incremental from accumulated values

[Bill Gustafson <ivor@xxxxxxxxxxx>]

Hello,

I have a time series of accumulated precipitation output from a weather
model. I would like to do some calculations and plot time series of
incremental values of precip. What is the syntax to be able to do this in
Ferret? Essentially, I want something like:
   let prcpinc = prcpacc-prcpacc[l=`l-1`]
The problem is that `l-1` does not evaulate to a single scaler since it
changes with every time.

Thanks for any suggestions.

-Bill

____________________________________________________________
                        Bill Gustafson
_____________Ph.D. Candidate, Atmospheric Science___________

Land, Air and Water Resources
University of California, Davis
1 Shields Avenue
Davis, CA 95616-8627                           (530)752-1868
U.S.A.                                      ivor@ucdavis.edu
____________________________________________________________