Well not necessarily always.
The datm guide here explains two common cases (this is for CPLHIST but CLMNCEP operates the same way):
Caseroot variable definitons for DATM2.1 in CESM2
www.cesm.ucar.edu
DATM_CPLHIST_YR_ALIGN | -999 | integer | run_component_datm | env_run.xml |
- Valid Values []
- Description Simulation year corresponding to DATM_CPLHIST_YR_START (only used
when DATM_MODE is CPLHIST). A common usage is to set this to
RUN_STARTDATE. With this setting, the forcing in the first year of
the run will be the forcing of year DATM_CPLHIST_YR_START. Another
use case is to align the calendar of transient forcing with the
model calendar. For example, setting
DATM_CPLHIST_YR_ALIGN=DATM_CPLHIST_YR_START will lead to the
forcing calendar being the same as the model calendar. The forcing
for a given model year would be the forcing of the same year. This
would be appropriate in transient runs where the model calendar is
setup to span the same year range as the forcing data.
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Another case would be where one doesn't have forcing data for the full duration of a desired transient run.
For example, our land-only transient simulations run 1850-2014, but we only have forcing data for 1901-2014.
So, we've decided we want to use the first 20 years of our forcing data (1901-1920) for simulation years 1850-1900. Then we want to use our 1901-2014 forcing data for simulation years 1901-2014.
For the first part of the simulation we set:
RUN_STARTDATE=1850-01-01
DATM_CLMNCEP_YR_ALIGN=1901
DATM_CLMNCEP_YR_START=1901
DATM_CLMNCEP_YR_END=1920
So here we are telling the datm that we want model year 1901 to (eventually) line up with year 1901 of the forcing data.
The datm will actually figure out that it needs to start with 1910 forcing data for the first year of simulation (1850), eg.,
Model year 1850 1859 1860 1861 1881 1901
Atm year 1910 1919 1920 1901 1901 1901
We run the simulation through the end of 1900 with this setup, then we switch to using atm years that correspond to model years for simulation years 1901-2014. RUN_STARTDATE stays the same because we are doing a restart (CONTINUE_RUN - True).
DATM_CLMNCEP_YR_ALIGN=1901
DATM_CLMNCEP_YR_START=1901
DATM_CLMNCEP_YR_END=2014