Climate Data Operators (CDO)

https://code.zmaw.de/projects/cdo/wiki/Cdo

Introduction and History • A single command with hundreds of operators

• CDO was inspired by NCO – providing a range of climate data-related operations through the command-line

• Designed to operate on netCDF3/4, GRIB1/2 primarily

• Much functionality can be used for any NetCDF/gridded data

• Very efficient for specific tasks

• Manages memory effectively

CDO: a command-line tool CDO is a single command-line tool. It is used as follows: $ cdo <operator> [options] <files>

Types of CDO Operator 1. File information and file operations 2. Selection and Comparison 3. Modification of metadata 4. Arithmetic operations 5. Statistical analysis 6. Regression and Interpolation 7. Vector and spectral Transformations 8. Formatted I/O 9. Climate indices

Many, many operators CDO breaks down its functionality into individual operators. There are over 650 at the time of writing this. For example: showstdname Show standard names sellonlatbox Select a longitude/latitude box setmissval Set a new missing value monadd Add monthly time series zonstd Zonal standard deviation eca_hd Heating degree days per time period

Getting help on an operator To see help just type: cdo -h <operator> $ cdo -h showparam NAME showformat, showcode, showname, showstdname, showlevel, showltype, showyear, showmon, showdate, showtime, showtimestamp - Show variables, levels or times SYNOPSIS <operator> ifile DESCRIPTION This module prints the format, variables, levels or times of the input dataset.

Operators: 1. File Information $ cdo infov ifile

This is an example result of a dataset with one 2D variable over 3 time steps: -1 : Date Time Varname Level Size Miss : Minimum Mean Maximum 1 : 1987−01−31 12:00:00 SST 0 2048 1361 : 232.77 266.65 305.31 2 : 1987−02−28 12:00:00 SST 0 2048 1361 : 233.64 267.11 307.15 3 : 1987−03−31 12:00:00 SST 0 2048 1361 : 225.31 267.52 307.67

Operators: 1. File Information $ cdo showtimestamp ggas2014121200_00-18.nc 2014-12-12T00:00:00 2014-12-12T06:00:00 2014-12-12T12:00:00 2014-12-12T18:00:00 $ cdo pardes ggas2014121200_00-18.nc -1 CI Sea-ice cover [(0 - 1)] -2 SSTK Sea surface temperature [K] -3 MSL Mean sea-level pressure [Pa] -4 TCC Total cloud cover [(0 - 1)] -5 U10 10 metre U wind component [m s**-1] -6 V10 10 metre V wind component [m s**-1] -7 SKT Skin temperature [K]

Operators: 1. File Information It is simple to see a summary of the numerical difference between 2 files: $ cdo diff tas_rcp45_2055_ann_95p_change.nc tas_rcp45_2055_ann_05p_change.nc Date Time Param Level Size Miss : S Z Max_Absdiff Max_Reldiff 1 : 2065-12-30 12:00:00 -1 0 64800 0 : T F 16.693 0.99997 1 of 1 records differ

Operators: 1. File Operations The full list of file operations covers: • Copying and concatenating • Merging fields/times • Splitting fields by variable/level/grid/time

Some examples follow

Operators: 1. File Operations To copy a file and convert the output to NetCDF: $ cdo −f nc copy ifile ofile.nc To merge all files along the time axis: $ cdo mergetime ifile1 ifile2 ifile3 ofile

Operators: 2. Selection Data can be selected by a number of different methods including: • Variable code, name or attribute • Levels • Time • Spatial region (latitude/longitude)

Operators: 2. Selection To select variables with ids: "SSTK" and "CI": $ cdo selname,SSTK,CI infile.nc outfile.nc

To select a lat/lon bounding box: $ cdo sellonlatbox,120,−90,20,−20 infile.nc outfile.nc

To select a date/time range: $ cdo seldate,2014-12-12T12:00:00, 2015-01-31T18:00:00 infile.nc outfile.nc

Operators: 3. Modification To set the time axis to 1987-01-16 12:00 with an increment of one month for each time step use: $ cdo settaxis,1987-01-16,12:00,1mon ifile ofile

To invert the latitudes of a 2D field from N-to-S to S-to-N use: $ cdo invertlat ifile ofile

Operators: 4. Arithmetic To calculate the square root for all field elements use: $ cdo sqrt ifile ofile

To sum all input fields with the constant -273.15 use: $ cdo addc,-273.15 ifile ofile

Operators: 5. Statistics To compute the zonal mean of all input fields use: $ cdo zonmean ifile ofile

Assume an input dataset has monthly means over several years. In order to compute seasonal means from monthly means the first two months must be skipped: $ cdo timselmean,3,2 ifile ofile

Operators: 5. Statistics Multi-year daily running percentile values: • To calculate a percentile the minimum and

maximum bounds must be known. Hence the signature of the command is:

$ cdo ydrunpctdl,p,nts infile minfile maxfile outfile

This operator writes running percentile values for each day of year in infile to outfile. The minimum and maximum bounds are provided in minfile and maxfile.

Operators: 5. Statistics So to calculate a multi-year daily running percentile values you need multiple steps: $ cdo ydrunmin,5 ifile minfile $ cdo ydrunmax,5 ifile maxfile $ cdo ydrunpctl,90,5 ifile minfile maxfile ofile

Operators: 6. Interpolation To remap all fields bilinear to a T42 Gaussian grid type: $ cdo remapbil,t42grid ifile ofile

To interpolate hybrid model level data to pressure levels of 925, 850, 500 and 200 hPa use: $ cdo ml2pl,92500,85000,50000,20000 ifile ofile

Operators: 9. Climate Indices To get the largest number of consecutive dry days of a time series of daily precipitation amounts use: $ cdo eca_cdd rrfile ofile

Where rrfile is a time series of daily precipitation amounts RR, then counted is the largest number of consecutive days where RR is less than 1 mm.

Operators: 9. Climate Indices

To get the number of tropical nights of a time series of daily minimum temperatures use: $ cdo eca_tr tnfile ofile

Where tnfile is a time series of daily minimum temperatures TN, then counted is the number of days where TN > T. The number T is an optional parameter with default T = 20°c.

What else can CDO do? That was a whistle-stop tour of some of the functionality provided by CDO. There are other useful features worth knowing about.

CDO Command-line switches CDO has many generic command-line switches that can be used with (most) operators. Here is a selection:

-a Generate an absolute time axis -f <format> Specify format ("grb", "nc", "nc4" etc) -m <val> Set the default missing value -Q Sort netCDF variable names -r Generate a relative time axis -s Silent mode -V Version of CDO -v Verbose - print extra details. -z zip Deflate compression of netCDF4 vars.

Combining operators

All operators with a fixed number of input streams and one output stream can pipe the result directly to an other operator. The operator must begin with "-", in order to combine it with others. This can improve the performance by: • Reducing unnecessary disk I/O • Parallel processing

Combining operators: example 1

For example, we can replace: $ cdo timavg ifile1 tmp1 $ cdo dayavg ifile2 tmp2 $ cdo sub tmp2 tmp1 ofile $ rm tmp1 tmp2

With... $ cdo sub -dayavg ifile2 -timavg ifile1 ofile

Combining operators: example 2 The three commands to calculate a multi-year daily running percentile values you need multiple steps: $ cdo ydrunmin,5 ifile minfile $ cdo ydrunmax,5 ifile maxfile $ cdo ydrunpctl,90,5 ifile minfile maxfile ofile

…can be replaced with a single command: $ cdo ydrunpctl,90,5 ifile −ydrunmin ifile −ydrunmax ifile ofile

Further information

CDO Home page: https://code.zmaw.de/projects/cdo CDO Tutorial: https://code.zmaw.de/projects/cdo/wiki/Tutorial CDO Documentation: https://code.zmaw.de/projects/cdo/wiki#Documentation