--- MITgcm/pkg/generic_advdiff/gad_dst3fl_adv_x.F	2005/08/19 22:19:35	1.7
+++ MITgcm/pkg/generic_advdiff/gad_dst3fl_adv_x.F	2005/10/18 16:03:55	1.8
@@ -1,4 +1,4 @@
-C $Header: /home/ubuntu/mnt/e9_copy/MITgcm/pkg/generic_advdiff/gad_dst3fl_adv_x.F,v 1.7 2005/08/19 22:19:35 heimbach Exp $
+C $Header: /home/ubuntu/mnt/e9_copy/MITgcm/pkg/generic_advdiff/gad_dst3fl_adv_x.F,v 1.8 2005/10/18 16:03:55 jmc Exp $
 C $Name:  $
 
 #include "GAD_OPTIONS.h"
@@ -36,11 +36,17 @@
       INTEGER i,j
       _RL Rjm,Rj,Rjp,cfl,d0,d1,psiP,psiM,thetaP,thetaM
       _RL uFld
+      _RL thetaMax
+      PARAMETER( thetaMax = 1.D+20 )
+
+C- jmc: an alternative would be to compute directly psiM*Rj & psiP*Rj
+C       (if Rj*Rjm < 0 => psiP*Rj = 0 , elsef Rj > 0 ... , else  ... )
+C       with no need to compute thetaM (might be easier to differentiate)
 
       DO j=1-Oly,sNy+Oly
-       uT(1-Olx,j)=0.D0
-       uT(2-Olx,j)=0.D0
-       uT(sNx+Olx,j)=0.D0
+       uT(1-Olx,j)=0. _d 0
+       uT(2-Olx,j)=0. _d 0
+       uT(sNx+Olx,j)=0. _d 0
        DO i=1-Olx+2,sNx+Olx-1
         Rjp=(tracer(i+1,j)-tracer( i ,j))*maskLocW(i+1,j)
         Rj =(tracer( i ,j)-tracer(i-1,j))*maskLocW( i ,j)
@@ -50,20 +56,37 @@
         uFld = uTrans(i,j)*recip_dyG(i,j,bi,bj)
      &       *recip_drF(k)*recip_hFacW(i,j,k,bi,bj)
         cfl=abs(uFld*deltaTloc*recip_dxC(i,j,bi,bj))
-        d0=(2.D0-cfl)*(1.D0-cfl)*oneSixth
-        d1=(1.D0-cfl*cfl)*oneSixth
+        d0=(2. _d 0 -cfl)*(1. _d 0 -cfl)*oneSixth
+        d1=(1. _d 0 -cfl*cfl)*oneSixth
+
+C-      the old version: can produce overflow, division by zero,
+c       and is wrong for tracer with low concentration:
+c       thetaP=Rjm/(1.D-20+Rj)
+c       thetaM=Rjp/(1.D-20+Rj)
+C-      the right expression, but not bounded:
 c       thetaP=0.D0
-c       IF (Rj.NE.0.D0) thetaP=Rjm/Rj
-        thetaP=Rjm/(1.D-20+Rj)
-        psiP=d0+d1*thetaP
-        psiP=max(0.D0, min(min(1.D0,psiP),
-     &       (1.D0-cfl)/(1.D-20+cfl)*thetaP))
-        thetaM=Rjp/(1.D-20+Rj)
 c       thetaM=0.D0
+c       IF (Rj.NE.0.D0) thetaP=Rjm/Rj
 c       IF (Rj.NE.0.D0) thetaM=Rjp/Rj
+C-      prevent |thetaP,M| to reach too big value:
+        IF ( ABS(Rj)*thetaMax .LE. ABS(Rjm) ) THEN
+          thetaP=SIGN(thetaMax,Rjm*Rj)
+        ELSE
+          thetaP=Rjm/Rj
+        ENDIF
+        IF ( ABS(Rj)*thetaMax .LE. ABS(Rjp) ) THEN
+          thetaM=SIGN(thetaMax,Rjp*Rj)
+        ELSE
+          thetaM=Rjp/Rj
+        ENDIF
+
+        psiP=d0+d1*thetaP
+        psiP=MAX(0. _d 0, MIN(MIN(1. _d 0,psiP),
+     &                        thetaP*(1. _d 0 -cfl)/(cfl+1. _d -20) ))
         psiM=d0+d1*thetaM
-        psiM=max(0.D0, min(min(1.D0,psiM),
-     &       (1.D0-cfl)/(1.D-20+cfl)*thetaM))
+        psiM=MAX(0. _d 0, MIN(MIN(1. _d 0,psiM),
+     &                        thetaM*(1. _d 0 -cfl)/(cfl+1. _d -20) ))
+
         uT(i,j)=
      &   0.5*(uTrans(i,j)+abs(uTrans(i,j)))
      &      *( Tracer(i-1,j) + psiP*Rj )