Extract one-day data from monthly combined CMAQ output files


I am trying to get the initial condition for my CMAQ simulation (my simulation started on 01/01/2015 and the initial condition needed to be 12/31/2014). I searched the CMAS data warehouse and found there were hourly CMAQ output files for 2014. However, they were combined for every month. Is there any way to extract the output only for 12/31/2014 from the combined file (ACONC_CMAQv5.2_201412.nc) to serve as the initial condition in my simulation?


You could use M3Tools program m3cple

But why bother: the files use a selective direct access programming interface, not a sequential one. By analogy, if you had 100-year-long 1900-1999 hourly meteorology for the US, it takes exactly the same amount of time to read the meteorology for July 27, 1987 15:00:00 that it does for Jan 1, 1900 0:00:00…
In each case, the system goes directly to the data requested and reads it.

Thanks for your reply. If I understood it correctly, I could use the combined file (ACONC_CMAQv5.2_201412.nc) directly as the initial condition file. I actually tried it before posting this thread, but it failed. The error is:


There is only one layer in ACONC_CMAQv5.2_201412.nc but CMAQ may need a 35-layer dimension (based on my observation of previous initial condition files I used). Is ACONC.nc the right file to use as the initial condition file?


No, this is not the right file to use to create CMAQ initial conditions for two reasons:

  • The concentrations in this file are a subset of post-processed and aggregated CMAQ concentrations meant for comparison to observations, not the full set of original CMAQ species

  • As you noted, this file contains concentrations for the surface layer only while you need a 3D concentration file to create initial conditions

Thanks for explaining it! I will generate the initial condition by myself.