gotm-model · bderembl · Dec 8, 2025
diff --git a/src/airsea/CMakeLists.txt b/src/airsea/CMakeLists.txt
@@ -12,6 +12,7 @@ add_library(airsea
    longwave_radiation.F90
    fairall.F90
    kondo.F90
+   constant_cd.F90
    solar_zenith_angle.F90
    albedo_water.F90
    shortwave_radiation.F90

diff --git a/src/airsea/airsea.F90 b/src/airsea/airsea.F90
@@ -169,6 +169,7 @@ subroutine init_airsea_yaml()
 ! {\tt fluxes\_method}   & Select which parameterisation to use for latent and sensible fluxes:   \\
 !                          & 1: Kondo (1975)                                                        \\
 !                          & 2: Fairall et al. (1996)                                               \\
+!                          & 3: user input exchange coefs (cdd, chd, ced)                           \\
 ! {\tt longwave\_radiation\_method}   & Select which parameterisation to use:                        \\
 !                          & 3: Clark et al. (1974)                                                 \\
 !                          & 4: Hastenrath and Lamb (1978)                                          \\
@@ -248,7 +249,7 @@ subroutine init_airsea_yaml()
 
    twig => branch%get_typed_child('fluxes', 'heat and momentum fluxes')
    call twig%get(fluxes_method, 'method', 'method to calculate fluxes from meteorological conditions', &
-                options=(/option(0, 'use prescribed fluxes', 'off'), option(1, 'Kondo (1975)', 'kondo'), option(2, 'Fairall et al. (1996)', 'fairall')/), default=0)
+                options=(/option(0, 'use prescribed fluxes', 'off'), option(1, 'Kondo (1975)', 'kondo'), option(2, 'Fairall et al. (1996)', 'fairall'), option(3, 'prescribed exchange coefs', 'constant_cd')/), default=0)
    call twig%get(heat_input, 'heat', 'prescribed total heat flux (sensible, latent and net longwave-radiation)', 'W/m^2', &
                 default=0._rk)
    call twig%get(tx_input, 'tx', 'prescribed momentum flux in West-East direction', 'Pa', &
@@ -300,6 +301,9 @@ subroutine init_airsea_yaml()
                 default=0._rk, display=display_advanced)
    call branch%get(sss_input, 'sss', 'observed surface salinity', 'psu', &
                 default=0._rk, display=display_advanced)
+   call branch%get(const_cdd, 'cdd', 'momentum exchange coefficient', '1', default=0._rk)
+   call branch%get(const_chd, 'chd', 'heat exchange coefficient', '1', default=0._rk)
+   call branch%get(const_ced, 'ced', 'humidity exchange coefficient', '1', default=0._rk)
    LEVEL2 'done'
    return
    end subroutine init_airsea_yaml
@@ -326,6 +330,7 @@ subroutine post_init_airsea(lat,lon)
 ! {\tt fluxes\_method}   & Select which parameterisation to use for latent and sensible fluxes:   \\
 !                          & 1: Kondo (1975)                                                        \\
 !                          & 2: Fairall et al. (1996)                                               \\
+!                          & 3: user input cdd, chd, ced                                            \\
 ! {\tt longwave\_radiation\_method}   & Select which parameterisation to use:                        \\
 !                          & 1: Clark et al. (1974)                                                 \\
 !                          & 2: Hastenrath and Lamb (1978)                                          \\
@@ -481,6 +486,11 @@ subroutine post_init_airsea(lat,lon)
             LEVEL3 'using Kondo formulation'
          case(2)
             LEVEL3 'using Fairall et. all formulation'
+         case(3)
+            LEVEL3 'user input exchange coefficients'
+            LEVEL3 'cdd=',const_cdd
+            LEVEL3 'chd=',const_chd
+            LEVEL3 'ced=',const_ced
          case default
       end select
       select case (ql_input%method)

diff --git a/src/airsea/airsea_fluxes.F90 b/src/airsea/airsea_fluxes.F90
@@ -38,6 +38,8 @@ subroutine airsea_fluxes(method,sst,airt,u10,v10,precip, &
          call kondo(sst,airt,u10,v10,precip,evap,taux,tauy,qe,qh)
       case (2) ! Fairall et. all
          call fairall(sst,airt,u10,v10,precip,evap,taux,tauy,qe,qh)
+      case (3) ! Constant cd
+         call constant_cd(sst,airt,u10,v10,precip,evap,taux,tauy,qe,qh)
       case default
    end select
 

diff --git a/src/airsea/airsea_variables.F90 b/src/airsea/airsea_variables.F90
@@ -35,6 +35,7 @@ module airsea_variables
    REALTYPE, public                    :: L
    REALTYPE, public                    :: rhoa
    REALTYPE, public, target            :: ta              ! 2m air temperature (degree_Celsius)
+   REALTYPE, public                    :: const_cdd, const_ced, const_chd
    logical, public                     :: rain_impact
    logical, public                     :: calc_evaporation
 !

diff --git a/src/airsea/constant_cd.F90 b/src/airsea/constant_cd.F90
@@ -0,0 +1,136 @@
+#include "cppdefs.h"
+!-----------------------------------------------------------------------
+!BOP
+!
+! !ROUTINE: Heat and momemtum fluxes computed with constant transfert coefs
+!
+! !INTERFACE:
+   subroutine constant_cd(sst,airt,u10,v10,precip,evap,taux,tauy,qe,qh)
+!
+! !DESCRIPTION:
+!  Based on the model sea surface temperature, the wind vector
+!  at 10 m height, the air pressure at 2 m, the dry air
+!  temperature and the air pressure at 2 m, and the relative
+!  humidity (either directly given or recalculated from the
+!  wet bulb or the dew point temperature),
+!  this routine the surface
+!  momentum flux vector, $(\tau_x^s,\tau_y^s)$,
+!  the latent heat flux, $Q_e$,
+!  and the sensible heat flux, $Q_h$, according to the 
+!  bulk formulae.
+!
+!  \begin{equation}
+!  \begin{array}{rcl}
+!  \tau_x^s &=& c_{dd} \rho_a W_x W \\ \\
+!  \tau_y^s &=& c_{dd} \rho_a W_y W \\ \\
+!  Q_e &=& c_{ed} L \rho_a W (q_s-q_a) \\ \\
+!  Q_h &=& c_{hd} C_{pa} \rho_a W (T_w-T_a)
+!  \end{array}
+!  \end{equation}
+!
+!  with the air density $\rho_a$, the wind speed at 10 m, $W$,
+!  the $x$- and the $y$-component of the wind velocity vector,
+!  $W_x$ and $W_y$, respectively, the specific evaporation heat of sea water,
+!  $L$, the specific saturation humidity, $q_s$, the actual
+!  specific humidity $q_a$, the specific heat capacity of air at constant
+!  pressure, $C_{pa}$, the sea surface temperature, $T_w$ and the
+!  dry air temperature, $T_a$.
+!
+! !USES:
+   use airsea_variables, only: kelvin,const06,rgas,rho_0
+   use airsea_variables, only: qs,qa,rhoa
+   use airsea_variables, only: cpa,cpw
+   use airsea_variables, only: rain_impact,calc_evaporation
+   use airsea_variables, only: const_cdd, const_ced, const_chd
+   IMPLICIT NONE
+!
+! !INPUT PARAMETERS:
+   REALTYPE, intent(in)                :: sst,airt,u10,v10,precip
+!
+! !INPUT/OUTPUT PARAMETERS:
+   REALTYPE, intent(inout)             :: evap
+!
+! !OUTPUT PARAMETERS:
+   REALTYPE, intent(out)               :: taux,tauy,qe,qh
+!
+! !REVISION HISTORY:
+!  Original author(s): Bruno Deremble
+!
+! !LOCAL VARIABLES:
+   REALTYPE                  :: w,L
+
+   REALTYPE                  :: x1,x2,x3
+   REALTYPE                  :: ta,ta_k,tw,tw_k
+   REALTYPE                  :: cdd,chd,ced
+   REALTYPE                  :: tmp,rainfall,cd_rain
+   REALTYPE, parameter       :: eps=1.0e-12
+!EOP
+!-----------------------------------------------------------------------
+!BOC
+   w = sqrt(u10*u10+v10*v10)
+   L = (2.5-0.00234*sst)*1.e6
+
+   if (sst .lt. 100.) then
+      tw  = sst
+      tw_k= sst+kelvin
+   else
+      tw  = sst-kelvin
+      tw_k= sst
+   end if
+
+   if (airt .lt. 100.) then
+      ta_k  = airt + kelvin
+      ta = airt
+   else
+      ta  = airt - kelvin
+      ta_k = airt
+   end if
+
+
+   cdd = const_cdd
+   chd = const_chd
+   ced = const_ced
+
+
+   qh=-chd*cpa*rhoa*w*(sst-airt)       ! sensible
+   qe=-ced*L*rhoa*w*(qs-qa)            ! latent
+
+!  compute sensible heatflux correction due to rain fall
+   if (rain_impact) then
+!     units of qs and qa - should be kg/kg
+      rainfall=precip * 1000. ! (convert from m/s to kg/m2/s)
+      x1 = 2.11e-5*(ta_k/kelvin)**1.94
+      x2 = 0.02411*(1.0+ta*(3.309e-3-1.44e-6*ta))/(rhoa*cpa)
+      x3 = qa * L /(rgas * ta_K * ta_K)
+      cd_rain = 1.0/(1.0+const06*(x3*L*x1)/(cpa*x2))
+      cd_rain = cd_rain*cpw*((tw-ta) + (qs-qa)*L/cpa)
+      qh = qh - rainfall * cd_rain
+   end if
+
+!  calculation of evaporation/condensation in m/s
+   if (rain_impact .and. calc_evaporation) then
+!     ced from latent heatflux for moisture flux
+      evap = rhoa/rho_0*ced*w*(qa-qs)
+   else
+      evap = _ZERO_
+   end if
+
+   tmp = cdd*rhoa*w
+   taux = tmp*u10
+   tauy = tmp*v10
+
+!  Compute momentum flux (N/m2) due to rainfall (kg/m2/s).
+!  according to Caldwell and Elliott (1971, JPO)
+   if ( rain_impact ) then
+      tmp  = 0.85d0 * rainfall
+      taux  = taux + tmp * u10
+      tauy  = tauy + tmp * v10
+   end if
+
+   return
+   end subroutine constant_cd
+!EOC
+
+!-----------------------------------------------------------------------
+! Copyright by the GOTM-team under the GNU Public License - www.gnu.org
+!-----------------------------------------------------------------------