Heli-Boy1 mentioned earlier;
The BERP blades fitted generate far more lift at the tips. This coupled with the considerable down wash (high mass helicopter) means that recirculation occurs further away from the fuselage.
There is still a huge amount of recirculating dust and to say there is no brownout is not really true. Rather the effect is a 'bubble' of relatively clear air close to th aircraft.
That is my view - having said that, Westlands themselves did not have a real answer to the effect when this question was asked of tem recently!
The following may support your
"The BERP blades fitted generate far more lift at the tips. .........."
From:~ Art of the Helicopter, by John Watkinson;
”The presence of the rotor head and blade root cut-out area creates a hole in the middle of the disk through which air can escape upward in ground effect. This is actually another form of tip loss, which is called the fountain effect. In practice the fountain effect can be beneficial because it puts some of the hull into upflow and reduces the hull download.”
This presentation by AgustaWestland may be of interest.
Dave