SESSION: CLIMATE MODELS & PARAMETERS

• A question was raised on the complexity of developing COSP simulator for TRUTHS; it was pointed out that the number of spectral bands is only runtime issue, not adding to the development complexity; also sampling is done via mask, so it is doable in principle. Furthermore, it was pointed out it would be useful however to analyse with existing multispectral data and what their benefits are when modelling in COSP (short-term goal, before using full hyperspectral capabilities of TRUTHS) 
• CMIP7: Eleanor made the comment that the CMIP7 goals are preliminary, not fully agreed yet. 
• Furthermore, it was pointed out that the bottom-up development of models will include feedback from new observations (sensitivities can be measured, not cloud feedbacks directly) to better constrain models  

Talk 2: Chris Smith (Uni. Leeds & International Institute for Applied Systems Analysis),“Observations in support of radiative forcing and Earth’s energy imbalance”

• Energy imbalance is the radiative forcing plus any feedbacks; the relevant equation is: ΔN= ΔF+λΔT, with ΔN being the Total Radiative Imbalance, which is an observable, ΔF being the radiative forcing, and λΔT the radiative responses. Here ΔT can be measured, while λ cannot, so the right hand side cannot be measured directly. Radiative forcing is mostly derived from modelling and quite uncertain (particularly aerosol forcing), but policy relevant/climate change. 
• Aerosol forcing: There is a better agreement between observations and models in AR6 than AR5, but still large uncertainties on both (see Smith et al, 2021); Uncertainties are so large that for same emission scenario temperature increase could be below 1.5K or above 2.0K (Watson-Parris and Smith 2022) 
• CERES has much higher planetary heat uptake in SW than CMIP6 models; 
• Climate feedback parameter, λ, tells us about climate sensitivity; 
• Decreases in OLR in CO2, CH4 and N2O bands from AIRS observations: evidence of greenhouse gas radiative forcing -shows value of hyperspectral measurements (see also Raghuraman et al., 2023) 
• CERES follow-on: Libera (launch 2027) 

• It was pointed out that the high internal variability is also important; 
• the talk focused on long term averages, not going to details of pattern effect 
• Major discrepancy is in shortwave, so focus should be there; TRUTHS will be valuable to solve the question which part of shortwave spectrum is causing discrepancy. 
• The suggestion was made to use CAIRT and ESA-study on long-lived greenhouse gases 

• TRUTHS objectives are presented: 
• provide data from which trends in radiation can directly be derived 
• provide data to calibrate other sensors which then are used to derive climate time series 
• However even with TRUTHS we'd need about 20 years of data to detect climate signal 
• It was pointed out that there is a continuity issue: 
• GCOS action B1.5: establish long-term space-based reference calibration system (high spectral resolution reflected solar and infrared plus GNSS radio occultations) 
• CLARREO is delayed to spring 2026 with ISS stopping operations in 2030 
• TRUTHS not before 2030 
• SI-Traceable space observations need to be in WMO WIGOS 
• Geo-Ring applications: Martin Stengel (DWD) 
• EUMETSAT-NOAA Geo-Ring project using historical data back to 1979 
• Maybe possible to recalibrate backwards in time, but errors increase the further one goes back in time 

• The question was raised whether there is any process to add calibration system to WMO IGOS?One way is via CGMS plenary and expert teams; Another one is SITSats Task Team under WMO umbrella (Thorsten Fehr is member, Nigel Fox is co-chair) 
• It was also pointed out that the TRUTHS measurements would be beneficial for accuracy of lunar models 

• in 2015 the IAU declared total solar irradiance to be 1361 W/m2, based on two independent instruments (SORCE/TIM and PICARD/PRESMOS) which agree very well 
• PMOD/WRC provides a composite time series of all TSI observations from space; TRUTHS will be added to this time series 
• Variability with solar cycles is about +/-0.7 W/m2 
• Solar spectral irradiance references, previously the ATLAS 3 spectrum has been used; recently the TSIS-SIM spectrum has been adopted; it is higher in VIS, lower in NIR by several W/m2 than CMIP6 assumptions; this reference spectrum will be used in CMIP7 (see e.g., Funke et al., 2024, GMD) 

• it has been pointed out that the VIS reference in CMIP6 was a combination of SATIRE TS and NRLSSI2
• Nigel is surprised about large difference TSIS-SIM to CMIP6 in VIS, 
• while TSIS-SIM vs Thuillier has only 0.1 to 0.2% difference in VIS 
• Climate model runs have studied sensitivity to changes in SSI for the past, this should also be done for the future 
• The question was raised whether there are any possibly synergies of TRUTHS solar and heliospheric missions 
• Langley extrapolation of high-altitude solar spectra? Getting better results than in the past, but extra-terrestrial observations still superior 

• The question was raised if there are any attempts to go beyond calibration and looking at retrievals. 
• Annual gridded radiances from TRUTHS would be useful for climate modellers; Spatial resolution very much depends on application within CMIP and requires discussion with them (model benchmarking, process studies,...) 
• Richard: A better use would be to improve other spectral measurements, as we'd be missing out on a lot of detail in the annual averages 
• Potential application: Improving benchmarking of radiative transfer schemes in climate models