Fix phenology issue with heavy snow: In the Arctic, following a heavy snow winter, snow-melt may last past the summer solstice. This likely causes no PFT onset stage even when a short non-snow season occurs.

Add a flag for switching plant growth onset gdd summation end option in phenology. The switch is for extending onset stage up to start of offset in Fall, rather than to summer solstice by default.

Author: fmyuan

components/elm/bld/namelist_files/namelist_definition.xml

@@ -1801,6 +1801,15 @@ Specifies the method for decomposing CLM grids across processors.
     pflotran file prefix.
 </entry>
 
+<!-- ========================================================================================  -->
+<!-- Namelist options related to plant phenology                                               -->
+<!-- ========================================================================================  -->
+
+<entry id="onset_gdd_extension" type="logical" category="default_settings"
+    group="elm_inparm" valid_values="">
+    Runtime flag to turn on/off stress_deciduous PFT's onset gdd summation end from summer solstice to offset starting.
+</entry>
+
 <!-- ========================================================================================  -->
 <!-- Namelist options related to Variably Saturated Flow Model (VSFM)                          -->
 <!-- ========================================================================================  -->

components/elm/src/biogeochem/PhenologyMod.F90

@@ -659,6 +659,7 @@ subroutine CNSeasonDecidPhenology (num_soilp, filter_soilp, cnstate_vars)
       !$acc routine seq
     use shr_const_mod   , only: SHR_CONST_TKFRZ, SHR_CONST_PI
     use elm_varcon      , only: secspday
+    use elm_varctl      , only: onset_gdd_extension
     !
     ! !ARGUMENTS:
     integer                  , intent(in)    :: num_soilp       ! number of soil patches in filter
@@ -905,11 +906,27 @@ subroutine CNSeasonDecidPhenology (num_soilp, filter_soilp, cnstate_vars)
                ! In that case, it will take until the next winter solstice
                ! before the growing degree-day summation starts again.
 
-               if (onset_gddflag(p) == 1._r8 .and. ws_flag == 0._r8) then
-                  onset_gddflag(p) = 0._r8
-                  onset_gdd(p) = 0._r8
+               ! A note by F-M Yuan (2020-02-21): when having heavy snow winter,
+               ! snow melting may be lasting into July in some Arctic area (esp. East Russia)
+               ! So better to let ONSET stage up to 'offset' starting criteria date, if this feature is on.
+
+               if (onset_gdd_extension) then
+                  if (onset_gddflag(p) == 1._r8 .and. ws_flag == 0._r8 &
+                    .and. dayl(g) < PhenolParamsInst%crit_dayl) then
+                     onset_gddflag(p) = 0._r8
+                     onset_gdd(p) = 0._r8
+
+                  end if
+
+               else
+
+                  if (onset_gddflag(p) == 1._r8 .and. ws_flag == 0._r8) then
+                     onset_gddflag(p) = 0._r8
+                     onset_gdd(p) = 0._r8
+                  end if
                end if
 
+
                ! if the gdd flag is set, and if the soil is above freezing
                ! then accumulate growing degree days for onset trigger
 

components/elm/src/main/controlMod.F90

@@ -285,6 +285,9 @@ subroutine control_init( )
     ! bgc & pflotran interface
     namelist /elm_inparm/ use_elm_interface, use_elm_bgc, use_pflotran
 
+    ! onset_gdd_extension in plant phenology
+    namelist /elm_inparm/ onset_gdd_extension
+
     namelist /elm_inparm/ use_dynroot
 
     namelist /elm_inparm/ use_var_soil_thick, use_lake_wat_storage
@@ -891,6 +894,9 @@ subroutine control_spmd()
     call mpi_bcast (use_elm_bgc, 1, MPI_LOGICAL, 0, mpicom, ier)
     call mpi_bcast (use_pflotran, 1, MPI_LOGICAL, 0, mpicom, ier)
 
+    ! phenology
+    call mpi_bcast (onset_gdd_extension, 1, MPI_LOGICAL, 0, mpicom, ier)
+
     !cpl_bypass
      call mpi_bcast (metdata_type,   len(metdata_type),   MPI_CHARACTER, 0, mpicom, ier)
      call mpi_bcast (metdata_bypass, len(metdata_bypass), MPI_CHARACTER, 0, mpicom, ier)

components/elm/src/main/elm_varctl.F90

@@ -462,6 +462,15 @@ module elm_varctl
   !$acc declare copyin(initth_pf2clm)
   !$acc declare copyin(pf_clmnstep0 )
 
+  !-----------------------------------------------------------------------
+  ! onset_gdd summation ending switch in phenology
+  ! by default, onset_gdd will reset to zero passing summer-solstice
+  ! This may cause non-growing if heavy winter snowing which results in
+  ! snow-melting passing summer-solstice in arctic.
+  ! When this switches, it will end by offset critical daylength (i.e. vegetation senescence starts normally in Fall)
+  logical, public :: onset_gdd_extension = .false.
+  !$acc declare create(onset_gdd_extension)
+
   ! cpl_bypass
    character(len=fname_len), public :: metdata_type   = ' '    ! metdata type for CPL_BYPASS mode
    character(len=fname_len), public :: metdata_bypass = ' '    ! met data directory for CPL_BYPASS mode (site, qian, cru_ncep)