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In step 3.7 why do we need to extract dekad 1,2 and 3 into separate files when we want to calculate monthly SPI values? Aren't we getting 3 different SPI values for a single month in the process (each for decad 1, 11 and 21)?
I am not familiar what cdo is doing under the hood, so kind of confused with the process of converting the decad data into monthly data. Could you please elaborate on this?

We are trying to calculate SPI for an area with the help of your tutorial and we are getting the following error when running the following command :

Cmd : spi --periodicity monthly --scales 1 3 6 9 12 24 --calibration_start_year 1981 --calibration_end_year 2021 --netcdf_precip ./nc_files/tr_chirps-v2.0.monthly_3.nc --var_name_precip precip --output_file_base /datadrv/chirps_data/output_nc/tr_chirps --multiprocessing all --save_params /datadrv/chirps_data/tr_chirps_fitting.nc --overwrite

Error : ValueError: conflicting sizes for dimension 'lat': length 12 on the data but length 170 on coordinate 'lat'

The output of ncdump of the input file is as follow :

netcdf tr_chirps-v2.0.monthly_3 {
dimensions:
time = UNLIMITED ; // (486 currently)
lon = 434 ;
lat = 170 ;
variables:
float time(time) ;
time:standard_name = "time" ;
time:units = "days since 1980-1-1 0:0:0" ;
time:calendar = "gregorian" ;
time:axis = "T" ;
float lon(lon) ;
lon:standard_name = "longitude" ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
lon:axis = "X" ;
float lat(lat) ;
lat:standard_name = "latitude" ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
lat:axis = "Y" ;
float precip(time, lat, lon) ;
precip:standard_name = "convective precipitation rate" ;
precip:long_name = "Climate Hazards group InfraRed Precipitation with Stations" ;
precip:units = "mm" ;
precip:_FillValue = -9999.f ;
precip:missing_value = -9999.f ;
precip:time_step = "month" ;
precip:geostatial_lat_min = -50.f ;
precip:geostatial_lat_max = 50.f ;
precip:geostatial_lon_min = -180.f ;
precip:geostatial_lon_max = 180.f ;

We can't find out the problem. We also tried to do analysis with GeoTiff option and we got the same error. Any suggestions?

I have one question, do the SPI-3 and SPI-6 values obtained through this process of rolling window accumulation account for the seasonality of precipitation?