Stability correction factor in the Wavewatch III wave model
The stability correction factor ASF is related to the effects of atmospheric stability (function of buoyancy and shear) on wave growth, and has been implemented in Wavewatch3 in the Tolman and Chalikov 1996 input source term. The code where the correction happens can be found in w3updtmd.ftn:

! 5.  Stability correction ( !/STAB2 )
!     Original settings :
!
!     SHSTAB =    1.4
!     OFSTAB =   -0.01
!     CCNG   =   -0.1
!     CCPS   =    0.1
!     FFNG   = -150.
!     FFPS   =  150.
!
!/STAB2      STAB0  = ZWIND * GRAV / 273.
!
!/STAB2      DO ISEA=1, NSEA
!/STAB2        STAB   = STAB0 * AS(ISEA) / MAX(5.,U10(ISEA))**2
!/STAB2        STAB   = MAX ( -1. , MIN ( 1. , STAB ) )
!
!/STAB2        THARG1 = MAX ( 0. , FFNG*(STAB-OFSTAB))
!/STAB2        THARG2 = MAX ( 0. , FFPS*(STAB-OFSTAB))
!/STAB2        COR1   = CCNG * TANH(THARG1)
!/STAB2        COR2   = CCPS * TANH(THARG2)
!
!/STAB2        ASF(ISEA) = SQRT ( (1.+COR1+COR2)/SHSTAB )
!/STAB2        U10(ISEA) = U10(ISEA) / ASF(ISEA)
!/STAB2        END DO


The correction essentially accounts for atmospheric stability based on air-sea temperature difference, and is applied to calculate the "effective" wind speed at 10-m height, from the input wind speed. Note that this code is active only when /STAB2 switch is enabled (optional), and works only with the /ST2 switch (Tolman and Chalikov 1996 source term package). It's implementation and test results have been described in Tolman 2002.

This is also documented in the manual for version 4.18, see equations (2.86)-(2.89) in section 2.3.7.

References:


Tolman, H. L. and D. V. Chalikov, 1996: Source terms in a third-generation wind-wave model. J. Phys. Oceanogr., 26, 2,497–2,518.
Tolman, H. L., 2002: Validation of WAVEWATCH III version 1.15 for a global domain. Tech. Note 213, NOAA/NWS/NCEP/OMB, 33 pp.