In ERA-Interim there are two data fields for photosynthetically available radiation (PAR):
- surface photosynthetically active radiation (par, code 58)
- clear sky surface photosynthetically active radiation (parcs, code 20)
Both are cumulative fields for the 12 hour periods 00:00 to 12:00 and 12:00 to 24:00, giving units of J m**-2.
However, the surface PAR value (code 58) seems erroneously low. For example, in locations in the Celtic Sea, surface PAR is typically around 20% to 25% of the clear sky value (code 20), and about a third of in-situ measurement of surface PAR.
ERA-Interim is produced by the ECMWF Integrated Forecasting System (IFS), which we suspect has a bug in the calculation of surface PAR.
Details: We suspect there is a bug in the way surface PAR is calculated in that it looks like it is taken from the wrong parts of the spectrum. We have shortwave bands that include 0.442-0.625 micron, 0.625-0.778 micron and 0.778-1.24 micron. PAR is coded as if it was intending to sum all of the radiation in the first of these and 0.42 of the second (to account for the fact that PAR is normally defined to stop at 0.7 microns. However, PAR is in fact calculated from the sum of the second band plus 0.42 of the third.
We will try to fix this in a future cycle, but it is not possible to correct previously released.
The bug only affects surface PAR (code 58). Clear sky surface PAR (code 20) has a good fit with in-situ measurements and can be deemed trustworthy.
In ERA5, which is generated by a later version of the same forecasting system, the bug is still present, hence in ERA5 the parameters surface PAR and clear sky surface PAR are not published.
A rough estimate of PAR can be calculated from SSRD*AFACT where SSRD is the "Surface solar radiation downwards" and AFACT is a multiplier that depends on location, season, atmospheric conditions etc.
There is no single best method to estimate AFACT, but for guidance see for example:
Jacovides et al.: Global photosynthetically active radiation and its relationship with global solar radiation in the Eastern Mediterranean basin. Theor. Appl. Climatol. (2003) 74: 227. doi:10.1007/s00704-002-0685-5