Gridded Datasets

Gridded data files organise data in blocks with each block representing a common period and field, with the rows representing latitudes and the columns longitudes. Compulsory tags are:

  • cpt:T= time period for which the data are valid (in ISO8601 format )
  • cpt:nrow= number of latitudes
  • cpt:ncol= number of longitudes
  • cpt:row=Y (representing latitudes)
  • cpt:col=X (representing longitudes)

Optional tags are:

  • cpt:field= abbreviated name of the field variable
  • cpt:model= name of model
  • cpt:M= ensemble member number
  • cpt:S= start date (in ISO8601 format ) (indicating when the forecast was initialised, cf. the compulsory cpt:T tag, which gives the target season of the forecast)
  • cpt:units= units in which the data are stored
  • cpt:missing= missing value flag

Any additional tags are ignored. For further details, see the section on CPT tags .

Immediately after this tag line there is a line listing all the longitudes from west to east. The longitudes must be between -180 and 360, with negative values representing the western hemisphere. There must be ncol longitudes. The data follow in the subsequent nrow rows, with the first value representing the latitude. Latitudes must be between 90 and -90, with negative values representing the southern hemisphere. The data can be from north to south or from south to north.

If there is only one field and there are no lagged fields (e.g., more than one season per year) then the data for the subsequent date should follow. All tag values should remain the same or be omitted, except for cpt:T= time (and cpt:S= time , if present). For example, if the data are SST anomalies in Celsius, the file might look something like the following: 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:nfields=1
cpt:field=ssta, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.40977  -1.40228  -1.01278  -1.03197
-61.25  -3.65581  -3.18779  -2.28418  -1.45435
-63.75  -3.54459  -3.04635  -2.07969  -1.32451
cpt:T=1980-01
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.22299  -0.68736   0.27909  -0.42638
-61.25  -1.13313  -0.56869  -1.02204  -1.09111
-63.75  -1.40550  -1.10838  -0.37565   0.88614

It is possible for some of the dates to be missing, although there are some restrictions. For further details see the section on missing values .

If there are lagged fields the additional lags can either be included as subsequent dates, or listed as a separate field. For example, in the example below data for January and February of each year are included. Since CPT is looking for one value per year, the February data will be treated as a lagged field. In effect the February data are an extra field, but since they have a different date rather than being a different meteorological parameter they are handled as a lagged field. Because the February values are recognised as lagged fields, there are only two years of data available for the training period in the example. CPT will identify the number of years in the file, recognising that there is one lagged field. The latitudes and longitudes for different lagged fields do not have to be the same, but they do need to be consistent from year to year. 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:nfields=1
cpt:field=ssta, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.40977  -1.40228  -1.01278  -1.03197
-61.25  -3.65581  -3.18779  -2.28418  -1.45435
-63.75  -3.54459  -3.04635  -2.07969  -1.32451
cpt:T=1979-02
        -18.75    -16.25    -13.75    -11.25
-58.75  -1.34563  -0.23456  -0.98857  -0.37327
-61.25  -2.23454  -3.21124  -1.99463  -2.44437
-63.75  -4.00043  -3.05441  -2.12645  -1.40078
cpt:T=1980-01
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.22299  -0.68736   0.27909  -0.42638
-61.25  -1.13313  -0.56869  -1.02204  -1.09111
-63.75  -1.40550  -1.10838  -0.37565   0.88614
cpt:T=1980-02
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.34256  -0.23346   0.26537  -0.67278
-61.25  -1.07636  -0.96765  -0.85677  -1.14531
-63.75  -1.34254  -1.04553  -0.25831   0.82646

In the example above, cpt:nfields could have been set as 2, in which case February would have been treated as a separate field rather than as a lagged field. The results are exactly the same either way, but the results for the different months will be accessed slightly differently.

If there is a second field the first data for the second field can come either immediately after the first data for the first field ("unstacked"), or all the data for the second field can come after all the data for the first field have been listed ("stacked"). If there are lagged fields, and the data for the second field are unstacked, the second field should come before the first data for any lagged field. The latitudes and longitudes for the two fields do not have to be identical. The missing value flag can be unique to each field.

Thus a file with two unstacked fields may look something like the following: 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:nfields=2
cpt:field=ssta, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.40977  -1.40228  -1.01278  -1.03197
-61.25  -3.65581  -3.18779  -2.28418  -1.45435
-63.75  -3.54459  -3.04635  -2.07969  -1.32451
cpt:field=mslp, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.2   1000.6   1000.9
-61.25   1002.8   1003.2   1001.0
-63.75   1004.4   1002.6   1002.5
cpt:field=ssta, cpt:T=1980-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.22299  -0.68736   0.27909  -0.42638
-61.25  -1.13313  -0.56869  -1.02204  -1.09111
-63.75  -1.40550  -1.10838  -0.37565   0.88614
cpt:field=mslp, cpt:T=1980-01, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.9   1000.6   1000.5
-61.25   1003.7   1001.5   1000.7
-63.75   1001.3   1001.6   1000.7

If the fields are stacked the same data would be formatted as follows: 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:nfields=2
cpt:field=ssta, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.40977  -1.40228  -1.01278  -1.03197
-61.25  -3.65581  -3.18779  -2.28418  -1.45435
-63.75  -3.54459  -3.04635  -2.07969  -1.32451
cpt:T=1980-01
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.22299  -0.68736   0.27909  -0.42638
-61.25  -1.13313  -0.56869  -1.02204  -1.09111
-63.75  -1.40550  -1.10838  -0.37565   0.88614
cpt:field=mslp, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.2   1000.6   1000.9
-61.25   1002.8   1003.2   1001.0
-63.75   1004.4   1002.6   1002.5
cpt:T=1980-01
        -18.75    -16.25    -13.75
-58.75   1001.9   1000.6   1000.5
-61.25   1003.7   1001.5   1000.7
-63.75   1001.3   1001.6   1000.7

No specific indication is given to CPT as to whether the fields are stacked or unstacked. CPT will identify the stacking automatically.

A file with 2 fields, each with one lagged field must be ordered as follows: 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:nfields=2
cpt:field=ssta, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.40977  -1.40228  -1.01278  -1.03197
-61.25  -3.65581  -3.18779  -2.28418  -1.45435
-63.75  -3.54459  -3.04635  -2.07969  -1.32451
cpt:field=mslp, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.2   1000.6   1000.9
-61.25   1002.8   1003.2   1001.0
-63.75   1004.4   1002.6   1002.5
cpt:field=ssta, cpt:T=1979-02, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -1.34563  -0.23456  -0.98857  -0.37327
-61.25  -2.23454  -3.21124  -1.99463  -2.44437
-63.75  -4.00043  -3.05441  -2.12645  -1.40078
cpt:field=mslp, cpt:T=1979-02, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.0   1000.7   1000.7
-61.25   1002.0   1003.9   1001.6
-63.75   1004.4   1002.6   1002.7
cpt:field=ssta, cpt:T=1980-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.22299  -0.68736   0.27909  -0.42638
-61.25  -1.13313  -0.56869  -1.02204  -1.09111
-63.75  -1.40550  -1.10838  -0.37565   0.88614
cpt:field=mslp, cpt:T=1980-01, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.9   1000.6   1000.5
-61.25   1003.7   1001.5   1000.7
-63.75   1001.3   1001.6   1000.7
cpt:field=ssta, cpt:T=1980-02, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.34256  -0.23346   0.26537  -0.67278
-61.25  -1.07636  -0.96765  -0.85677  -1.14531
-63.75  -1.34254  -1.04553  -0.25831   0.82646
cpt:field=mslp, cpt:T=1980-02, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-9999
        -18.75    -16.25    -13.75
-58.75   1001.7   1000.6   1000.1
-61.25   1003.7   1001.8   1000.5
-63.75   1001.5   1001.5   1000.6

Alternatively the fields can be stacked as follows: 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:nfields=2
cpt:T=1979-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.40977  -1.40228  -1.01278  -1.03197
-61.25  -3.65581  -3.18779  -2.28418  -1.45435
-63.75  -3.54459  -3.04635  -2.07969  -1.32451
cpt:T=1979-02
        -18.75    -16.25    -13.75    -11.25
-58.75  -1.34563  -0.23456  -0.98857  -0.37327
-61.25  -2.23454  -3.21124  -1.99463  -2.44437
-63.75  -4.00043  -3.05441  -2.12645  -1.40078
cpt:T=1980-01
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.22299  -0.68736   0.27909  -0.42638
-61.25  -1.13313  -0.56869  -1.02204  -1.09111
-63.75  -1.40550  -1.10838  -0.37565   0.88614
cpt:T=1980-02
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.34256  -0.23346   0.26537  -0.67278
-61.25  -1.07636  -0.96765  -0.85677  -1.14531
-63.75  -1.34254  -1.04553  -0.25831   0.82646
cpt:field=mslp, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.2   1000.6   1000.9
-61.25   1002.8   1003.2   1001.0
-63.75   1004.4   1002.6   1002.5
cpt:T=1979-02
        -18.75    -16.25    -13.75
-58.75   1001.0   1000.7   1000.7
-61.25   1002.0   1003.9   1001.6
-63.75   1004.4   1002.6   1002.7
cpt:T=1980-01
        -18.75    -16.25    -13.75
-58.75   1001.9   1000.6   1000.5
-61.25   1003.7   1001.5   1000.7
-63.75   1001.3   1001.6   1000.7
cpt:T=1980-02
        -18.75    -16.25    -13.75
-58.75   1001.7   1000.6   1000.1
-61.25   1003.7   1001.8   1000.5
-63.75   1001.5   1001.5   1000.6

If lagged fields are included and there are multiple fields, all fields must have the same number of lagged fields. If this restriction is undesirable, then the lagged fields could be listed as separate fields, For example, if SST anomaly data for January and February are wanted, but MSLP data are wanted only for one month then the file could be constructed as follows: 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:nfields=3
cpt:field=ssta, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.40977  -1.40228  -1.01278  -1.03197
-61.25  -3.65581  -3.18779  -2.28418  -1.45435
-63.75  -3.54459  -3.04635  -2.07969  -1.32451
cpt:field=ssta, cpt:T=1979-02, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -1.34563  -0.23456  -0.98857  -0.37327
-61.25  -2.23454  -3.21124  -1.99463  -2.44437
-63.75  -4.00043  -3.05441  -2.12645  -1.40078
cpt:field=mslp, cpt:T=1979-03/04, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.2   1000.6   1000.9
-61.25   1002.8   1003.2   1001.0
-63.75   1004.4   1002.6   1002.5
cpt:field=ssta, cpt:T=1980-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.22299  -0.68736   0.27909  -0.42638
-61.25  -1.13313  -0.56869  -1.02204  -1.09111
-63.75  -1.40550  -1.10838  -0.37565   0.88614
cpt:field=ssta, cpt:T=1980-02, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.34256  -0.23346   0.26537  -0.67278
-61.25  -1.07636  -0.96765  -0.85677  -1.14531
-63.75  -1.34254  -1.04553  -0.25831   0.82646
cpt:field=mslp, cpt:T=1980-03/04, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.9   1000.6   1000.5
-61.25   1003.7   1001.5   1000.7
-63.75   1001.3   1001.6   1000.7

or as a stacked field: 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:nfields=3
cpt:field=ssta, cpt:T=1979-01, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.40977  -1.40228  -1.01278  -1.03197
-61.25  -3.65581  -3.18779  -2.28418  -1.45435
-63.75  -3.54459  -3.04635  -2.07969  -1.32451
cpt:T=1980-01
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.22299  -0.68736   0.27909  -0.42638
-61.25  -1.13313  -0.56869  -1.02204  -1.09111
-63.75  -1.40550  -1.10838  -0.37565   0.88614
cpt:field=ssta, cpt:T=1979-02, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=C, cpt:missing=-9999
        -18.75    -16.25    -13.75    -11.25
-58.75  -1.34563  -0.23456  -0.98857  -0.37327
-61.25  -2.23454  -3.21124  -1.99463  -2.44437
-63.75  -4.00043  -3.05441  -2.12645  -1.40078
cpt:T=1980-02
        -18.75    -16.25    -13.75    -11.25
-58.75  -2.34256  -0.23346   0.26537  -0.67278
-61.25  -1.07636  -0.96765  -0.85677  -1.14531
-63.75  -1.34254  -1.04553  -0.25831   0.82646
cpt:field=mslp, cpt:T=1979-03/04, cpt:nrow=3, cpt:ncol=3, cpt:row=Y, cpt:col=X, cpt:units=mb, cpt:missing=-10
        -18.75    -16.25    -13.75
-58.75   1001.2   1000.6   1000.9
-61.25   1002.8   1003.2   1001.0
-63.75   1004.4   1002.6   1002.5
cpt:T=1980-03/04
        -18.75    -16.25    -13.75
-58.75   1001.9   1000.6   1000.5
-61.25   1003.7   1001.5   1000.7
-63.75   1001.3   1001.6   1000.7

Note that the dates and the averaging periods for the fields do not have to be the same.

It is possible for some of the dates to be missing, although there are some restrictions. For further details see the section on missing values .

To verify gridded forecasts in CPT using the Probabilistic Forecast Verification (PFV) option the X file needs to contain the tag cpt:ncat=3, indicating that the file contains probabilities for three categories. The first block of data should then contain the probabilities for category 1, which is the below-normal category. The category number should be indicated using the cpt:C tag, and, optionally, the climatological probability for the category, by using the cpt:clim_prob tag. The forecasts for categories 2 and 3 should then follow immediately, as if they were unstacked fields. For example, a gridded probabilistic forecast input file might look something like the following: 

xmlns:cpt=http://iri.columbia.edu/CPT/v10/
cpt:ncats=3
cpt:field=prec, cpt:C=1, cpt:clim_prob=0.333333333333, cpt:T=1991-10/12, cpt:nrow=3, cpt:ncol=4, cpt:row=Y, cpt:col=X, cpt:units=%, cpt:missing=-9999
        32.25    32.75    33.25    33.75
5.75       50       50       45       40
5.25       40       35       35       25
4.75       35       25       25       15
cpt:C=2, cpt:clim_prob=0.333333333334
        32.25    32.75    33.25    33.75
5.75       35       30       35       40
5.25       35       40       45       35
4.75       40       35       30       25
cpt:C=3, cpt:clim_prob=0.333333333333
        32.25    32.75    33.25    33.75
5.75       15       20       20       20
5.25       25       25       20       40
4.75       25       40       45       60
cpt:C=1, cpt:clim_prob=0.333333333333, cpt:T=1992-10/12
...

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