Ferret "how to": sequence of files as a time series

[hankin@xxxxxxxxxxxxxxxxxxx (Steve Hankin)]

Ferret how-to: treating a sequence of independent files as a time series

On Jul 13, 11:18am, Ms S Sajani wrote:
> Subject: @ave from 12 nc files.
>
>
> Hi Netters,
>
> 	I have a simple question. I have 12 .nc
> files consists of daily data of 12 months. I
> wanted to compute the yearly average. How I can
> append the whole time points other than writing
> them into a single .nc output file.



Hi Sajani,

The above example could be addressed "brute force" as follows:

	yes? USE file1.nc, file2.nc, file3.nc, ...
	yes? LET ave = (v[d=1]+v[d=2]+v[d=3]+...+v[d=12])/12

Alternatively, here is a general technique for converting a sequence of files
into a time series, where each file contains a single time step of data. The
technique applies equally well to NetCDF, ASCII, or binary files.

The trick to this technique is to "time stamp" the files by defining a variable
with a value everywhere of zero but which is defined on the appropriate time
axis.

==========

! GOAL: combine variable "V" from 12 separate files into a monthly time series

! initialize the files
USE file1.cdf, file2.cdf, file3.cdf, ..., file12.cdf

! define the time axis
DEFINE AXIS/T=15-JAN:15-DEC/npoints=12 tmonth		! note 1

! define the "zero" value
LET zero = T[gt=tmonth] * 0				! note 2

! define the time-stamped variable
LET v_stamped = v + zero

! save all 12 months in a time series
REPEAT/L=1:12 (SET DATA `L`; SAVE/APPEND/FILE=T_series.cdf v_stamped)



Note 1: A better way to define a climatological monthly time axis is
	yes? SET DATA coads_climatology
	yes? SHOW GRID sst      <== use the time axis name

Note 2:  look at what you defined with        yes? list/l=1:12 zero



	Happy Ferreting - steve


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