Answer from Airbus:
9. WIND DISPLAYED AT 100 KNOTS DURING TAKEOFF ON THE ND
During take-off roll, where is the wind derived from at 100kts on the ND? As the
aeroplane is still on the ground, how does it work?
ANSWER:
The wind displayed on the ND corresponds to instant wind. It is computed by the
IRS, only from 100kts.
The computation of the wind direction and force uses the real time difference
between the heading and the track of the aircraft, as seen by the IRS. Therefore,
as long as the aircraft is on ground, the display corresponds to the longitudinal
component of the wind. Moreover during the take off roll, anemometric disturbance
may cause an erroneous wind value to be displayed on the ND.
Once airborne, since lateral drift can be measured, the IRS are able to compute
and display the wind direction and force.
Cheers
mcdhu