Next-generation radiance unfiltering process for the Clouds and Earth&rsquo;s Radiant Energy System instrument

Liang, Lusheng; Su, Wenying; Sejas, Sergio; Eitzen, Zachary A.; Loeb, Norman G.

doi:https://doi.org/10.5194/egusphere-2023-1670

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https://doi.org/10.5194/egusphere-2023-1670

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04 Aug 2023

| 04 Aug 2023

Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

Next-generation radiance unfiltering process for the Clouds and Earth’s Radiant Energy System instrument

Lusheng Liang, Wenying Su, Sergio Sejas, Zachary A. Eitzen, and Norman G. Loeb

Abstract. The filtered radiances measured by the Clouds and the Earth’s Radiant Energy System (CERES) instruments are converted to shortwave (SW), longwave (LW), and window unfiltered radiances based on regressions developed from theoretical radiative transfer simulations to relate filtered and unfiltered radiances. This paper describes an update to the existing Edition 4 CERES unfiltering algorithm (Loeb et al., 2001), incorporating the most recent developments in radiative transfer modeling, ancillary input datasets, and increased computational and storage capabilities during the past 20 years. Simulations are performed with MODTRAN 5.4. Over land and snow, the surface Bidirectional Reflectance Distribution Function (BRDF) is characterized by a kernel-based representation in the simulations, instead of the Lambertian surface used in the Edition 4 unfiltering process. Radiance unfiltering is explicitly separated into 4 seasonally dependent land surface groups based on the spectral radiation similarities of different surface types (defined by International Geosphere-Biosphere Programme); over snow, it is separated into fresh snow, permanent snow, and sea ice. It contrasts to the Edition 4 unfiltering process that one set of regressions for land and snow, respectively.

The instantaneous unfiltering errors are estimated with independent test cases generated from radiative transfer simulations in which the ‘true’ unfiltered radiances from radiative transfer simulations are compared with the unfiltered radiances calculated from the regressions. Overall, the relative errors are mostly within ±0.5 % for SW, within ±0.2 % for daytime LW, and within ±0.1 % for nighttime LW for both CERES Terra Flight Model 1 (FM1) and Aqua FM3 instruments. The unfiltered radiances are converted to fluxes and compared to CERES Edition 4 fluxes. The global mean instantaneous fluxes for Aqua FM3 are reduced by less than 0.42 Wm^-2 for SW and increased by less than 0.47 Wm^-2 for daytime LW; for Terra FM1, they are reduced by less than 0.31 Wm^-2 for SW and increased by less than 0.29 Wm^-2 for daytime LW, though regional differences can be as large as 2.0 Wm^-2. Nighttime LW flux differences are nearly negligible for both instruments.

Received: 24 Jul 2023 – Discussion started: 04 Aug 2023

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RC1: 'Comment on egusphere-2023-1670', Nicolas Clerbaux, 12 Sep 2023 reply

This is overall an interesting paper that suggests many improvements to the NASA CERES unfiltering process. The error analysis is very relevant (section 3). Although this work could be a step toward implementation in future CERES releases (eg Ed5), it does not fully explain the differences wrt to Ed4 that are shown in section 4. Additional validation/verification/documentation would be welcome before implementation in the CERES processing system.
I am not a native speaker but get the feeling that the English for some sentences could be improved. A careful review, eg by the journal editor, is suggested. Similarly, the authors should check the units and symbols (eg micrometer is sometime written as µm, or um, or mm).
I suggest the following points for an improved manuscript:
- Don't use "less than" in the abstract and text body (eg line 23: "... are reduced by less than 0.31 W/m² ..." should be "... are reduced by 0.31 W/m² ...").
- Typos line 16 ("process that used one set ..."), line 46 ("covering"), line 55 ("surface the surface"), line 91 ("relationships"), 132 ("µm").
- line 132, the wavelength range is given in µm and the spectral resolution in wavenumbers. It would be good to indicate how many wavelength steps have been used or to specify the wavelength increments, in µm, at lower end (0.25µm) and upper end (1000µm) of the wavelength range.
- For the MODTRAN radiative transfer calculation in the LW part of the spectrum it would be interesting to specify how the surface emissivity has been considered (especially over desert surface).
- The handling of the far infra-red region should be discussed in more details. The spectral responses seem to be defined until 140µm (or 200 µm?), please confirm. What is the assumed sensitivity beyond this limit? zero? In this case, why are the MODTRAN simulations performed until 1000 µm.
- The FM1 and FM3 have marked difference in terms of spectral response (Figure 1). A brief discussion of the difference would be welcome. Also, the far-IR leakage of the SW filter seems to have an identical effect on the SW spectral response for FM1 and FM3. Please confirm as it seems strange to have difference in TW responses and not in SW in the far IR.
- Figures 20-22 show 4 panels that are said to be for April (a), July (b), October (c) and December (d). This is visibly not the case (eg (a) should be Winter (December?)). Further, the text discusses the results for January (eg line 343). Please check and correct.
- The end-to-end sensitivity study of the unfiltering algorithm (section 4) is really interesting. Given the (significant) observed differences with the Ed4 fluxes, this sections would deserve a longer discussion as well on the methodology as on the interpretation of the observed differences.
- Comparing Figures 21 and 22, it seems that most of the daytime difference in LW flux is coming from the subtraction of the SW component in the TW channel. Please confirm this and consider performing additional studies to confirm this work is an improvement with respect to Ed4.

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Citation: https://doi.org/10.5194/egusphere-2023-1670-RC1

Lusheng Liang et al.

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https://doi.org/10.5194/egusphere-2023-1670-supplement

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Short summary

This paper describes an updated process to obtain unfiltered radiation from CERES satellite instruments by incorporating the most recent developments in radiative transfer modeling and ancillary input datasets (e.g., realistic representation of land surface radiation and climatology of surface temperatures and aerosols) during the past 20 years. The resulted global mean of instantaneous SW and LW fluxes are changed by less than 0.5 Wm^-2 with regional differences can be as large as 2.0 Wm^-2.


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