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Update the buoyancy variance

*INTERFACE:*

subroutine do_kb(nlev,dt,u_taus,u_taub,z0s,z0b,h,NN,SS)

*DESCRIPTION:
*

Based on the value of `kb_method`,
this routine calls the appropriate routines for
calculating (half) the buoyancy variance defined in (52).
The user has the choice between a simple algebraic expression,
described in section 4.18, and
a dynamic equation for , described in section 4.13.

*USES:*

IMPLICIT NONE

*INPUT PARAMETERS:*
integer, intent(in) :: nlev
REALTYPE, intent(in) :: dt,u_taus,u_taub,z0s,z0b
REALTYPE, intent(in) :: h(0:nlev)
REALTYPE, intent(in) :: NN(0:nlev),SS(0:nlev)

*REVISION HISTORY:*
Original author(s): Lars Umlauf

Karsten Bolding
2012-12-28