[ncl-talk] data match days before plot

[Debasish Hazra]

Many thanks Dave, able to follow the instructions and create the plot.

Debasish.

On Fri, Sep 4, 2026 at 3:30 PM Dave Allured - NOAA Affiliate <
dave.allured at noaa.gov> wrote:

> Debasish, attached is a demo program to align station data with model
> data.  Also see attached output.  There is more than one way to do this.  I
> decided to create integer dates in YYYYDDD format, to be able to
> cross-reference the dates.  This seemed like the simplest way, given the
> format of your input data.
>
> This demo adds an extra "column" for station data values, next to the
> model data values.  Then a search loop plus NCL's "ind" function is used,
> to find each position where the station date matches the model date.  You
> will need to find a way to add the year number to both model and station
> data that you showed.
>
>
> On Fri, Sep 4, 2026 at 9:27 AM Debasish Hazra <debasish.hazra5 at gmail.com>
> wrote:
>
>> Thanks Dave. Able to bring both data in julian day format and common
>> starting time. But having issues with two unequal records, one from the
>> model and one from the station observations (several days in obs are
>> missing). Attached are two examples of output. And the missing days are
>> random among different stations. Not able to use regline_stats etc because
>> of unequal length. How to match these two to create the same length
>> considering the random nature of missing days in Station obs. Any
>> help would be appreciated.
>>
>> Thanks
>> Debasish
>>
>> On Sun, Aug 30, 2026 at 11:56 AM Dave Allured - NOAA Affiliate <
>> dave.allured at noaa.gov> wrote:
>>
>>> Oops, I forgot that you are making a scatter plot.  This means that you
>>> need an X axis that is linear in time.  Therefore, you must use a numeric
>>> time line, not a decimal coded time line.  So use the method that I
>>> mentioned first, to convert both inputs to a common time line such as "days
>>> since".
>>>
>>>
>>> On Sat, Aug 29, 2026 at 10:53 AM Dave Allured - NOAA Affiliate <
>>> dave.allured at noaa.gov> wrote:
>>>
>>>> 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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