Hey @erydjunaedy ,
There isn’t anything too complex about this. The MRT that goes into the UTCI model is essentially essentially just longwave_mrt + shortwave_mrt_delta. Longwave MRT is dictated by the temperature of surrounding surfaces and the sky temperature. It’s essentially the outcome of heat exchange in really low frequency infrared wavelengths. Shortwave solar includes all of the visible solar energy from the sun as well as near infrared (and a little ultraviolet). It’s directly related to the solar irradaince (in W/m2) that we get out of Radiance. Essentially, the irradiance gets plugged into the ASHRAE-55 SolarCal model to convert it into an MRT delta experienced by a human. The documentation on the SolarCal model in Ladybug Tools can be found here along with appropriate references.
I just added the ability to do this in the very latest version of the LBT plugin that you can get with the LB Versioner. If you run that component and then restart Rhino, you’ll see the latest UTCI recipe has an env_conds output, which you can plug into a new component called “HB Read Environmental Matrix” to load either the MRT, Longwave MRT, and Shortwave MRT into Grasshopper.