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!**********************************************************************
! Copyright 1998,1999,2000,2001,2002,2005,2007,2008,2009,2010 *
! Andreas Stohl, Petra Seibert, A. Frank, Gerhard Wotawa, *
! Caroline Forster, Sabine Eckhardt, John Burkhart, Harald Sodemann *
! *
! This file is part of FLEXPART. *
! *
! FLEXPART is free software: you can redistribute it and/or modify *
! it under the terms of the GNU General Public License as published by*
! the Free Software Foundation, either version 3 of the License, or *
! (at your option) any later version. *
! *
! FLEXPART is distributed in the hope that it will be useful, *
! but WITHOUT ANY WARRANTY; without even the implied warranty of *
! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the *
! GNU General Public License for more details. *
! *
! You should have received a copy of the GNU General Public License *
! along with FLEXPART. If not, see <http://www.gnu.org/licenses/>. *
!**********************************************************************
real function psim(z,al)
!**********************************************************************
! *
! DESCRIPTION: CALCULATION OF THE STABILITY CORRECTION FUNCTION FOR *
! MOMENTUM AS FUNCTION OF HEIGHT Z AND OBUKHOV SCALE *
! HEIGHT L *
! *
!**********************************************************************
use par_mod
implicit none
real :: z,al,zeta,x,a1,a2
zeta=z/al
if(zeta.le.0.) then
! UNSTABLE CASE
x=(1.-15.*zeta)**0.25
a1=((1.+x)/2.)**2
a2=(1.+x**2)/2.
psim=log(a1*a2)-2.*atan(x)+pi/2.
else
! STABLE CASE
psim=-4.7*zeta
endif
end function psim
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