NetCDF spatially merging to global data

Question

Currently I use global precipitation (ppt) and potential evapotranspiration (pet) data to calculate SPEI. As I have limited hardware resources, I divided global ppt and ppt data into 32 parts, each file covering 45x45deg and contains 756 data - monthly from 1958-2020 (tile01.nc, tile02.nc, ... tile32.nc)

• For example to do this, I use cdo sellonlatbox,-180,-135,45,90 in.nc out.nc or ncks -d lat,45.,90. -d lon,-180.,-135. in.nc -O out.nc
• As required by SPEI script, I reorder and fixed the dimension from time,lat,lon to lat,lon,time using ncpdq and ncks.
• From the SPEI output, I got the output in lat,lon,time. So I did reorder the dimension so that it becomes time,lat,lon using ncpdq.
• Each tile SPEI output covering 45x45deg and contains 756 SPEI data - monthly from 1958-2020

Finally I need to merge all the output together (32 files) into one, so I will get the global SPEI output. I have try to use cdo mergegrid but the result is not what I expected. Is there any command from cdo or nco to solve this problem that has function similar to gdal_merge if we are dealing with geoTIFF format?

Below is the example of the SPEI output

UPDATE

I managed to merge all the data using cdo collgrid as suggested by Robert below. And here's the result:

Answer 1

I believe you want to use CDO's distgrid and collgrid methods for this.

First, run this:

cdo distgrid,4,8 in.nc obase

That will split the files up the way you want them.

Then do the post-processing necessary on the files.

Then use collgrid to merge the files:

cdo collgrid obase* out.nc

Potentially you can just use collgrid in place of mergegrid in your present work flow, depending on how you have split the files up.