7.3. Cell Methods
To describe the characteristic of a field that
is represented by cell values we define the
attribute of the variable. This
is a string attribute comprising a list
of blank-separated words of the form "
method". Each "
name: method" pair indicates that
for the axis identified by
name, the cell values
representing the field have been determined or
derived by the specified
method. The token name
can be a dimension of the variable, a scalar
coordinate variable, or a valid standard name. The
method should be selected from the list
Appendix E, Cell Methods,
Case is not
significant in the method name. Some methods
imply a change of units of
the variable, and this also is specified by
Appendix D, Dimensionless Vertical Coordinates.
It must be remembered that the
method applies only to the axis indicated, and
different methods may apply to other axes. If
a precipitation value in a longitude-latitude
cell is given the method maximum for these axes,
for instance, it means that it is the maximum
within these spatial cells, and does not imply
that it is also the maximum in time.
The default interpretation for variables that
have cells associated with their grid points,
but do not have the
specified, depends on whether the quantity is
extensive (which depends on the size of the cell)
or intensive (which doesn't). So, for example,
suppose the quantities "accumulated precipitation"
and "precipitation rate" each have a time axis
and that time intervals are associated with each
point on the time axis via a boundary variable. A
variable representing accumulated precipitation
is extensive in time and requires a time interval
to be completely specified. Hence its default
interpretation should be that the cell associated
with the grid point represents the time interval
over which the precipitation was accumulated. This
is indicated explicitly by setting the cell method
sum. A precipitation rate on the other hand is
intensive in time and could equally well represent
an instantaneous value or a mean value over the
time interval specified by the cell. However,
mean method is not specified then the
default interpretation for the quantity would be
instantaneous. The default method is indicated
explicity by setting the cell method to
Example 7.4. Methods applied to a timeseries
Consider 12-hourly timeseries of pressure, temperature and precipitation from a number of stations, where pressure is measured instantaneously, maximum temperature for the preceding 12 hours is recorded, and precipitation is accumulated in a rain gauge. For a period of 48 hours from 6 a.m. on 19 April 1998, the data is structured as follows:
dimensions: time = UNLIMITED; // (5 currently) station = 10; nv = 2; variables: float pressure(station,time); pressure:long_name = "pressure"; pressure:units = "kPa"; float maxtemp(station,time); maxtemp:long_name = "temperature"; maxtemp:units = "K"; maxtemp:cell_methods = "time: maximum"; float ppn(station,time); ppn:long_name = "depth of water-equivalent precipitation"; ppn:units = "mm"; double time(time); time:long_name = "time"; time:units = "h since 1998-4-19 6:0:0"; time:bounds = "time_bnds"; double time_bnds(time,nv); data: time = 0., 12., 24., 36., 48.; time_bnds = -12.,0., 0.,12., 12.,24., 24.,36., 36.,48.;
Note that in this example the
time axis values coincide with
the end of each interval. It is
sometimes desirable, however, to
use the midpoint of intervals as
coordinate values for variables
that are representative of an
interval. An application may
simply obtain the midpoint values
by making use of the boundary
If more than one cell method is to be indicated, they should be
arranged in the order they were applied. The left-most operation
is assumed to have been applied first. Suppose a quantity varies
in both longitude and time (dimensions lon and time) within each
gridbox. Values that represent the time-average of the zonal
maximum are labelled
cell_methods="lon: maximum time: mean",
i.e. find the largest value at each instant of time over all
longitudes, then average these maxima over time; values of the
zonal maximum of time-averages are labelled
cell_methods="time: mean lon: maximum". If the methods could
have been applied in any order without affecting the outcome,
they may be put in any order in the
If a data value is representative of variation over a combination
of axes, a single method should be prefixed by the names of all
the dimensions involved, whose order is immaterial. Dimensions
should be grouped in this way only if there is an essential
difference from treating them individually. For instance, the
standard deviation of topographic height within a
longitude-latitude gridbox would have
cell_methods="lat: lon: standard_deviation". This is not the
cell_methods="lon: standard_deviation lat: standard_deviation",
which would mean finding the standard deviation along each
parallel of latitude within the zonal extent of the gridbox,
and then the standard deviation of these values over latitude.
To indicate more precisely how the cell method was applied,
extra information may be included in parentheses () after the
identification of the method. This information includes
standardized and non-standarized parts. Currently the only
stardardized information is to provide the typical interval
between the original data values to which the method was applied,
in the situation where the present data values are statistically
representative of original data values which had a finer spacing.
The syntax is
(interval: value unit),
value is a numerical
is a string that can be recognized by UNIDATA's
Udunits package [UDUNITS].
unit does not have to be
dimensionally equivalent to the unit of the corresponding
dimension name, although it often will be. Recording the original
interval is particularly important for standard deviations.
For example, the standard deviation of daily values could be
cell_methods="time: standard_deviation (interval: 1 day)"
and of annual values
cell_methods="time: standard_deviation (interval: 1 year)".
If the cell method applies to a combination of axes, they may
have a common original interval
cell_methods="lat: lon: standard_deviation (interval: 10 km)".
Alternatively, they may have separate intervals, which are
matched to the names of axes by position
cell_methods="lat: lon: standard_deviation (interval: 0.1 degree_N interval: 0.2 degree_E)",
in which 0.1 degree applies to latitude and 0.2 degree to longitude.
If there is both standardized and non-standardized information,
the non-standardized follows the standardized information and
comment:. For instance, an area-weighted mean over
latitude could be indicated as
lat: mean (area-weighted)
lat: mean (interval: 1 degree_north comment: area-weighted).
A dimension of size one may be the result of "collapsing" an axis by some statistical operation, for instance by calculating a variance from time series data. We strongly recommend that dimensions of size one be retained and used to document the method and its domain.
Example 7.5. Surface air temperature variance
The variance of the diurnal cycle on 1 January 1990 has been calculated from hourly instantaneous surface air temperature measurments. The time dimension of size one has been retained.
dimensions: lat=90; lon=180; time=1; nv=2; variables: float TS_var(time,lat,lon); TS_var:long_name="surface air temperature variance" TS_var:units="K2"; TS_var:cell_methods="time: variance (of hourly instantaneous)"; float time(time); time:units="days since 1990-01-01 00:00:00"; time:bounds="time_bnds"; float time_bnds(time,nv); data: time=.5; time_bnds=0.,1.;
Notice that a parenthesized comment in the
cell_methods attribute provides
the nature of the samples used to calculate the variance.
The convention of specifying a cell method for a
standard_name rather than for
a dimension with a coordinate variable is to allow
one to provide an indication that a particular cell
method is relevant to the data without having to
provide a precise description of the corresponding cell.
There are two reasons for doing this.
If the cell coordinate range cannot be precisely defined. For example, the Levitus ocean climatology uses any data that exists. It is a time mean but the time range is not well defined, so cannot be stated.
For convenience, if the cell extends over all valid coordinates. This is permitted only for the standard names
latitude. Methods specified for these standard names are assumed to apply to the complete range of longitude and latitude respectively. If in addition the data variable has a dimension with a corresponding labeled axis that specifies a geographic region Section 6.1.1, “Geographic Regions”, the implied range of longitude and latitude is the valid range for each specified region.
We recommend that whenever possible cell bounds should be supplied by giving the variable a dimension of size one and attaching bounds to the associated coordinate variable.