about transient CO2 file for CLMBGC simulation

[liliyao]

Hi, I am generating the transient CO2 file for my offline CLM5BGC simulation. I downloaded those CO2 files from here https://svn-ccsm-inputdata.cgd.ucar.edu/trunk/inputdata/atm/datm7/CO2/
as benchmarks, but I am confused by the variable "time" in these transient CO2 files. This time is the days since a specific date, for example "days since 1750-01-01 00:00:00"
This data is different in different CO2 files. For example:
In "fco2_datm_globalSSP5-8.5__simyr_2014-2501_CMIP6_c190506.nc", the variable "time" is the "days since 1850-01-01 00:00:00"
in "fco2_datm_globalSSP1-1.9_simyr_1750-2015_CMIP6_c190514.nc", the variable "time" is the "days since 1750-01-01 00:00:00"
So I am not sure how to determine this date when calculating the "time" variable for my transient CO2 file.

Any help will be much appreciated. Thanks!

 

[oleson]

I'm not sure what time period you are trying to generate your transient CO2 file for. However, the datm will be able to figure out what time slice to use in the file that corresponds to the current year of the model run. As long as the time attribute "days since..." is consistent with the time data. For example, if your data starts in 1850, you could use "days since 1850-01-01 00:00:00" and the first value of "time" would be "182.5", since the data is yearly.

 

[liliyao]

Thank you for the clarification!
I am curious why the first value of "time" would be "182.5" (half a year), instead of "0".

 

[oleson]

The yearly data is linearly interpolated to the time step of the model. Setting it to the middle of the year allows that to work properly.

 

[liliyao]

Got it. Thanks!