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I am following the paper to calculate UTCI using ERA5 meteorological variables.

https://link.springer.com/article/10.1007/s00484-020-01900-5?shared-article-renderer

In the paper, the authors have mentioned that they are using ERA5 product. And specifically mentioned to use the direct and diffuse component of shortwave (SW) radiation.

The problem is, ERA5 data does not provide the direct and diffuse SW separately. Rather it only provides Surface solar radiation downwards which includes both the direct and diffuse component.

My question would be, is there a way to calculate the direct and diffuse component of SW from ERA5. The dataset I have is similar to ERA5 and has the Downward SW and Net SW. I have no problem in obtaining the Long waves BTW.

Any help would be much appreciated. TIA

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    $\begingroup$ In the Era5 hourly data on single levels from 1979 to present, I can find Total Sky (and Clear-sky) direct solar radiation at surface which "is the amount of direct radiation from the Sun (...) reaching the surface of the Earth". Subtracting from what you've got should yield what you need. I noticed a mistake in the paper. Compare eq(4) with the first Symbol/Equation in their Table 1. $\endgroup$
    – Joscha Fregin
    Jul 2, 2020 at 20:01
  • $\begingroup$ Thanks a lot. Yes, I was also assuming the equation 4 is wrong. $\endgroup$
    – Golam Rabbani Fahad
    Jul 2, 2020 at 23:58
  • $\begingroup$ @J.Fregin if I have only clear sky surface SW, surface SW, surface net SW, would it be possible to get direct and diffused component?? Detailed variable for the radiation for my dataset are in page 44 of this pdf link. gmao.gsfc.nasa.gov/pubs/docs/Bosilovich785.pdf $\endgroup$
    – Golam Rabbani Fahad
    Jul 3, 2020 at 8:51
  • $\begingroup$ I'm no expert in radiation. What's the difference between surface net SW and, surface SW? Does surface net SW include reflections? However my guess is that you can't calculate the diffusive SW component from the variables given in your comment. $\endgroup$
    – Joscha Fregin
    Jul 3, 2020 at 19:58
  • $\begingroup$ @GolamRabbaniFahad, for MERRA-2, which does not provide direct SW, you will need to use the technique in the approved answer. The comment Joscha Fregin is the best answer for ERA5 data as it doesn't rely on an empirical partitioning relationship. $\endgroup$
    – Klimaat
    Nov 5, 2022 at 19:32

1 Answer 1

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This paper By Fiddes & Gruber (TopoSCALE: downscaling gridded climate data in complex terrain) provides a method to partition direct and diffuse from ERA-Interim. The results could easily be applied to ERA5.

It uses a clearness index approximation from Ruiz-Arias et al. (2010)

$k_t = SW / SW_{toa}$ (eq C11) where $ SW_{toa} $ is the shortwave at the top-of-atmosphere.

then,

$SW_{diffuse} = 0.952 - 1.041 e^{-exp(2.3 - 4.702k_t)}$ (eq C12)

You should be able to get top-of-atmosphere SW from ERA5, or possibly calculate it based on solar geometry.

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  • $\begingroup$ Wow, this is exactly what I was looking for. Thanks a lot. $\endgroup$
    – Golam Rabbani Fahad
    May 13, 2022 at 17:10
  • $\begingroup$ please consider marking as the accepted answer. $\endgroup$
    – alex_danielssen
    Jul 18, 2022 at 21:19
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    $\begingroup$ @GolamRabbaniFahad The Ruiz-Arias relationship is one of many such approximations. Depending upon your region, there may be better options, as reviewed here:e Gueymard, Christian A., and Jose A. Ruiz-Arias. “Extensive Worldwide Validation and Climate Sensitivity Analysis of Direct Irradiance Predictions from 1-Min Global Irradiance.” Solar Energy 128 (April 2016): 1–30. doi.org/10.1016/j.solener.2015.10.010. $\endgroup$
    – Klimaat
    Nov 5, 2022 at 19:36

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