1.5.11. Running with anomaly forcing
Because performing fully coupled climate simulations is computationally expensive, an alternate method of running land-only simulations forced by future climate projections was developed for CTSM called “anomaly forcing.” The anomaly forcing method uses a previously-completed, fully-coupled simulation to create monthly anomalies, relative to the present day, of near-surface atmospheric states and fluxes. These anomalies, representing the evolution of future climate projections, are applied to a repeating cycle of present day atmospheric forcing data, either as an additive (for states) or multiplicative (for fluxes) quantity. Thus, high-frequency variability is obtained from the present day atmospheric forcing data, while the long-term evolution of the climate is determined by the anomaly forcing dataset.
Anomaly climate forcings are automatically enabled for ISSP
compsets (e.g., ISSP585
). After the namelist has been created, CaseDocs/datm.streams.xml
will have an entry like this pointing to the anomaly forcing file being used:
<stream_info name="Anomaly.Forcing.cmip6.ssp585">
...
<datafiles>
<file>/glade/campaign/cesm/cesmdata/inputdata/atm/datm7/anomaly_forcing/CMIP6-SSP5-8.5/af.allvars.CESM.SSP5-8.5.2015-2100_c20220628.nc</file>
</datafiles>
<datavars>
<var>huss Sa_shum_af</var>
<var>pr Faxa_prec_af</var>
<var>ps Sa_pbot_af</var>
<var>rlds Faxa_lwdn_af</var>
<var>rsds Faxa_swdn_af</var>
<var>tas Sa_tbot_af</var>
<var>uas Sa_u_af</var>
<var>vas Sa_v_af</var>
</datavars>
...
</stream_info>
To use alternative data, add a user_nl_datm_streams
namelist file to your case with contents like so:
Anomaly.Forcing.cmip6.ssp585:datafiles = /path/to/your/datafile
Anomaly.Forcing.cmip6.ssp585:meshfile = /path/to/meshfile/for/your/datafile
! List of Data types to use
! Remove the variables you do NOT want to include in the anomaly forcing:
! pr is precipitation
! tas is temperature
! huss is humidity
! uas and vas are U and V winds
! rsds is solare
! rlds is LW down
Anomaly.Forcing.cmip6.ssp585:datavars = pr Faxa_prec_af, \
tas Sa_tbot_af, \
ps Sa_pbot_af, \
huss Sa_shum_af, \
uas Sa_u_af, \
vas Sa_v_af, \
rsds Faxa_swdn_af, \
rlds Faxa_lwdn_af
To instead disable anomaly forcing in ISSP
compsets, the following can be added to the user_nl_datm
file:
anomaly_forcing = 'none'
Note that other inputs are also set automatically for ISSP
compsets, including CO2 (co2tseries
), ozone (preso3
), N deposition (presndep
), and aerosols (presaero
).
The first and last years over which the present-day (base) climate should cycle are set through the DATM_YR_START
and DATM_YR_END
XML variables.