[ncl-talk] data match days before plot

[Dave Allured - NOAA Affiliate]

Put both the obs and model data on a common time line.  For the obs
(station) data, it looks like you need to combine the year number into the
calculation.  You did not show any time information for the model data, but
I am sure it is something simple like "days since" or "hours since".  I
suggest convert both to "days since", since the obs data is already based
on day numbers.

Also when calculating the new time lines, you must pick a common starting
point.  I suggest YYYY January 1, where YYYY is the starting year of either
data set.

Alternatively, you could use a decimal coded time line such as YYYYDDD or
YYYYMMDD, rather than a pure numeric time line.  With this type, dates are
absolute in one sense, so there is no need to pick a common starting date.

For the scatter plot, you compare both time lines and pick out the dates in
common.  This is possible with any kind of time line, as long as both are
computed accurately, and roundoff error is resolved.  My final advice is,
pick whichever method seems easiest to create the time lines.

NCL has date functions to assist with creating and converting time lines.
https://www.ncl.ucar.edu/Document/Functions/date.shtml


On Thu, Aug 27, 2026 at 2:20 PM Debasish Hazra via ncl-talk <
ncl-talk at mailman.ucar.edu> wrote:

> Hi,
> I am trying to plot a scatter plot using Ncl and plotting daily averaged
> station data vs model output using more than 2 years data. However, I am
> seeing more than one data gaps in station data, like below (first value is
> in Julian days, second is precip value for a station of that day), but
> model data has values in all days.
>
> 350,0.418833
> 351,0.494576
> 355,0.389460
> 356,0.406334
> ...SNIP...
> 362,0.113083
> 363,0.124250
> 364,0.081895
> 365,0.159218
>   2,0.125749
>   3,0.217326
>   4,0.276310
>
> How to match those two data set in terms of  exact days before the scatter
> plot ? Any help ?
> Thanks
> Debasish
>
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