See the notes in
Takeoff and Landing on Wet and Slippery Runways which quotes data from testing published in NASA Technical Paper 2917.
The ‘effective friction’ is compared across dry, wet, and damp runway surfaces (page 25 - ).
The summary (page 31), states that over a range of damp surfaces the friction was reduced compared to dry, but may be better than wet; but that damp is a subjective term.
Data for grooved runways is also given (page 28).