the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Aug 2024
Radiative Closure Assessment of Retrieved Cloud and Aerosol Properties for the EarthCARE Mission: The ACMB-DF Product
Abstract. Measurements made by three instruments aboard the EarthCARE satellite, plus data from auxiliary sources, will be used to synergistically retrieve estimates of cloud and aerosol properties. The ACMB-DF processor consists of a continuous radiative closure assessment of these retrievals and is both described and demonstrated in this study. The closure procedure begins with 3D radiative transfer models (RTMs) acting on retrieved and auxiliary data. These models yield upwelling shortwave and longwave broadband radiances commensurate with measurements made by EarthCARE’s multi-angle broadband radiometer (BBR). Measured and modelled radiances are averaged up to “assessment domains”, that measure ~21 km along-track by no more than 5 km across-track, centred on the retrieved cross-section of ~1 km profiles, and are then combined, by angular distributions models (ADMs), to produce “effective” upwelling fluxes at the top-of-atmosphere, denoted as FBBR and FRTM, respectively. Last, the probability 𝑝ΔF^ of |FRTM – FBBR| being less than ΔF^ W m-2 is estimated recognizing as many sources of, assumed normally distributed, uncertainties as possible. For historical/programmatic reasons, ΔF^ is set to 10 W m-2, but that might change during EarthCARE’s commissioning phase and with Sun angle. The closure process is demonstrated up to calculation of 𝑝ΔF^ using four 400 km-long portions of one of EarthCARE’s test frames for which simulated passive measurements were computed by 3D RTMs. Note that this study, like the ACMB-DF process with real EarthCARE observations, does not comment explicitly on performance of retrieval algorithms.
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RC1: 'Comment on egusphere-2024-1651', Anonymous Referee #2, 02 Sep 2024
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The authors present the radiative closure method used for the EarthCARE ACMB-DF product. In the real processes, EarthCARE’s angular distribution models are applied to both observed shortwave and longwave broadband radiances. The probability of the difference of the irradiances being within a pre-determined threshold is computed. Only nadir view BBR radiances are used for the assessment. Because there are currently no measurements, the authors use broadband radiances computed by 3D radiative transfer models as measured BBR and MSI radiances to demonstrate the process. The assessment domain size of 5 km by 21 km is used to compare irradiances converted from modeled radiances with EarthCARE’s angular distribution models and 3D modeled irradiances at a 20 km altitude. The differences are smaller for smaller shortwave irradiances and for larger longwave irradiances. These are likely to be clear-sky scenes. Scenes causing larger differences will be investigated in the future. The probability of the difference being within the threshold is smaller than that reported in an earlier study (Illingworth et al.). The accuracy of temperature, humidity profiles, surface albedos and emissivities that are not variables derived from EarhCARE observations also influence the difference.
As the authors state in Introduction, the primally reason for using 3D radiative transfer in the radiative closure is because of the EarthCARE’s goal of achieving TOA irradiance error less than 10 Wm-2. Retrieved cloud fields are tested with realistic radiative transfer, i.e. 3D, to be compared with observations to assess retrieval errors. This is a different goal for some other projects who try to maintain the consistency of algorithms used in retrievals and forward models. If cloud retrievals use 1D models, then using 1D models in the closure brings back observed narrowband radiances used in the retrievals. In this sense, the goal of the forward model is to minimize broadband radiance (or irradiances) errors given a set of narrow-band radiances and active sensor observations.
The manuscript is well written for the purpose of describing the process of EarthCARE’s radiative closure. I do not have any major issues but I have a few clarifications and questions.
On page 6, the authors state that this study is to demonstrate end-to-end simulations with realistic radiances. To me, end-to-end simulations mean that 1) use modeled cloud fields and compute 3D radiative transfer to model BBR and MSI radiances (true radiances). Input these radiances to retrieval algorithms to retrieve cloud and aerosol properties. Use retrieved cloud and aerosol properties combined with a scene construction algorithm to construct 3D cloud fields. Compute TOA radiances using these retrieved cloud fields (retrieved radiances). Apply angular distribution models to both true and retrieved radiances. Assess the difference of true irradiances and retrieved irradiances. I do not think that this is done in this study.
While what I described above is end-to-end simulations of EarhCARE processes, perhaps what the authors described in this manuscript is end-to-end simulation of EarthCARE radiative closure process. If so, the difference shown in Figures 6 and 9 are caused by not knowing scenes outside of the instrument field of view that measures radiances. Is this correct? Or do cloud and aerosol retrieval errors contribute to the differences? If the retrieval errors do not contribute to the differences, the authors need to state in the manuscript.
As stated by the authors, Figures 6 and 9 show that clear-sky scenes have a smaller difference. But looking Figure 6, I cannot clearly link the smaller difference to a larger probability of closure statistics shown in Figure 6c. The authors need to link and show that smaller differences lead to a larger probability for a sanity check.
Does the radiation closure for the ACMB-DF product use 1D radiative transfer models too? If so and if the purpose of this manuscript is to demonstrate the process, could you describe how 1D models will be used in the process?
Citation: https://doi.org/10.5194/egusphere-2024-1651-RC1
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