A question about the calculation of friction velocity (or roughness length)

[Rachel201810]

Hi,
I found the friction velocity (UST) and the suface roughness length (Z0) have abrupt changes at about 30m/s wind speed, using ST4. It seems for wind speed larger than 30m/s, the friction velocity (roughness) is only related to the wind speed and not to the sea state (see the following figure). I found in the codes the limit for max wind speed is 50m/s, but not 30m/s. I want to know why there is such a abrupt change at about 30m/s wind speed?
Thanks!

[Rachel201810]

I think the abrupt change at about 30m/s may because these lines in w3src4md.ftn:
REAL, PARAMETER :: TAUWMAX = 2.2361 !SQRT(5.)
...
TAUW_LOCAL=MAX(MIN(TAUW,TAUWMAX),0.)
But I can't understand why there is such a limit for TAUW? Can I set TAUWMAX a larger constent (e.g., 5 or 10)?