|CESM Research Tools: CLM4 in CESM1.0.4 User's Guide Documentation|
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When running "I" compsets with CLM you use the DATM model to give atmospheric forcing data to CLM. There are four ways to customize DATM:
DATM Main Namelist (datm_in)
DATM Stream Namelist (datm_atm_in)
DATM stream files
DATM template file (Tools/Templates.datm.cpl7.template)
Because, they aren't useful for work with CLM we will NOT discuss any of the options for the main DATM namelist. Use the DATM Users Guide at the link above to find details of that. For the streams namelist we will discuss three items:
mapalgo sets the spatial interpolation method to go from the
DATM input data to the output DATM model grid. The default is
bilinear. For CLM1PT we set it to nn to just
select the nearest neighbor. This saves time and we also had problems running the
interpolation for single-point mode.
taxmode is the time axis mode. For CLM we usually have it
set to cycle which means that once the end of the data is reached
it will start over at the beginning. The extend modes is used
have it use the last time-step of the forcing data once it reaches the end of forcing
data (or use the first time-step before it reaches where the forcing data starts).
See the warning below about the extend mode.
THE extend OPTION NEEDS TO BE USED WITH CAUTION! It is only invoked by default for the CLM1PT mode and is only intended for the supported urban datasets to extend the data for a single time-step. If you have the model run extensively through periods in this mode you will effectively be repeating that last time-step over that entire period. This means the output of your simulation will be worthless.
offset is the time offset in seconds to give to each stream
of data. Normally it is NOT used because the time-stamps for data is set correctly
for each stream of data. Note, the
offset may NEED to be
adjusted depending on the
taxmode described above, or it may
need to be adjusted to account for data that is time-stamped at the END of an
interval rather than the middle or beginning of interval. The
offset can is set in the stream file rather than on the
stream namelist. For data with a
taxmode method of
coszen the time-stamp needs to be for the beginning of the interval,
while for other data it should be the midpoint. The
offset can be
used to adjust the time-stamps to get the data to line up correctly.
tintalgo is the time interpolation algorithm. For CLM we usually
use one of three modes: coszen, nearest, or
linear. We use coszen for solar data,
nearest for precipitation data, and linear
for everything else. If your data is half-hourly or hourly, nearest
will work fine for everything. The coszen scaling is useful for
longer periods (three hours or more) to try to get the solar to match the cosine of
the solar zenith angle over that longer period of time. If you use
linear for longer intervals, the solar will cut out at night-time
anyway, and the straight line will be a poor approximation of the cosine of the
solar zenith angle of actual solar data. nearest likewise would
be bad for longer periods where it would be much higher than the actual values.
Note: For coszen the time-stamps of the data should correspond to the beginning of the interval the data is measured for. Either make sure the time-stamps on the datafiles is set this way, or use the
offsetdescribed above to set it.
Note: For nearest and linear the time-stamps of the data should correspond to the middle of the interval the data is measured for. Either make sure the time-stamps on the datafiles is set this way, or use the
offsetdescribed above to set it.
DATM_MODEoptions and what the above DATM settings are for each. This gives you examples of actual usage for the settings. We also describe in what ways you might want to customize them for your own case.
In CLM_QIAN mode the Qian dataset is used which has 6-hourly
solar and precipitation data, and 3-hourly for everything else.
The dataset is divided into those three data streams: solar, precipitation,
and everything else (temperature, pressure, humidity and wind). The time-stamps
of the data were also adjusted so that they are the beginning of the interval
for solar, and the middle for the other two. Because, of this the
offset is set to zero, and the
is: coszen, nearest, and
linear for the solar, precipitation and other data
taxmode is set to cycle
mapalgo is set to bilinear so that
the data is spatially interpolated from the input T62 grid to the grid the atmosphere
model is being run at.
Normally you wouldn't customize the CLM_QIAN settings, but you might replicate it's use for your own global data that had similar temporal characteristics.
In CLM1PT mode the model is assumed to have half-hourly or hourly data
for a single-point. For the supported datasets that is exactly what it has.
But, if you add your own data you may need to make adjustments accordingly.
CLM_USRDAT_NAME option you can easily extend this mode for your own
datasets that may be regional or even global and could be at different temporal
frequencies. If you do so you'll need to make adjustments to your DATM settings.
The dataset has all data in a single stream file. The time-stamps
of the data were also adjusted so that they are at the middle of the interval.
Because, of this the
offset is set to zero, and the
tintalgo is set to nearest.
taxmode is set to extend
mapalgo is set to nn so that
simply the nearest point is used.
If you are using your own data for this mode and it's not at least hourly you'll want to adjust the DATM settings for it. If the data is three or six hourly, you'll need to divide it up into separate streams like in CLM_QIAN mode which will require fairly extensive changes to the DATM namelist and streams files. For an example of doing this see Example 5-7.
In CPLHIST3HrWx mode the model is assumed to have 3-hourly for a global grid from
a previous CESM simulation. Like CLM_QIAN mode the data is divided into
three streams: one for precipitation, one for solar, and one for everything else.
The time-stamps for Coupler history files for CESM is at the end of the interval,
so the offset needs to be set in order to adjust the time-stamps to what it needs
to be for the
tintalgo settings. For precipitation
taxmode is set to nearest so the
offset is set to -5400 seconds so that
the ending time-step is adjusted by an hour and half to the middle of the interval.
taxmode is set to coszen so the
offset is set to -10800 seconds so that
the ending time-step is adjust by three hours to the beginning of the interval.
For everything else
taxmode is set to
linear so the
offset is set to
-5400 seconds so that the ending time-step is adjusted by an
hour and half to the middle of the interval.
Normally you wouldn't modify the DATM settings for this mode. However, if you
had data at a different frequency than 3-hours you would need to modify the
offset and possibly the
taxmode. The other
two things that you might modify would be the path to the data (which you can
change in the DATM template see Appendix A) or
the domain file for the resolution (which is currently hardwired to f09). For
data at a different input resolution you would need to change the domain file
in the streams file to use a domain file to the resolution that the data comes in