Use total irradiance to calculate UV index

Question

I am using pvlib to calculate diffuse and direct incoming solar radiation at the surface and this works well. I account for clouds, albedo, sea-ice etc. when calculating the total incoming shortwave radiation at the surface.

....
transmittance = (1.0 - cloud_covers) * 0.75
# irrads is a DataFrame containing ghi, dni, dhi

irrads = pvlib.irradiance.liujordan(solpos['apparent_zenith'].to_numpy(), transmittance, am_abs_array)

total_irrad = pvlib.irradiance.get_total_irradiance(surface_tilt,
                                                    surface_azimuth,
                                                    apparent_zenith,
                                                    azimuth,
                                                    irrads['dni'],
                                                    irrads['ghi'],
                                                    irrads['dhi'],
                                                    albedo=albedo)
sw_dr = total_irrad['poa_direct']
sw_df = total_irrad['poa_diffuse']

Now, using the direct (sw_dr) and diffuse (sw_df) shortwave radiation, I wanted to calculate the UV index by 1) Calculating the spectral distribution of the incoming solar radiation by multiplying by the solar energy spectrum 2) Consider only the contribution from the irradiance in the wavelengths 280-400 nm (sw_dr_λ_280-400).

The contribution of each wavelength interval dλ to the direct and diffuse shortwave radiation is weighted by its amount of solar energy under the standard solar spectra ASTM E-490 AM0 (Shanmugam and Ahn, 2007), which is available as a table in the supplementary of Séférian et al. 2018 for wavelengths 200-4000 nm. My understanding is that this gives me the direct and diffuse shortwave radiation as a function of wavelength at 10 nm intervals. To calculate the UV index I used the equation found here:

uvi = np.sum(sw_dr_λ_280_400*erythema_spectrum_λ) * 40.0

Where the erythema_spectrum is the weighting per wavelength band (10 nm) for the effect of erythema.

When I calculate the UVI values using this approach, the values are too high and I wonder if my approach is flawed or whether I am doing something wrong here. Is there anyone else who has calculated UVI using pvlib?

I appreciate your help. Cheers, Trond

Update: I fixed the function above to be more correct. When I calculate UVI north of 30N (0-360E) for January 15 I get an average UVI of 14 but a maximum of 264. This is what it looks like:

Update 2: Based on @Cliff H's comment below I divided my values by 25. The new values are within the range for UVI so this seems correct. See plot for UVI for June 2015.

Answer 1

I don't recognize a conceptual error. What do you get when you integrate the spectral direct irradiance? I'd expect to recover the sw_dr broadband value. Something to check. The line of code that calculates uvi looks odd. sw_dr from total_irrad is a Series, sw_dr(λ[280:400]) indicates that sw_dr is a function.