### Satellite PAR formula and Gregg-Carder light model

Posted:

**Wed Nov 24, 2021 2:57 pm America/New_York**Hi everyone!

I'm modelling hourly surface irradiance from 400-700nm using the Gregg-Carder model and trying to figure out how to properly convert it to the same units as the satellite PAR product so it can be scaled, but I'm having trouble following the steps in the PAR model. I would like to use the satellite planar value in mW/cm2/um instead of the scalar value in Einsteins/m2/day.

For Gregg-Carder irradiance, I end up with a matrix of irradiance values in Watts/m2/nm that I can convert to mW/cm2/um, where each row is an hour interval and each column is a waveband.

For satellite PAR, my understanding is that the satellite PAR value in mW/cm2/um is an average value over the whole PAR spectrum and the whole day, and that multiplying it by a factor of 1.193 accounts for the transformation of units, planar to scalar geometry, and integration over waveband and time, with a little bit of error, giving us the final PAR product in Einsteins/m2/day.

If this is right, then I should divide satellite PAR by 1.193 to get it back to units of mW/cm2/um, and take the average value of my Gregg-Carder matrix in mW/cm2/um units for comparison.

Can anyone confirm is this is the correct way to do it?

Thanks,

Steph

I'm modelling hourly surface irradiance from 400-700nm using the Gregg-Carder model and trying to figure out how to properly convert it to the same units as the satellite PAR product so it can be scaled, but I'm having trouble following the steps in the PAR model. I would like to use the satellite planar value in mW/cm2/um instead of the scalar value in Einsteins/m2/day.

For Gregg-Carder irradiance, I end up with a matrix of irradiance values in Watts/m2/nm that I can convert to mW/cm2/um, where each row is an hour interval and each column is a waveband.

For satellite PAR, my understanding is that the satellite PAR value in mW/cm2/um is an average value over the whole PAR spectrum and the whole day, and that multiplying it by a factor of 1.193 accounts for the transformation of units, planar to scalar geometry, and integration over waveband and time, with a little bit of error, giving us the final PAR product in Einsteins/m2/day.

If this is right, then I should divide satellite PAR by 1.193 to get it back to units of mW/cm2/um, and take the average value of my Gregg-Carder matrix in mW/cm2/um units for comparison.

Can anyone confirm is this is the correct way to do it?

Thanks,

Steph