Thanks for the reply.
If I have understood correctly, Q2, T2 and PSFC are read directly from the wrf output file.
I also found the formulas for calculating relative humidity, however!
When I request through RIP the mixing ratio (qvp) at the isobaric level equal of PSFC, I get a different number from the Q2 (I do not mean hPa and Pa)
The relative humidity is therefore different but much closer to the value I get from RIP directly when I request the rhu at the PSFC level (plus, this is closer to my real surface values).
Which value should I use, the one I get from RIP at the PSFC level, the one I calculate when using Q2 or the one using qvp?
Sorry for the mess, but I am a bit at loss