Add documentation for M21C surface met forcing #794

doc/README.MetForcing_and_BCS.md

@@ -73,6 +73,11 @@ COMMONLY USED values for `MET_PATH`:
   MET_PATH : /discover/nobackup/projects/gmao/merra/iau/merra_land/MERRA2_land_forcing/ 
 ```
 
+#### MERRA-21C forcing 
+```
+  MET_PATH : [TBD]
+```
+
 Note: Used for SMAP Nature Run v05, v7.2, v8.1, v8.3, v9.1, v10.0 (before 1/1/2015).
 
 #### GEOS FP forcing with "seamless" file names, for use with MET_TAG=GEOS.fp.asm[__prec*] (__PREFERRED__)
@@ -177,6 +182,74 @@ COMMONLY USED values for `MET_TAG`:
    MET_TAG  : M2INT_400                                         ! stream 4 only
    ```
 
+#### MERRA-21C (M21C)
+
+ - IMPORTANT: M21C forcing option is experimental.  As of Jan 2025, M21C is in production.  Publication is planned for 2026/2027.
+
+ - All M21C options use native M21C "lfo" files for all surface met forcing fields *except* precipitation.
+
+ - Option 1a: 
+   Use corrected precip seen by the land w/in the M21C system (i.e., native M21C "lfo" files).
+   Precip is as corrected within the M21C system.  Closest to land-only M21C replay. 
+
+   Use 1152-by-721 lat/lon gridded forcing: 
+   ```
+   MET_TAG  : M21CCOR_cross                                     ! all streams
+   MET_TAG  : M21CCOR_jan98                                     ! stream 1 only
+   MET_TAG  : M21CCOR_jan08                                     ! stream 2 only
+   MET_TAG  : M21CCOR_jan18                                     ! stream 3 only
+   ```
+
+   Use c360 cube-sphere gridded forcing (works only if using matching c360 tile space for simulation): 
+   ```
+   MET_TAG  : M21CCSCOR_cross                                   ! all streams
+   MET_TAG  : M21CCSCOR_jan98                                   ! stream 1 only
+   MET_TAG  : M21CCSCOR_jan08                                   ! stream 2 only
+   MET_TAG  : M21CCSCOR_jan18                                   ! stream 3 only
+   ```
+
+ - Option 1b: 
+   Use corrected precip forcing [xxx] constructed in post-processing using M21C as background. 
+   Background precip is typically from M21C "int" data, but corrected precip is stored
+   in files that look like M21C "lfo" files.
+
+   Use 1152-by-721 lat/lon gridded forcing: 
+   ```
+   MET_TAG  : M21CCOR_cross__prec[xxx]                          ! all streams
+   MET_TAG  : M21CCOR_jan98__prec[xxx]                          ! stream 1 only
+   MET_TAG  : M21CCOR_jan08__prec[xxx]                          ! stream 2 only
+   MET_TAG  : M21CCOR_jan18__prec[xxx]                          ! stream 3 only
+   ```
+
+   Use c360 cube-sphere gridded forcing (works only if using matching c360 tile space for simulation): 
+   ```
+   MET_TAG  : M21CCSCOR_cross__prec[xxx]                        ! all streams
+   MET_TAG  : M21CCSCOR_jan98__prec[xxx]                        ! stream 1 only
+   MET_TAG  : M21CCSCOR_jan08__prec[xxx]                        ! stream 2 only
+   MET_TAG  : M21CCSCOR_jan18__prec[xxx]                        ! stream 3 only
+   ```
+
+ - Option 2:
+   Use uncorrected precip generated by the AGCM w/in the M21C system (i.e., native 
+   M21C "int" files).
+
+   Use 1152-by-721 lat/lon gridded forcing: 
+   ```
+   MET_TAG  : M21CINT_cross                                     ! all streams
+   MET_TAG  : M21CINT_jan98                                     ! stream 1 only
+   MET_TAG  : M21CINT_jan08                                     ! stream 2 only
+   MET_TAG  : M21CINT_jan18                                     ! stream 3 only
+   ```
+
+   Use c360 cube-sphere gridded forcing (works only if using matching c360 tile space for simulation): 
+   ```
+   MET_TAG  : M21CCSINT_cross                                   ! all streams
+   MET_TAG  : M21CCSINT_jan98                                   ! stream 1 only
+   MET_TAG  : M21CCSINT_jan08                                   ! stream 2 only
+   MET_TAG  : M21CCSINT_jan18                                   ! stream 3 only
+   ```
+
+
 #### RP-IT/FP-IT (d591)
   ```
   MET_TAG  : cross_d591_RPFPIT                                  ! all streams