Merge pull request ESCOMP#2053 from slevis-lmwg/small_snocan_to_zero

Change small snocan to zero

doc/ChangeLog

@@ -1,4 +1,80 @@
 ===============================================================
+Tag name: ctsm5.1.dev141
+Originator(s): slevis (Samuel Levis,UCAR/TSS,303-665-1310)
+Date: Wed Sep 13 13:58:04 MDT 2023
+One-line Summary: Change small snocan to zero
+
+Purpose and description of changes
+----------------------------------
+
+ Issues #2041 and #2048 discuss and resolve a test failure in the ctsm5.2
+ branch. The failure goes away when we reset small snocan to zero.
+
+ Bill Sacks recommended merging this change in ctsm5.1 and then updating
+ the ctsm5.2 branch to the latest ctsm5.1.
+
+
+Significant changes to scientifically-supported configurations
+--------------------------------------------------------------
+
+Does this tag change answers significantly for any of the following physics configurations?
+(Details of any changes will be given in the "Answer changes" section below.)
+
+    [Put an [X] in the box for any configuration with significant answer changes.]
+
+[ ] clm5_1
+
+[ ] clm5_0
+
+[ ] ctsm5_0-nwp
+
+[ ] clm4_5
+
+
+Bugs fixed or introduced
+------------------------
+CTSM issues fixed (include CTSM Issue #):
+ Fixes #2041
+ Fixes #2048
+
+
+Testing summary:
+----------------
+
+ [PASS means all tests PASS; OK means tests PASS other than expected fails.]
+
+  regular tests (aux_clm: https://github.com/ESCOMP/CTSM/wiki/System-Testing-Guide#pre-merge-system-testing):
+
+    cheyenne ---- OK
+    izumi ------- OK
+
+
+Answer changes
+--------------
+
+Changes answers relative to baseline:
+ Yes
+
+  Summarize any changes to answers, i.e.,
+    - what code configurations: All
+    - what platforms/compilers: All
+    - nature of change: roundoff
+
+ The answer changes are expected to be roundoff-level because the code change
+ just truncates roundoff-level greater-than-zero states to exactly zero for
+ snocan that most likely needed to be zero anyway.
+
+ The answer changes grow to greater than roundoff, but the
+ cprnc.out file from a 20-year izumi test-suite case does not contain
+ differences of concerning magnitude.
+
+Other details
+-------------
+Pull Requests that document the changes (include PR ids):
+ https://github.com/ESCOMP/ctsm/pull/2053
+
+===============================================================
+===============================================================
 Tag name: ctsm5.1.dev140
 Originator(s): afoster (Adrianna Foster)
 Date: Tue Sep 12 14:47:06 MDT 2023

doc/ChangeSum

@@ -1,5 +1,6 @@
 Tag                   Who      Date  Summary
 ============================================================================================================================
+    ctsm5.1.dev141   slevis 09/13/2023 Change small snocan to zero
     ctsm5.1.dev140  afoster 09/12/2023 add lai_streams capability for FATES
     ctsm5.1.dev139   slevis 08/28/2023 Fix problems uncovered by nag -nan tests
     ctsm5.1.dev138   slevis 08/25/2023 Refactor max_patch_per_col and maxsoil_patches loops

src/biogeophys/CanopyFluxesMod.F90

@@ -47,6 +47,7 @@ module CanopyFluxesMod
   use EDTypesMod            , only : ed_site_type
   use SoilWaterRetentionCurveMod, only : soil_water_retention_curve_type
   use LunaMod               , only : Update_Photosynthesis_Capacity, Acc24_Climate_LUNA,Acc240_Climate_LUNA,Clear24_Climate_LUNA
+  use NumericsMod           , only : truncate_small_values
   !
   ! !PUBLIC TYPES:
   implicit none
@@ -422,6 +423,7 @@ subroutine CanopyFluxes(bounds,  num_exposedvegp, filter_exposedvegp,
     real(r8) :: uuc(bounds%begp:bounds%endp)             ! undercanopy windspeed
     real(r8) :: carea_stem                               ! cross-sectional area of stem
     real(r8) :: dlrad_leaf                               ! Downward longwave radition from leaf
+    real(r8) :: snocan_baseline(bounds%begp:bounds%endp)  ! baseline of snocan for use in truncate_small_values
 
     ! Indices for raw and rah
     integer, parameter :: above_canopy = 1         ! Above canopy
@@ -1599,6 +1601,9 @@ subroutine CanopyFluxes(bounds,  num_exposedvegp, filter_exposedvegp,
          cgrndl(p) = cgrndl(p) + forc_rho(c)*wtgq(p)*wtalq(p)*dqgdT(c)
          cgrnd(p)  = cgrnds(p) + cgrndl(p)*htvp(c)
 
+         ! save before updating
+         snocan_baseline(p) = snocan(p)
+
          ! Update dew accumulation (kg/m2)
          if (t_veg(p) > tfrz ) then ! above freezing, update accumulation in liqcan
             if ((qflx_evap_veg(p)-qflx_tran_veg(p))*dtime > liqcan(p)) then ! all liq evap
@@ -1616,6 +1621,12 @@ subroutine CanopyFluxes(bounds,  num_exposedvegp, filter_exposedvegp,
          end if
 
       end do
+
+      ! Remove snocan that got reduced by more than a factor of rel_epsilon
+      ! snocan < rel_epsilon * snocan_baseline will be set to zero
+      ! See NumericsMod for rel_epsilon value
+      call truncate_small_values(fn, filterp, begp, endp, &
+         snocan_baseline(begp:endp), snocan(begp:endp))
 
       if ( use_fates ) then