Climatological statistics may be derived from corresponding portions of the annual cycle in a set of years, e.g., the average January temperatures in the climatology of 1961-1990, where the values are derived by averaging the 30 Januarys from the separate years. Portions of the climatological cycle are specified by references to dates within the calendar year. However, a calendar year is not a well-defined unit of time, because it differs between leap years and other years, and among calendars. Nonetheless for practical purposes we wish to compare statistics for months or seasons from different calendars, and to make climatologies from a mixture of leap years and other years. Hence we provide special conventions for indicating dates within the climatological year. Climatological statistics may also be derived from corresponding portions of a range of days, for instance the average temperature for each hour of the average day in April 1997. In addition the two concepts may be used at once, for instance to indicate not April 1997, but the average April of the five years 1995-1999.
Climatological variables have a climatological
time axis. Like an ordinary time axis, a
climatological time axis may have a dimension
of unity (for example, a variable containing the
January average temperatures for 1961-1990), but
often it will have several elements (for example,
a climatological time axis with a dimension of
12 for the climatological average temperatures in
each month for 1961-1990, a dimension of 3 for the
January mean temperatures for the three decades
1961-1970, 1971-1980, 1981-1990, or a dimension of
24 for the hours of an average day). Intervals of
climatological time are conceptually different
from ordinary time intervals; a given interval
of climatological time represents a set
of subintervals which are not necessarily
contiguous. To indicate this difference, a
climatological time coordinate variable does
not have a
bounds
attribute. Instead, it has a
climatology
attribute, which names a variable
with dimensions (n,2), n being the dimension of
the climatological time axis. Using the units and
calendar of the time coordinate variable, element
(i,0) of the climatology variable specifies the
beginning of the first subinterval and element
(i,1) the end of the last subinterval used to
evaluate the climatological statistics with index
i in the time dimension. The time coordinates
should be values that are representative
of the climatological time intervals, such
that an application which does not recognise
climatological time will nonetheless be able to
make a reasonable interpretation.
The COARDS standard offers limited support
for climatological time. For compatibility with
COARDS, time coordinates should also be recognised
as climatological if they have a
units
attribute
of time-units relative to midnight on 1 January
in year 0 i.e. since 0-1-1 in udunits syntax , and
provided they refer to the real-world calendar. We
do not recommend this convention because (a)
it does not provide any information about the
intervals used to compute the climatology, and
(b) there is no standard for how dates since year
1 will be encoded with units having a reference
time in year 0, since this year does not exist;
consequently there may be inconsistencies among
software packages in the interpretation of the
time coordinates. Year 0 may be a valid year in
non-real-world calendars, and therefore cannot be
used to signal climatological time in such cases.
A climatological axis may use different
statistical methods to represent variation among
years, within years and within days. For example,
the average January temperature in a climatology
is obtained by averaging both within years and
over years. This is different from the average
January-maximum temperature and the maximum
January-average temperature. For the former,
we first calculate the maximum temperature in
each January, then average these maxima; for the
latter, we first calculate the average temperature
in each January, then find the largest one. As
usual, the statistical operations are recorded
in the
cell_methods
attribute, which may have
two or three entries for the climatological
time dimension.
Valid values of the
cell_methods attribute must be
in one of the forms from the following list. The
intervals over which various statistical methods
are applied are determined by decomposing the date
and time specifications of the climatological time
bounds of a cell, as recorded in the variable
named by the
climatology
attribute. (The date
and time specifications must be calculated from
the time coordinates expressed in units of "time
interval since reference date and time".) In the
descriptions that follow we use the abbreviations
y,
m,
d,
H,
M,
and S for year, month, day, hour,
minute, and second respectively. The suffix 0
indicates the earlier bound and 1 the latter.
within years time: method2 over yearsmethod1 is applied to the time intervals (mdHMS0-mdHMS1) within individual years and method2 is applied over the range of years (y0-y1).
within days time: method2 over daysmethod1 is applied to the time intervals (HMS0-HMS1) within individual days and method2 is applied over the days in the interval (ymd0-ymd1).
within days time: method2 over days time: method3 over yearsmethod1 is applied to the time intervals (HMS0-HMS1) within individual days and method2 is applied over the days in the interval (md0-md1), and method3 is applied over the range of years (y0-y1).
The methods which can be specified are those
listed in Appendix E, Cell Methods and each entry in the
cell_methods
attribute may also, as usual, contain
non-standardised information in parentheses after
the method. For instance, a mean over ENSO years
might be indicated by
"time: mean over years (ENSO years)".
When considering intervals within years, if the earlier climatological time bound is later in the year than the later climatological time bound, it implies that the time intervals for the individual years run from each year across January 1 into the next year e.g. DJF intervals run from December 1 0:00 to March 1 0:00. Analogous situations arise for daily intervals running across midnight from one day to the next.
When considering intervals within days, if the earlier time of day is equal to the later time of day, then the method is applied to a full 24 hour day.
We have tried to make the examples in this section easier to understand by translating all time coordinate values to date and time formats. This is not currently valid CDL syntax.
Example 7.6. Climatological seasons
This example shows the metadata for the average seasonal-minimum temperature for the four standard climatological seasons MAM JJA SON DJF, made from data for March 1960 to February 1991.
dimensions:
time=4;
nv=2;
variables:
float temperature(time,lat,lon);
temperature:long_name="surface air temperature";
temperature:cell_methods="time: minimum within years time: mean over years";
temperature:units="K";
double time(time);
time:climatology="climatology_bounds";
time:units="days since 1960-1-1";
double climatology_bounds(time,nv);
data: // time coordinates translated to date/time format
time="1960-4-16", "1960-7-16", "1960-10-16", "1961-1-16" ;
climatology_bounds="1960-3-1", "1990-6-1",
"1960-6-1", "1990-9-1",
"1960-9-1", "1990-12-1",
"1960-12-1", "1991-3-1" ;
Example 7.7. Decadal averages for January
Average January precipitation totals are given for each of the decades 1961-1970, 1971-1980, 1981-1990.
dimensions:
time=3;
nv=2;
variables:
float precipitation(time,lat,lon);
precipitation:long_name="precipitation amount";
precipitation:cell_methods="time: sum within years time: mean over years";
precipitation:units="kg m-2";
double time(time);
time:climatology="climatology_bounds";
time:units="days since 1901-1-1";
double climatology_bounds(time,nv);
data: // time coordinates translated to date/time format
time="1965-1-15", "1975-1-15", "1985-1-15" ;
climatology_bounds="1961-1-1", "1970-2-1",
"1971-1-1", "1980-2-1",
"1981-1-1", "1990-2-1" ;
Example 7.8. Temperature for each hour of the average day
Hourly average temperatures are given for April 1997.
dimensions:
time=24;
nv=2;
variables:
float temperature(time,lat,lon);
temperature:long_name="surface air temperature";
temperature:cell_methods="time: mean within days time: mean over days";
temperature:units="K";
double time(time);
time:climatology="climatology_bounds";
time:units="hours since 1997-4-1";
double climatology_bounds(time,nv);
data: // time coordinates translated to date/time format
time="1997-4-1 0:30", "1997-4-1 1:30", ... "1997-4-1 23:30" ;
climatology_bounds="1997-4-1 0:00", "1997-4-30 1:00",
"1997-4-1 1:00", "1997-4-30 2:00",
...
"1997-4-1 23:00", "1997-5-1 0:00" ;
Example 7.9. Temperature for each hour of the typical climatological day
This is a modified version of the previous example. It now applies to April from a 1961-1990 climatology.
variables:
float temperature(time,lat,lon);
temperature:long_name="surface air temperature";
temperature:cell_methods="time: mean within days ",
"time: mean over days time: mean over years";
temperature:units="K";
double time(time);
time:climatology="climatology_bounds";
time:units="days since 1961-1-1";
double climatology_bounds(time,nv);
data: // time coordinates translated to date/time format
time="1961-4-1 0:30", "1961-4-1 1:30", ..., "1961-4-1 23:30" ;
climatology_bounds="1961-4-1 0:00", "1990-4-30 1:00",
"1961-4-1 1:00", "1990-4-30 2:00",
...
"1961-4-1 23:00", "1990-5-1 0:00" ;
Example 7.10. Monthly-maximum daily precipitation totals
Maximum of daily precipitation amounts for each of the three months June, July and August 2000 are given. The first daily total applies to 6 a.m. on 1 June to 6 a.m. on 2 June, the 30th from 6 a.m. on 30 June to 6 a.m. on 1 July. The maximum of these 30 values is stored under time index 0 in the precipitation array.
dimensions:
time=3;
nv=2;
variables:
float precipitation(time,lat,lon);
precipitation:long_name="Accumulated precipitation";
precipitation:cell_methods="time: sum within days time: maximum over days";
precipitation:units="kg";
double time(time);
time:climatology="climatology_bounds";
time:units="days since 2000-6-1";
double climatology_bounds(time,nv);
data: // time coordinates translated to date/time format
time="2000-6-16", "2000-7-16", "2000-8-16" ;
climatology_bounds="2000-6-1 6:00:00", "2000-7-1 6:00:00",
"2000-7-1 6:00:00", "2000-8-1 6:00:00",
"2000-8-1 6:00:00", "2000-9-1 6:00:00" ;