HC3 Persistence Forecast methodology

Last update: Jul. 2017


In brief:

  • Operational since March 2015
  • Access: automatic-only
  • Time steps: all the 15 minute and hourly data of the current day "D"
  • database: HelioClim-3v4 radiation values in real time (H-15 min) and short-term forecasts using a model of persistence until end of the current day. NB: Version 5 can not be available in real time since the data are only available at "D-2".
  • News mi-March 2016: the service starts as soon as the sun elevation is above 2° (instead of originally 12°). The advantage is to deliver a data from sunrise onwards.

Methodology: The principle of the HC3v4 real-time and short-term forecast service is the persistence of the clear-sky index Kc, defined as the ratio of the GHI by the GHI in clear-sky conditions. Figure 1 illustrates this principle by giving an example of 15 min slots available until 11:25 AM (Figure 1 a)) for a given day d. The last slot available gives half-cloudy half-sunny weather; the main assumption of the Kc persistence states that the same type of weather will be available until the end of the current day, modulated by the sun position (Figure 1 b). Finally, a new MSG image is acquired, and finally, the weather is nicer than planned 15 min ago. The prevision is consequently adjusted (Figure 1 c)). Every 15 min, the service provides the data until the end of the day, with estimates based on all the slots available until the current instant, and forecasts based on local persistence afterwards.

The HelioClim-3 (version 4) Persistence Forecast is based on the "persistence of Kc". We remind that the Kc index corresponds to the Global Horizontal component divided by the GHI in clear sky conditions (GHI/GHI_cls). You may see it as the "percentage of clouds". This method computes the last available Kc value and extend this value for the remaining slots of the day, taking into account of course the time of the day. As a consequence, at least one slot of the day should be available. The HelioClim-3v4 Persistence Forecast predicts the current day and is available only after sunrise.

Figure 1: Illustration of the principle of the HC3v4 real-time and short-term forecast service

NB: We remind that the last image acquired by our system is 20:45 UT. As a consequence, after that time, the service in real time delivers the same outputs as the standard service at d-1, and the values are not updated anymore.