Update ChangeLog

doc/ChangeSum

@@ -1,5 +1,6 @@
 Tag                   Who      Date  Summary
 ============================================================================================================================
+release-clm5.0.15    sacks 12/06/2018 Option for rain-to-snow to immediately run off in some regions
 release-clm5.0.14     erik 11/29/2018 Update cime and fix surface dataset for f05 1850 non-crop case
 release-clm5.0.13     erik 11/14/2018 Update externals with new CO2/presearo/rtm/mosart, add science_support, change testing
 release-clm5.0.12     erik 11/03/2018 New IC files for clm45/clm50 coupled cases, add 2010 compset

doc/release-clm5.0.ChangeLog

@@ -1,4 +1,178 @@
 ===============================================================
+Tag name:  release-clm5.0.15
+Originator(s):  sacks (Bill Sacks)
+Date: Thu Dec  6 10:14:30 MST 2018
+One-line Summary: Option for rain-to-snow to immediately run off in some regions
+
+Purpose of this version:
+------------------------
+
+Up until now: When repartition_rain_snow is .true. (which is the default
+for CLM5), rain that falls when the near-surface temperature is cold is
+converted to snow. This repartitioning was put in place for two
+reasons: (1) Downscaling to elevation classes: changing the balance
+between rain and snow for different elevation classes; (2) Correcting
+problems in CAM. However, members of the Land Ice Working Group would
+like to change this behavior so that, when CAM produces cold-temperature
+rain, this rain immediately runs off rather than being converted to
+snow. The purpose of this is to reduce the too-high SMB over portions of
+Greenland in CESM2 coupled runs (which results in part from CAM's
+generation of liquid precipitation despite very cold temperatures).
+
+This new behavior is implemented in a glacier region-specific manner,
+based on a new namelist flag, glacier_region_rain_to_snow_behavior. It
+is not at all ideal to make this aspect of the physics differ by region,
+but this has been requested by members of the Land Ice Working Group in
+order to address biases over Greenland while having minimal impact on
+the climate (so that the climate can stay very similar to that of the
+official CMIP6 runs). Note that, unlike other glacier region-specific
+behaviors, this one applies to all landunits, not just glaciers. This
+also seems a bit non-ideal, but we want the physics to be the same for
+all landunit types in a given region, and we also want this behavior to
+apply to vegetated columns because they are used for glacial
+inception (and we want this alternate behavior to apply to glacial
+inception, too, in order to decrease some instances of inception).
+
+The justification for this new physics is: In the case of (1) above: If
+CAM is generating rain at a given elevation / temperature, that doesn't
+necessarily imply that an equal water equivalent of snow would be
+generated at a higher elevation / lower temperature: indeed, in reality,
+there might not be any precipitation falling at that higher elevation /
+lower temperature. In the case of (2) above: There seem to be problems
+with CAM's microphysics that cause it to produce too much rain when
+temperatures are very cold; it seems (at least to some people) equally
+justifiable to throw this cold rain away (by sending it to the ocean as
+runoff) as it is to convert this cold rain to snow.
+
+Note: I don't think any changes are needed in
+BalanceCheck (unfortunately), since BalanceCheck currently uses the
+post-downscaling precipitation fluxes, and the pre-lnd2atm runoff
+fluxes (i.e., the new runoff flux isn't included in the terms in
+BalanceCheck, and it doesn't need to be because BalanceCheck uses the
+post-downscaling precipitation fluxes). (See also
+https://github.com/ESCOMP/ctsm/issues/201#issuecomment-444264954 .)
+
+CTSM Master Tag This Corresponds To: N/A
+
+   At least for now, we are bringing this to the release branch but not to
+   master. Here is an excerpt from the email explaining this rationale:
+
+   My question is: Should I do this:
+
+   (1) Just on a branch off of the release-clm5.0 branch, with no plan to
+       bring it back to release-clm5.0 or master
+
+   (2) On the release-clm5.0 branch, but not bring it back to master
+
+   (3) On the release-clm5.0 branch and master
+
+   My inclination right now is towards (2). I don't really like (1) because
+   this change will be wanted for a number of CMIP6-related experiments,
+   and it feels like it could be a pain to keep this branch up-to-date with
+   the evolving release-clm5.0 branch. However, the changes are going to be
+   a bit messy and having this be region-specific isn't really physically
+   justifiable (it's just being done that way to keep the climate as close
+   as possible to the official CMIP6 runs), so I'm not sure we really want
+   this on master. If we did anything on master, I could imagine having a
+   globally-applicable switch controlling this behavior, rather than having
+   it apply to just certain glacier regions.
+
+   Bette: The main reason I could see for bringing this to master is if you
+   imagine needing to use this new option in isotope-enabled runs, since
+   the isotope-enabled version of CTSM won't be on the release-clm5.0
+   branch. Do you think it's likely that you'd need to do that, or would
+   the isotope-enabled runs use the standard CMIP6 physics settings in this
+   respect?
+
+Summary of changes:
+-------------------
+
+Science changes since: release-clm5.0.14
+   None by default: just introduces a new option
+
+Software changes since: release-clm5.0.14
+   Introduces a new option, glacier_region_rain_to_snow_behavior,
+   controlling whether would-be rain-to-snow conversion ends up as snow
+   or instead ends up as liquid runoff.
+
+Changes to User Interface since: release-clm5.0.14
+   Introduces a new option, glacier_region_rain_to_snow_behavior,
+   controlling whether would-be rain-to-snow conversion ends up as snow
+   or instead ends up as liquid runoff.
+
+Testing:
+--------
+
+ [PASS means all tests PASS and OK means tests PASS other than expected fails.]
+
+  build-namelist tests:
+
+    cheyenne - ok
+
+    Tests pass, namelists differ as expected
+
+  unit-tests (components/clm/src):
+
+    cheyenne - pass
+    hobart --- not run
+
+  tools-tests (components/clm/test/tools):
+
+    cheyenne - not run
+    hobart --- not run
+
+  PTCLM testing (components/clm/tools/shared/PTCLM/test):
+
+     cheyenne - not run
+     hobart --- not run
+
+  regular tests (aux_clm):
+
+    cheyenne_intel ---- pass
+    cheyenne_gnu ------ pass
+    hobart_nag -------- pass
+    hobart_pgi -------- pass
+    hobart_intel ------ pass
+
+  regular tests (prealpha):
+
+    cheyenne_intel - not run
+    cheyenne_gnu --- not run
+    hobart_nag ----- not run
+
+  regular tests (prebeta):
+
+    cheyenne_intel - not run
+    cheyenne_gnu --- not run
+    hobart_nag ----- not run
+
+Summary of Answer changes:
+-------------------------
+
+Baseline version for comparison: release-clm5.0.14
+
+Changes answers relative to baseline: NO
+
+Detailed list of changes:
+------------------------
+
+Externals being used:
+
+   cism:   release-cesm2.0.04
+   rtm:    release-cesm2.0.02
+   mosart: release-cesm2.0.03
+   cime:   cime_cesm2_1_rel_05
+   FATES:  fates_s1.8.1_a3.0.0
+   PTCLM:  PTCLM2_180611
+
+CTSM Tag versions pulled over from master development branch: none
+
+Pull Requests that document the changes (include PR ids):
+   ESCOMP/ctsm#586 - Option for rain-to-snow to immediately run off in
+   some regions
+
+===============================================================
+===============================================================
 Tag name:  release-clm5.0.14
 Originator(s):  erik (Erik Kluzek)
 Date: Thu Nov 29 11:46:41 MST 2018