MITgcm · amc2507 · Aug 8, 2025 · Aug 8, 2025 · Aug 13, 2025 · Aug 13, 2025
diff --git a/doc/phys_pkgs/seaice.rst b/doc/phys_pkgs/seaice.rst
@@ -192,6 +192,9 @@ General flags and parameters
   +------------------------------------+------------------------------+-------------------------------------------------------------------------+
   | :varlink:`SEAICEaEVPalphaMin`      | 5.0                          | aEVP lower limit of alpha and beta (non-dim.), see :cite:`kimmritz:16`  |
   +------------------------------------+------------------------------+-------------------------------------------------------------------------+
+  | :varlink:`SEAICE_evpAreaReg`       | -1.0                         | a minimun ice fraction for regularizations of denomU/V in EVP;          |
+  |                                    |                              | off by default (-1), 1.E-5 is a useful value for high-res simulation    |
+  +------------------------------------+------------------------------+-------------------------------------------------------------------------+
   | :varlink:`SEAICE_elasticParm`      | 0.33333333                   | EVP parameter :math:`E_0` (non-dim.), sets relaxation timescale         |
   |                                    |                              | :varlink:`SEAICE_evpTauRelax` =                                         |
   |                                    |                              | :varlink:`SEAICE_elasticParm` * :varlink:`SEAICE_deltaTdyn`             |

diff --git a/pkg/seaice/SEAICE_PARAMS.h b/pkg/seaice/SEAICE_PARAMS.h
@@ -372,6 +372,10 @@ C     SEAICEaEVPcStar    :: multiple of stabilty factor: alpha*beta=cstar*gamma
 C     SEAICEaEVPalphaMin :: lower limit of alpha and beta, regularisation
 C                           to prevent singularities of system matrix,
 C                           e.g. when ice concentration is too low.
+C     SEAICE_evpAreaReg  :: Specifies a minimun ice fraction for the purposes
+C                           of regularizations in the calculation of denomU/V,
+C                           to enhance the stability of EVP; off by default,
+C                           turn on with a sensible value, e.g. 1e-5
 C     SEAICEnonLinTol    :: non-linear tolerance parameter for implicit solvers
 C     JFNKgamma_lin_min/max :: tolerance parameters for linear JFNK solver
 C     JFNKres_t          :: tolerance parameter for FGMRES residual
@@ -560,7 +564,7 @@ C
       _RL SEAICE_evpAlpha, SEAICE_evpBeta
       _RL SEAICE_evpDampC, SEAICE_zetaMin, SEAICE_zetaMaxFac
       _RL SEAICEaEVPcoeff, SEAICEaEVPcStar, SEAICEaEVPalphaMin
-      _RL SEAICEpresH0
+      _RL SEAICE_evpAreaReg, SEAICEpresH0
       _RL SEAICEdiffKhArea, SEAICEdiffKhHeff, SEAICEdiffKhSnow
       _RL SEAICEdiffKhSalt
       _RL SEAICE_tauAreaObsRelax
@@ -576,7 +580,7 @@ C
      &    SEAICE_evpAlpha, SEAICE_evpBeta,
      &    SEAICEaEVPcoeff, SEAICEaEVPcStar, SEAICEaEVPalphaMin,
      &    SEAICE_evpDampC, SEAICE_zetaMin, SEAICE_zetaMaxFac,
-     &    SEAICEpresH0,
+     &    SEAICE_evpAreaReg, SEAICEpresH0,
      &    SEAICE_monFreq, SEAICE_dumpFreq, SEAICE_taveFreq,
      &    SEAICE_initialHEFF,
      &    SEAICE_rhoAir, SEAICE_rhoIce, SEAICE_rhoSnow, ICE2WATR,

diff --git a/pkg/seaice/seaice_evp.F b/pkg/seaice/seaice_evp.F
@@ -883,12 +883,23 @@ SUBROUTINE SEAICE_EVP( myTime, myIter, myThid )
             tmp        = evpStarFac*recip_deltaT
             betaFacP1V = betaFacV + tmp
             betaFacP1U = betaFacU + tmp
-            denomU = seaiceMassU(i,j,bi,bj)*betaFacP1U
+            IF ( SEAICE_evpAreaReg .GT. 0. _d 0 ) THEN
+             denomU = MAX(seaiceMassU(i,j,bi,bj),SEAICE_evpAreaReg
+     &           * seaice_rhoIce)*betaFacP1U
+     &           + 0.5 _d 0*( DWATN(i,j,bi,bj) + DWATN(i-1,j,bi,bj) )
+     &           * COSWAT * MAX(areaW(i,j,bi,bj),SEAICE_evpAreaReg)
+             denomV = MAX(seaiceMassV(i,j,bi,bj),SEAICE_evpAreaReg
+     &           * seaice_rhoIce)*betaFacP1V
+     &           + 0.5 _d 0*( DWATN(i,j,bi,bj) + DWATN(i,j-1,bi,bj) )
+     &           * COSWAT * MAX(areaS(i,j,bi,bj),SEAICE_evpAreaReg)
+            ELSE
+             denomU = seaiceMassU(i,j,bi,bj)*betaFacP1U
      &           + 0.5 _d 0*( DWATN(i,j,bi,bj) + DWATN(i-1,j,bi,bj) )
      &           * COSWAT * areaW(i,j,bi,bj)
-            denomV = seaiceMassV(i,j,bi,bj)*betaFacP1V
+             denomV = seaiceMassV(i,j,bi,bj)*betaFacP1V
      &           + 0.5 _d 0*( DWATN(i,j,bi,bj) + DWATN(i,j-1,bi,bj) )
      &           * COSWAT * areaS(i,j,bi,bj)
+            ENDIF
 #ifdef SEAICE_ALLOW_BOTTOMDRAG
             denomU = denomU + areaW(i,j,bi,bj)
      &           * 0.5 _d 0*( CbotC(i,j,bi,bj) + CbotC(i-1,j,bi,bj) )

diff --git a/pkg/seaice/seaice_init_fixed.F b/pkg/seaice/seaice_init_fixed.F
@@ -471,10 +471,14 @@ SUBROUTINE SEAICE_INIT_FIXED( myThid )
       ENDIF
 
       CALL EXCH_UV_XY_RL( coastRoughU, coastRoughV, .FALSE., myThid )
-      CALL WRITE_FLD_XY_RL( 'coastRoughUnormalized',
-     &     ' ', coastRoughU, -1, myThid )
-      CALL WRITE_FLD_XY_RL( 'coastRoughVnormalized',
-     &     ' ', coastRoughV, -1, myThid )
+C     Write coastRoughU/V fields only we are actually going to use them
+C     to reduce unnecessary output
+      IF ( SEAICEsideDrag .NE. 0. _d 0 ) THEN
+       CALL WRITE_FLD_XY_RL( 'coastRoughUnormalized',
+     &      ' ', coastRoughU, -1, myThid )
+       CALL WRITE_FLD_XY_RL( 'coastRoughVnormalized',
+     &      ' ', coastRoughV, -1, myThid )
+      ENDIF
 #endif /* SEAICE_ALLOW_SIDEDRAG */
 
 #ifdef ALLOW_SHELFICE

diff --git a/pkg/seaice/seaice_readparms.F b/pkg/seaice/seaice_readparms.F
@@ -136,6 +136,7 @@ SUBROUTINE SEAICE_READPARMS( myThid )
      & SEAICE_evpDampC, SEAICEnEVPstarSteps,
      & SEAICE_evpAlpha, SEAICE_evpBeta,
      & SEAICEaEVPcoeff, SEAICEaEVPcStar, SEAICEaEVPalphaMin,
+     & SEAICE_evpAreaReg,
      & SEAICE_zetaMin, SEAICE_zetaMaxFac, SEAICEusePicardAsPrecon,
      & SEAICEuseLSRflex, SEAICEuseKrylov, SEAICEuseJFNK,
      & SEAICEnonLinIterMax, SEAICElinearIterMax, SEAICEnonLinTol,
@@ -373,6 +374,7 @@ SUBROUTINE SEAICE_READPARMS( myThid )
       SEAICEaEVPcoeff    = UNSET_RL
       SEAICEaEVPcStar    = UNSET_RL
       SEAICEaEVPalphaMin = UNSET_RL
+      SEAICE_evpAreaReg  = -1.
       SEAICE_initialHEFF = ZERO
 #ifdef SEAICE_ITD
 C     Coefficients used to calculate sea ice thickness category limits

diff --git a/pkg/seaice/seaice_summary.F b/pkg/seaice/seaice_summary.F
@@ -306,6 +306,8 @@ SUBROUTINE SEAICE_SUMMARY( myThid )
      & 'SEAICEnEVPstarSteps =', ' /* num. of EVP* steps */')
        CALL WRITE_0D_L ( SEAICEuseEVPpickup, INDEX_NONE,
      & 'SEAICEuseEVPpickup=', ' /* start EVP solver with EVP pickup*/')
+       CALL WRITE_0D_RL ( SEAICE_evpAreaReg, INDEX_NONE,
+     & 'SEAICE_evpAreaReg   =', ' /* EVP DenomU/V regularizations*/')
 #endif /* SEAICE_ALLOW_EVP */
       ENDIF
 

diff --git a/verification/seaice_obcs/input.regDenom/data.seaice b/verification/seaice_obcs/input.regDenom/data.seaice
@@ -0,0 +1,27 @@
+# SEAICE parameters
+ &SEAICE_PARM01
+#- seaice dynamics params:
+# turn on aEVP with some sensible parameters
+ SEAICEaEVPcoeff     = 0.5,
+ SEAICEnEVPstarSteps = 500,
+# test regularisation of EVP (helps especially for high resolution)
+ SEAICE_evpAreaReg = 0.00001,
+#- seaice state & transport params:
+ SEAICEadvScheme    = 7,
+# add small diffKh to test diffusion with multi-dim advect.
+ SEAICEdiffKhArea   = 20.,
+ SEAICE_saltFrac    = 0.3,
+#- seaice thermodyn params:
+#SEAICE_multDim     = 7, # this is the default
+#- constant seawater freezing point:
+ SEAICE_tempFrz0    = -1.96,
+ SEAICE_dTempFrz_dS = 0.,
+# use the new standard way of specifying ocean-ice turb. flux coeff:
+# defaults:
+# SEAICE_mcPheePiston= MCPHEE_TAPER_FAC*STANTON_NUMBER*USTAR_BASE=8.75e4
+# and SEAICE_frazilFrac = 1.
+ SEAICE_mcPheePiston= 0.00004,
+ SEAICE_frazilFrac  = 0.3,
+#- seaice I/O params:
+ SEAICEwriteState   = .TRUE.,
+ &
diff --git a/verification/seaice_obcs/input.regDenom/eedata.mth b/verification/seaice_obcs/input.regDenom/eedata.mth
@@ -0,0 +1,12 @@
+# Example "eedata" file  for multi-threaded test
+#   (copy "eedata.mth" to "eedata" to use it)
+# Lines beginning "#" are comments
+# nTx - No. threads per process in X
+# nTy - No. threads per process in Y
+ &EEPARMS
+ nTx=2,
+ nTy=1,
+#debugMode=.TRUE.,
+ &
+# Note: Some systems use & as the namelist terminator (as shown here).
+#       Other systems use a / character.