Have datm send ndep to surface components

[billsacks]

(Copying this from ESMCI/cime#3549, originally opened by @ekluzek.)

Both CLM and the ocean model need Nitrogen deposition to be sent in. We've handled this by having it sent in by CAM when WACCM is run -- but having each surface component handle prescribed datasets themselves. This means that OCN and LND could have inconsistent datasets. And it also leads to certain problems when surface components don't know if ndep is provided by the ATM model or not (see for example: ESCOMP/CTSM#946). Since, ndep physically is something in the atmosphere, it makes the most sense if ndep would ALWAYS be provided by the ATM component just like prescribed aerosols are now.

The CAM issue that relates to this is here:

ESCOMP/CAM#104

Once CAM is able to send ndep, the surface components could stop having to prescribe it and just always rely on what is being sent from that ATM component.

[billsacks]

This would also be very good so that we could test the sending of ndep in I compsets, avoiding issues like ESCOMP/CTSM#1233

[mvertens]

@ekluzek - I have added the stream capability in datm and cam so that it can be removed from ctsm and pop for ingesting ndep data. However, I am confused regarding the following setting in namelist_defaults_ctsm.xml.
The new implementation has the following new scenarios for ndep forcing in datm:

<entry id="DATM_PRESNDEP">
   <type>char</type>
   <valid_values>none,clim_1850,clim_2000,clim_2010,hist,SSP1-2.6,SSP2-4.5,SSP3-7.0,SSP5-3.4,SSP5-8.5</valid_values>
   <default_value>clim_2000</default_value>
   <values match="last">
     <value compset="^1850_"   >clim_1850</value>
     <value compset="^2000_"   >clim_2000</value>
     <value compset="^2010_"   >clim_2010</value>
     <value compset="^SSP126_" >SSP1-2.6</value>
     <value compset="^SSP245_" >SSP2-4.5</value>
     <value compset="^SSP370_" >SSP3-7.0</value>
     <value compset="^SSP585_" >SSP5-8.5</value>
     <value compset="^HIST_"   >hist</value>
     <value compset="^20TR_"   >hist</value>
     <value compset="_DATM%CPLHIST">cplhist</value>
     <value compset="_DATM.*_DICE.*_DOCN.*_DROF">none</value>
   </values>
 </entry_id>

I am a bit confused as to the following settings in ctsm. I think that the default setting should take care of this (since it will be clim_2000). What compset using these sim_year and sim_year_range settings? Do you agree with my assumption?

<stream_year_first_ndep use_cn=".true."   sim_year="1000" >2000</stream_year_first_ndep>
<stream_year_last_ndep  use_cn=".true."   sim_year="1000" >2000</stream_year_last_ndep>

<stream_year_first_ndep use_cn=".true."   sim_year="constant" sim_year_range="1000-1002" >2000</stream_year_first_ndep>
<stream_year_last_ndep  use_cn=".true."   sim_year="constant" sim_year_range="1000-1002" >2000</stream_year_last_ndep>

<stream_year_first_ndep use_cn=".true."   sim_year="constant" sim_year_range="1000-1004" >2000</stream_year_first_ndep>
<stream_year_last_ndep  use_cn=".true."   sim_year="constant" sim_year_range="1000-1004" >2000</stream_year_last_ndep>

[ekluzek]

@mvertens the 1000 to 1004 case was a specific test case that was setup to test some of the transient changes. I think we can drop it, at least here. But, I need to do some checking on that. So only worry about this, if I come back and say there is some reason to keep that test case.

[mvertens]

@ekluzek - thanks for the clarification. I think even if you need it - the default would be 2000 I think so that should be fine.

[billsacks]

@mvertens has done this.

[ekluzek]

I just want to comment that we'll want to pull this into CTSM and MOM and probably remove our current mechanisms for ndep in those models. As part of that process we will want to compare answers and make sure it's identical or at least similar. So we should add a task for this in CTSM to start taking advantage of this.