No temp FLEX on contaminated runways for AB.
But Airbus do not determine that Frost is a contaminant.
If the layer of contaminant on the runway is thin enough, the runway is not considered contaminated, but only wet.
FLEX takeoff is not allowed from a contaminated runway.
As far as performance determination is concerned, the following guidelines should be considered, as mentioned in the FCOM:
In terms of performance a fluid contamination is equivalent to wet, up to and including a maximum depth of 3 mm (1/8") of:
- dry snow
- wet snow
- standing water slush.
"Frost" is equivalent to wet.
Same reference (note that the Getting to Grips with Cold Weather easily available online is the older version which does not have these references).
Out of genuine interest, do you have a Boeing reference for frost? Performance on contaminated runways is a hot topic in some parts of the world and the subject of much recent study.