How obtain UVI

[jmcabrera]

Hi everyone,

I'm working in Python to compute the UVI (Ultraviolet Index). Here’s my approach:

I activated the solar diagnostics in WRF to obtain SZA (Solar Zenith Angle) and CLRNIDX (Clearness Index).

So, the formula is:

UVI = 40 × FCLR × CLRNIDX

If the wrfout files contain valid values, this works fine. However, in my case all CLRNIDX values were -999.0, so I used another expression to estimate the clearness index:

CLRNIDX = SWDOWN / GHI_clear

• SWDOWN = downward shortwave radiation at the surface (from WRF)
• GHI_clear = clear-sky global horizontal irradiance (computed with pvlib)

FCLR is the clear-sky erythemal fraction, derived from Madronich (2007):

• μ₀ = cos(SZA)
• Ω = total ozone in Dobson Units (200–400 DU for SZA 0–60°, with ~10% uncertainty)

It’s also possible to add corrections for altitude, albedo, and aerosols.

If anyone has a better approach, I’d really appreciate it if you could share it here.