Hello Sarah,

During the previous year research, using the cross correlation function between Heliosat daily data of solar radiation and measured data in horizontal plane, we concluded that they seem to exhibit one day lag. The reasearch was conducted for the region of Athens, Grecce.

You can take a look on the next diagrams ( http://prntscr.com/jrnqx9 ) , or if you want more deatails, read the presentation "Investigation of the stochastic nature of solar radiation for renewable resources management"

Hello Giannis,

Thank you very much for your nice response and also feedback. Why do you think there is such kind of data lag between your measurements and the Heliosat model? Do you think this data lag related to some environmental parameters like clouds or something else? Best, Sahar

Hello Sahar,

I am sorry for the delayed answer. This is an interesting question, but I believe the answer does not only particular lie in the environmental parameters. I think that, as long as the values are almost identical, by exhibiting one day lag, this means that the cloudiness does not affect the Heliostat model. Otherwise, if the environmental condition may affect the model, we would observe different minimum and maximum values and possible a different pattern of daily solar radiation. Also, we must consider that heliostat model provides averaged daily modelled values. So, the fluctuation of solar radiation, in hourly or 10minute averaged scale, which is the direct result of the small scale dynamic behaviour of the cloudiness cannot be recognized. As a result, when you apply a daily model, you may lose all this small-scale information. To conclude, I would say that I do not know why we observed this one-day lag, but I constantly believe that the cloudiness is not the reason to look at, because the cloudiness process does not affect the daily scale so much. Nevertheless, the modelled data are almost the same with the observed ones so the dynamic behaviour of the environmental reasons may not be the answer.

You might also find this article on the comparison between solar data estimation models helpful:

Article Assessment of solar data estimation models for four cities in Iran

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