I'm studying the Wavewatch III wave model (http://polar.ncep.noaa.gov/waves/index2.shtml). I'm confused by the stability correction factor, named ASF in the code. What is its purpose? Is it related to the physics, or is it something to do with numerical methods and the CFL condition? I could not immediately figure this out from the code.
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
! 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.
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.