如何从每日 netcdf 计算每月数据

Question

I was able to open a netcdf file in R using the package "ncdf4" and extract rainfall (using ncvar_get). However I can't find information on how to calculate monthly values from daily values. Is this possible while staying in the netcdf format or should I convert it to something else? - but what format would keep the lat, long and time information associated with each rainfall value? I am not familiar with netcdfs so any help will be appreciated.

Answer 1

The exact command with cdo is

cdo monmean in.nc out.nc

EDIT:

As in the comment of JW you can also calculate the "average" instead of the "mean"

cdo monavg in.nc out.nc

The difference between the two is in the way they handle missing data. monmean ignores missing values, while monavg includes missing. Quoting the manual: "the mean of 1, 2, miss and 3 is (1+2+3)/3 = 2, whereas the average is (1+2+miss+3)/4 = miss/4 = miss" - see section 2.8 of the manual for more details.

Last, but not least, you can also use CDO commands directly from R using this package which provides R wrappers for CDO

Answer 2

I'm not an R programmer but I can recommend the command-line tools nco and/or cdo for these sorts of operations

http://nco.sourceforge.net/nco.html -- see averagers nces , ncra and ncwa . You probably want the record averager ncra

https://code.zmaw.de/projects/cdo -- see https://code.zmaw.de/projects/cdo/embedded/1.6.4/cdo_refcard.pdf for the commands for monthly, daily, seasonal, statistics

Answer 3

Quite late to the party, just wanted to add the following code in CDO can easily do the required job as well

cdo monsum dailydatafile.nc monfile.nc