance of the Earth, the ultimate driver of climate change. A reference black body for the WISG, developed in MetEOC-3, was compared with a primary ammonia reference heat pipe black body, an IRIS radiometer, and a previously used reference black body of the World Radiation Centre. Results showed consistency within the specified 0.5 Wm2. For details, see the previous update: Blackbody Comparison Makes Infrared Radiation Measurements of the Atmosphere More Reliable.
- Two prototype pyrgeometer designs, with diamond domes and nanotube coated thermopiles, were developed and are being tested at PMOD/WRC. Spectral transmission of the domes and the absorption of the thermopile coating were characterised.
- Initial diffraction calculations for the CSAR instrument (Cryogenic Solar Absolute Radiometer designed to measure Total Solar Irradiance to be used on board TRUTHS and as a terrestrial reference to replace the world radiometric reference of the WMO) were performed. This accounted for different atmospheric properties such as aerosols, ozone, and water vapour.
Routes to impact
Facilitating the take-up of technologies and measurement infrastructures developed in the project is another aim of MetEOC-4. Beneficiaries could become apparent in the measurement supply chain (such as calibration laboratories, instrument manufacturers), standards developing organisations and end users (such as environmental monitoring and regulatory bodies including the World Meteorology Organisation (WMO) and Group of Earth Observations (GEO).
Progress towards implementing such developments through Meteoc-4 in the ‘field’ will be described in forthcoming meteoc.org updates.