! SPDX-FileCopyrightText: FLEXPART 1998-2019, see flexpart_license.txt
! SPDX-License-Identifier: GPL-3.0-or-later

real function scalev(ps,t,td,stress)

  !********************************************************************
  !                                                                   *
  !                       Author: G. WOTAWA                           *
  !                       Date:   1994-06-27                          *
  !                       Update: 1996-05-21 A. Stohl                 *
  !                                                                   *
  !********************************************************************
  !                                                                   *
  !     This Programm calculates scale velocity ustar from surface    *
  !     stress and air density.                                       *
  !                                                                   *
  !********************************************************************
  !                                                                   *
  !     INPUT:                                                        *
  !                                                                   *
  !     ps      surface pressure [Pa]                                 *
  !     t       surface temperature [K]                               *
  !     td      surface dew point [K]                                 *
  !     stress  surface stress [N/m2]                                 *
  !                                                                   *
  !********************************************************************
  
  use par_mod

  implicit none

  real :: ps,t,td,e,ew,tv,rhoa,stress

  e=ew(td)                       ! vapor pressure
  tv=t*(1.+0.378*e/ps)           ! virtual temperature
  rhoa=ps/(r_air*tv)              ! air density
  scalev=sqrt(abs(stress)/rhoa)

end function scalev