ESR3 - TRUTHS

for Climate Workshop

27-28 June 2024 | ESA-ECSAT


SESSION: SCIENCE & SUPPORTING ELEMENTS

The discussions on the science and supporting elements needing studies and developments to accelerate the products uptake, optimise the Cal/Val phase, and maximise benefits from Day-1 were preceded by three introductory talks:

Salient points from the presentations include:

  • Current RTMs rely on old historical assumptions and approximations 
    • 1D radiative transfer, plan parallel atmosphere 
    • Flat uniform surface 
    • Simple empirical model ode surface bidirectional reflectance 
    • Pixel-based processing, ignoring neighbouring context/computations 
    • Atmospheric composition parametrised with up to 6 profiles from the AFGL US standard atmosphere (1976), at the best rescaled based on e.g. TCWV or TCO3 content 
  • The uncertainties from this legacy approach have so far been mostly compatible with radiometric uncertainty ambitioned with EO missions, incl. current operational meteorological missions (~5% accuracy). 
  • RTM intercomparison studies showed good agreement in the atmospheric window spectral regions, but biases of about 2 and even exceeding 3% in absorption regions. 
  • Correctly parametrising the atmospheric constituents is critical for RT accuracy. Differences between RTM simulations following the historical approach (US standard atmosphere) and using CAMS profiles exceed 5% for molecular transmittance beyond 75% (affecting nearly all absorption bands), even reaching 15% with 50% molecular transmittance. 
  • The work done as part of Eradiate project to compare hyperspectral observations and simulations with Rayference RTM demonstrate the benefits of scene-dependent parametrisation the atmospheric state, e.g. with CAMS for constituents and aerosol optical thickness, of modelling a spherical Earth and accounting for illumination and viewing geometry. The OBS-CALC departures computed for different calibration test sites and missions are considerably reduced and well within 5% most of the time, except for a few absorption subregions. 
  • There is confidence that 1% uncertainty in RT computations can be reached by TRUTHS’ launch date if R&D efforts are pursued. 
  • TRUTHS, with its exceptional radiometric accuracy, will overcome the current radiometric accuracy ambiguity of approximately 3% between observation and radiative transfer modelling.
  • Latest results indicate that the aerosol forcing spread among climate models is of about 50%.
  • The aerosol absorption vs scattering and the aerosol vertical distribution, but also the aerosol type/mineralogy and size, are critical factors to aerosol-radiation interaction, the outgoing radiation and hence their role in climate system. They are currently subject to large uncertainties.
  • TRUTHS absolute radiometric uncertainty and spatio/spectral resolution will serve as reference towards constraining aerosol-related uncertainties in RTM and numerical simulations. 
  • Accurate aerosol representation and aerosol-radiation interactions also significantly improves NWP forecasts, mid-range and seasonal. 
  • Cloud-aerosol interactions and role in Earth system is subject to active research, combining climate modelling, campaign and satellite observations, in particular the recently launched EarthCare. 
  • Intercomparisons of EO missions with TRUTHS products can be made at radiance level, but also L2 level, to better understand the components of the retrieval error budget. 
  • RTTOV (NWP-SAF) operates in 0.24-100µm range, slow for Ryleigh multiple-scattering in UV. Planned improvements include enhanced VIR/IR cloud optical properties-heterogeneous surfaces-sea surface BRDF (end 2024) and aerosol size-enhanced cloud overlap-scattering (Q1-2026). PC-RTTOV is a very fast principal components (PC) version of RTTOV, implemented for IASI/IASI-NG and so far only retained for TIR. Extending to the UV-VIS is not in NWP-SAF plans.
  • PCRTM-solar is a code developed by NASA/X. Liu. Operates in 300-2500nm range. It models polarisation. Current open scientific areas are clouds, aerosol and surface BRDF. The PC training base would need to be adapted to TRUTHS. Comparisons between EMIT real observations and PC-RTTOV simulations both at spectrum and image levels are very promising. 
Discussions outome: 
  • Fostering R&D in RTM is critical to TRUTHS success and meet the required 1% radiometric uncertainty at RT level. It should be integral to TRUTHS’ programme development and science plan. Studies should aim, e.g. at increasing understanding and improving uncertainty budget contributors in OBS-CALC: 
    • representativeness error from numerical model’s profiles 
    • modelling surface and surface/air interaction 
    • aerosol and clouds RT modelling 
    • disentangling EO missions’ observation uncertainties 
    • 1D vs 3D RT simulations 
    • … 
  • The uncertainties in the line spectroscopy are not considered being significant contributors to OBS-CALC budget. 
  • Two types of RTM will be needed to exploit TRUTHS data and shall be further fostered: 
    • a very fast RTM, e.g. with ML/statistical modelling, for NWP, re-analysis, climate modelling and rapid intercalibration assessments 
    • a full-physics modelling below 1% uncertainty for PICS characterisation and vicarious calibration, as well as direct sensor-to-sensor intercalibration possibly too. 
  • Adapting RTTOV (or PC-RTTOV) for TRUTHS cannot be borne by NWP-SAF unless a clear need for NWP and at least re-analysis is expressed and the project is retained (e.g. in CDOP-5). Given also TRUTHS’ climate purpose, evolving RTTOV could be discussed with the CM-SAF and, if endorsed, the request possibly piped by them to be funded via SAF federated activities.
  • Considerations to be reflected when preparing the TRUTHS Cal/Val plan: 
    • Current ground and airborne observations are not capable of validating TRUTHS radiance at the level of the mission objectives. Airborne measurements calibrated similarly to TRUTHS in-space may the closest we can get in such intercomparisons, acknowledging unavoidable representativeness errors. 
    • The framework to intercompare EO mission’s performances is GCICS and CEOS. So far uncertainties are larger than what is aimed at with TRUTHS. 
    • The acquisition of fiducial reference measurements and the development of advanced methodologies to validate the L2 surface reflectance should be further supported: to be able to demonstrate TRUTHS products performances, and exploit more than just pure clear-sky data (see RTM research priorities). 
  • New Space data currently have 10-20% uncertainties. They should be approached within GCICS and as part of TRUTHS user engagement plan.