The vertical friction (Source File: friction.F90)


    subroutine friction(kappa,avmolu,tx,ty)

This subroutine updates the bottom roughness

$\displaystyle z_0^b = 0.1 \frac{\nu}{u_*^b} + 0.03 h_0^b + z_a \quad .$ (23)

The first term on the right hand side of (23) represents the limit for hydraulically smooth surfaces, the second term the limit for completely rough surfaces. Note that the third term, $ z_a$, is the contribution of suspended sediments to the roughness length, see Smith and McLean (1977). It is updated during calls to the sediment-routines.

The law-of-the-wall relations are used to compute the friction velocity

$\displaystyle u_*^b = r \sqrt{U_1^2 + V_1^2} \; , \quad$ (24)

where $ U_1$ and $ V_1$ are the components of the mean velocity at the center of the lowest cell. We used the abbreviation

$\displaystyle r=\frac{\kappa}{\ln \left( \frac{0.5h_1+z_0^b}{z^b_0} \right)} \; , \quad$ (25)

where $ \kappa$ is the von Kármán constant and the index `1' indicates values at the center of the first grid box at the bottom (version 1). Another expression for $ r$ can be derived using the mean value of the velocity in the lowest grid box, and not its value in the middle of the box (version 2). Also this method is supported in friction() and can be activated by uncommenting one line in the code.

If no breaking surface waves are considered, the law of the wall also holds at the surface. The surface roughness length may be calculated according to the Charnock (1955) formula,

$\displaystyle z_0^s=\alpha \frac{(u_*^s)^2}{g} \quad .$ (26)

The model constant $ \alpha$ is read in as charnock_val from the meanflow namelist.


    use meanflow,      only: h,z0b,h0b,MaxItz0b,z0s,za
    use meanflow,      only: u,v,rho,gravity
    use meanflow,      only: u_taub,u_taus,drag,taub
    use meanflow,      only: charnock,charnock_val,z0s_min
    REALTYPE, intent(in)                :: kappa,avmolu,tx,ty
    Original author(s): Hans Burchard & Karsten Bolding
    $Log: friction.F90,v $
    Revision 1.11  2010-09-17 12:53:48  jorn
    extensive code clean-up to ensure proper initialization and clean-up of all variables
    Revision 1.10  2006-11-20 17:28:58  kbk
    [Cc]harnok -> [Cc]harnock - A. Jenkins
    Revision 1.9  2006-11-20 17:26:15  kbk
    [Cc]harnok -> [Cc]harnock - A. Jenkins
    Revision 1.8  2005/08/11 12:31:54  lars
    corrected error in documentation. Thanks to Patrizio Mariani
    Revision 1.7  2005/06/27 13:44:07  kbk
    modified + removed traling blanks
    Revision 1.6  2004/08/18 12:33:30  lars
    updated documentation
    Revision 1.5  2004/01/13 08:39:49  lars
    included roughness due to suspended sediments
    Revision 1.4  2003/03/28 09:20:35  kbk
    added new copyright to files
    Revision 1.3  2003/03/28 08:56:56  kbk
    removed tabs
    Revision 1.2  2003/03/10 08:50:06  gotm
    Improved documentation and cleaned up code
    Revision  2001/02/12 15:55:57  gotm
    initial import into CVS

Karsten Bolding 2012-01-24