Hi everyone!
I'm modelling hourly surface irradiance from 400700nm using the GreggCarder 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 GreggCarder 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 GreggCarder matrix in mW/cm2/um units for comparison.
Can anyone confirm is this is the correct way to do it?
Thanks,
Steph
Satellite PAR formula and GreggCarder light model

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Re: Satellite PAR formula and GreggCarder light model
Yes. PAR is calculated internally in mW/cm2/um, and at the very end of the code:
/* Convert to E/D/m^2 */
if (par[ip] != BAD_FLT)
par[ip] *= 1.193;
So...dividing by 1.193 will get you back into radiance.
Sean
/* Convert to E/D/m^2 */
if (par[ip] != BAD_FLT)
par[ip] *= 1.193;
So...dividing by 1.193 will get you back into radiance.
Sean
Re: Satellite PAR formula and GreggCarder light model
Thanks Sean!
I think my main confusion is the value that the satellite planar radiance represents, since I want to make sure I end up with the equivalent value modelled from GreggCarder. I was mixing up steps of integrating vs averaging and converting between energy and number of photons per waveband, and then confusing myself even more trying to relate it to the satellite model.
I just realized my method of calculating the average of the GreggCarder matrix is a little different too  I'm taking the average of the entire matrix, but I should be averaging over waveband at each hour interval, and then integrating over day and dividing by 24 hours, which gives a slightly different result. Can you confirm that the latter is the value I should be using from my GC matrix to get a value equivalent to the satellite radiance in mW/cm2/um?
Steph
I think my main confusion is the value that the satellite planar radiance represents, since I want to make sure I end up with the equivalent value modelled from GreggCarder. I was mixing up steps of integrating vs averaging and converting between energy and number of photons per waveband, and then confusing myself even more trying to relate it to the satellite model.
I just realized my method of calculating the average of the GreggCarder matrix is a little different too  I'm taking the average of the entire matrix, but I should be averaging over waveband at each hour interval, and then integrating over day and dividing by 24 hours, which gives a slightly different result. Can you confirm that the latter is the value I should be using from my GC matrix to get a value equivalent to the satellite radiance in mW/cm2/um?
Steph

 User Services
 Posts: 1262
 Joined: Wed Sep 18, 2019 6:15 pm America/New_York
Re: Satellite PAR formula and GreggCarder light model
Steph,
Well, I can't *deny* that approach is valid...it's effectively what the satellite algorithm is doing
Sean
Well, I can't *deny* that approach is valid...it's effectively what the satellite algorithm is doing
Sean