<p>The stability correction factor <code>ASF</code> 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 <code>w3updtmd.ftn</code>:</p>

<pre><code>! 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
</code></pre>

<p>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 <code>/STAB2</code> switch is enabled (optional), and works only with the <code>/ST2</code> switch (Tolman and Chalikov 1996 source term package). It's implementation and test results have been described in Tolman 2002.</p>

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

<p>References:</p>

<ul>
<li><p>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.</p></li>
<li><p>Tolman, H. L., 2002: Validation of WAVEWATCH III version 1.15 for a global domain. Tech. Note 213, NOAA/NWS/NCEP/OMB, 33 pp.</p></li>
</ul>

CC BY-SA 3.0
2015-01-03T20:29:12.793Z
Thanks @milancurcic for the detailed reply and references! Yes, I checked and I am indeed using `STAB2` and `ST2` switches - they are included in the default `switch` file. From the manual, it seems that the `ASF` is a fudge factor to improve the accuracy of wave growth processes. Just to make sure (and I didn't find this mentioned in the manual), I should still input the wind speed at 10 m above the surface, and not at some other height, right? I think U10 is what WW3 takes in because it is mentioned "Note that this effective
wind speed was derived for winds at 10 m height." in the manual.
boxofchalk1
CC BY-SA 3.0
2015-01-04T11:43:57.050Z
It's great having experts on specific models here who can answer these sorts of questions :-)
Semidiurnal Simon
CC BY-SA 3.0
2015-01-04T22:04:05.690Z
@boxofchalk1 That is correct, keep the wind input fields at 10 m height.
milancurcic
CC BY-SA 3.0
2015-01-03T15:48:13.080Z
2015-01-03T15:56:30.697Z
2015-01-03T15:56:30.697Z
milancurcic
<p>I'm studying the Wavewatch III wave model (<a href="http://polar.ncep.noaa.gov/waves/index2.shtml">http://polar.ncep.noaa.gov/waves/index2.shtml</a>). 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.</p>

CC BY-SA 3.0
2015-01-03T05:56:00.780Z
Can you point to the specific section in the website or code that you are referring to?
Isopycnal Oscillation
CC BY-SA 3.0
2015-01-03T06:20:15.000Z
@IsopycnalOscillation It is not discussed on the website, but I will try to piece together the code segments that use `ASF` and report back.
boxofchalk1
<models><waves><numerical-modelling><wave-modeling>
Stability correction factor in the Wavewatch III wave model
boxofchalk1
# Stability correction factor in the Wavewatch III wave model

(Asked by: username_0 on 2015-01-03)

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.

**Tags:** models, waves, numerical-modelling, wave-modeling

> **Comments:**
> **username_1**: Can you point to the specific section in the website or code that you are referring to?  
> **username_0**: @username_1 It is not discussed on the website, but I will try to piece together the code segments that use `ASF` and report back.  

---

## 1 answers

### Answer by username_2 on 2015-01-03. Score: 12 | Accepted answer

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.

> **Comments:**
> **username_0**: Thanks @username_2 for the detailed reply and references! Yes, I checked and I am indeed using `STAB2` and `ST2` switches - they are included in the default `switch` file. From the manual, it seems that the `ASF` is a fudge factor to improve the accuracy of wave growth processes. Just to make sure (and I didn't find this mentioned in the manual), I should still input the wind speed at 10 m above the surface, and not at some other height, right? I think U10 is what WW3 takes in because it is mentioned "Note that this effective wind speed was derived for winds at 10 m height." in the manual.  
> **username_3**: It's great having experts on specific models here who can answer these sorts of questions :-)  
> **username_2**: @username_0 That is correct, keep the wind input fields at 10 m height.  

---

