## How do I create an annual composite of monthly data?

This
operation can be done with a few ingrid commands in expert mode and they
can be seen at the following link: http://iridl.ldeo.columbia.edu/expert/SOURCES/.UMD/.Carton/.goa/.beta7/.u/a:/T/%281980%29VALUES/:a:/T/%281970%29VALUES/T/120/shiftGRID/add/:a:/T/%281960%29VALUES/T/240/shiftGRID/add/:a:/T/%281950%29VALUES/T/360/shiftGRID/add/:a/4/div/

Let's go through this a bit of detail.

SOURCES .UMD .Carton .goa .beta7 .u

Selects the zonal wind variable
of the UMD Carton dataset.

a:
T (1980) VALUES
:a:
T (1970) VALUES
T 120 shiftGRID
add
:a:
T (1960) VALUES
T 240 shiftGRID
add
:a:
T (1950) VALUES
T 360 shiftGRID
add
:a

Each of the sections of text separated
by a: :a selects the months from the indicated years and adds them together.
The a: :a: :a: :a: :a construction allows you to select different
time periods. The shiftGRID commands (e.g., T 120 shiftGRID) artificially
shift the time grid to match among the different selections of data.
For example, 1950 is shifted 30 years forward using "T 360 shiftGRID" so
its values can be added to those of 1980.

4 div

This line divides the resulting
sum by 4 to find an average for the 4 year composite.

You can now view or download the
resulting composite by selecting one of the views links or the Data Files
link in the function bar.

*-- Michael Bell and Emily Grover*