surface: add namelist parameter for the minimum surface velocity

New variable min_horv in NAMSURFACE. Default is 0.1 (as was hardcoded before).
Used as a minimum value when calculating the magnitude of the horizontal surface
wind for drag and surface processes. Required for RCEMIP.

src/modsurface.f90

@@ -759,7 +759,7 @@ subroutine surface
       xpatches*ypatches,MPI_SUM, comm3d,mpierr)
 
       horvpatch = sqrt(((Supatch/SNpatch) + cu) **2. + ((Svpatch/SNpatch) + cv) ** 2.)
-      horvpatch = max(horvpatch, 0.1)
+      horvpatch = max(horvpatch, min_horv)
     endif
 
 
@@ -791,14 +791,14 @@ subroutine surface
             upcu  = 0.5 * (u0(i,j,1) + u0(i+1,j,1)) + cu
             vpcv  = 0.5 * (v0(i,j,1) + v0(i,j+1,1)) + cv
             horv  = sqrt(upcu ** 2. + vpcv ** 2.)
-            horv  = max(horv, 0.1)
+            horv  = max(horv, min_horv)
             ra(i,j) = 1. / ( Cs(i,j) * horv )
           else
             if (lhetero) then
               ra(i,j) = 1. / ( Cs(i,j) * horvpatch(patchx,patchy) )
             else
               horvav  = sqrt(u0av(1) ** 2. + v0av(1) ** 2.)
-              horvav  = max(horvav, 0.1)
+              horvav  = max(horvav, min_horv)
               ra(i,j) = 1. / ( Cs(i,j) * horvav )
             endif
           end if
@@ -838,9 +838,9 @@ subroutine surface
           upcu   = 0.5 * (u0(i,j,1) + u0(i+1,j,1)) + cu
           vpcv   = 0.5 * (v0(i,j,1) + v0(i,j+1,1)) + cv
           horv   = sqrt(upcu ** 2. + vpcv ** 2.)
-          horv   = max(horv, 0.1)
+          horv   = max(horv, min_horv)
           horvav = sqrt(u0av(1) ** 2. + v0av(1) ** 2.)
-          horvav = max(horvav, 0.1)
+          horvav = max(horvav, min_horv)
 
           if(lhetero) then
             patchx = patchxnr(i)
@@ -911,7 +911,7 @@ subroutine surface
             upcu  = 0.5 * (u0(i,j,1) + u0(i+1,j,1)) + cu
             vpcv  = 0.5 * (v0(i,j,1) + v0(i,j+1,1)) + cv
             horv  = sqrt(upcu ** 2. + vpcv ** 2.)
-            horv  = max(horv, 0.1)
+            horv  = max(horv, min_horv)
 
             dudz  (i,j) = ustar(i,j) * phimzf / (fkar*zf(1))*(upcu/horv)
             dvdz  (i,j) = ustar(i,j) * phimzf / (fkar*zf(1))*(vpcv/horv)
@@ -950,9 +950,9 @@ subroutine surface
           upcu   = 0.5 * (u0(i,j,1) + u0(i+1,j,1)) + cu
           vpcv   = 0.5 * (v0(i,j,1) + v0(i,j+1,1)) + cv
           horv   = sqrt(upcu ** 2. + vpcv ** 2.)
-          horv   = max(horv, 0.1)
+          horv   = max(horv, min_horv)
           horvav = sqrt(u0av(1) ** 2. + v0av(1) ** 2.)
-          horvav = max(horvav, 0.1)
+          horvav = max(horvav, min_horv)
           if( isurf == 4) then
             if(lmostlocal) then
               ustar (i,j) = fkar * horv  / (log(zf(1) / z0m(i,j)) - psim(zf(1) / obl(i,j)) + psim(z0m(i,j) / obl(i,j)))
@@ -1141,7 +1141,7 @@ subroutine getobl
           upcu    =   0.5 * (u0(i,j,1) + u0(i+1,j,1)) + cu
           vpcv    =   0.5 * (v0(i,j,1) + v0(i,j+1,1)) + cv
           horv2   =   upcu ** 2. + vpcv ** 2.
-          horv2   =   max(horv2, 0.01)
+          horv2   =   max(horv2, min_horv**2)
 
           if(lhetero) then
             patchx = patchxnr(i)
@@ -1228,7 +1228,7 @@ subroutine getobl
       MPI_SUM, comm3d,mpierr)
 
       horvpatch = sqrt(((Supatch/SNpatch) + cu) **2. + ((Svpatch/SNpatch) + cv) ** 2.)
-      horvpatch = max(horvpatch, 0.1)
+      horvpatch = max(horvpatch, min_horv)
 
       thlpatch  = thlpatch / SNpatch
       qpatch    = qpatch   / SNpatch
@@ -1287,7 +1287,7 @@ subroutine getobl
     thv    = thl0av(1) * (1. + (rv/rd - 1.) * qt0av(1))
 
     horv2 = u0av(1)**2. + v0av(1)**2.
-    horv2 = max(horv2, 0.01)
+    horv2 = max(horv2, min_horv**2)
 
     Rib   = grav / thvs * zf(1) * (thv - thvs) / horv2
     if (Rib == 0) then

src/modsurfdata.f90

@@ -222,6 +222,7 @@ module modsurfdata
   real, allocatable :: thlflux (:,:)          !<  Kinematic temperature flux [K m/s]
   real, allocatable :: qtflux  (:,:)          !<  Kinematic specific humidity flux [kg/kg m/s]
   real, allocatable :: svflux  (:,:,:)        !<  Kinematic scalar flux [- m/s]
+  real              :: min_horv = .1          !<  minimum surface wind speed for drag calculation
 
   ! Surface gradients of prognostic variables
   real, allocatable :: dudz  (:,:)            !<  U-wind gradient in surface layer [1/s]