When mig thermometers were used the regulation height was 2.5cm to 5cm above ground with the thermometer bulb “just touching the tips of the grass”. (Are mig thermometers still used?)
The radiation properties of the grass surface is dictated by the sub soil structure, sandy, clay, dry, wet, etc etc. As far as I’m aware, when artificial grass is installed, a thick layer of sand, then a weed resistant layer then the artificial turf is installed. This obviously makes all installations similar but none representative of the area. I’m sure artificial turf must have been trialed, but because of the sub soil differences in the area I can’t see how there’s any like for like comparison possible.