Re: Strange time series plot.

From: Adam Phillips <asphilli_at_nyahnyahspammersnyahnyah>
Date: Mon Aug 12 2013 - 16:33:49 MDT

Hi Dave,
Your plot is symmetric; but about the mean and not 0. You have means and
standard deviations that change at each timestep. You would not expect
your upper and lower bounds to be mirrored..

I do not see anything in your shortened test code, calculation or
plotting resource wise, that is incorrect. (Slight exception: You should
set -999 as the _FillValue for your std_slp_06_b and std_slp_06_b arrays.)

If you have any further questions on this issue please respond to
ncl-talk only.

On 08/12/2013 12:05 PM, David Jones wrote:
> Dear NCL Talk,
> I have a rather vexing issue with my time series plot that I hope you
> may be able
> to help with.
> I have a 20 member ensemble and am plotting an area averaged sea level
> pressure
> time series over a section of my domain.
> The ensemble mean is simply the arithmetic mean of all 20 time series.
> I then calculate the intra ensemble standard deviation, i.e. amongst
> the 20 members.
> This gives me another time series, of the same length, with the
> standard deviation
> between all 20 members at each time step.
> I then multiply this time series (standard deviations) by 1.96 and
> create two new time
> series by adding/subtracting this from the original time series, like
> creating 95%
> confidence intervals.
> The problem is, when I go to plot these three time series, the result
> is not as expected.
> I expect to see three time series, with the SLP time series in the
> middle and the other
> two above and below it. I expected that the upper and lower time
> series should be
> 'mirror images' of each other, through the central time series.
> However, as you can see
> in the attached .png file, the upper and lower time series are not
> symmetric about the
> central time series. In fact, the min/max peaks aren't even fully
> aligned. I can't
> understand this, as at each time step the same value is being added
> and subtracted.
> I have repeated the same plot, except this time subtracting just a
> constant from my entire
> SLP time series, and this does not give any problems. I have also
> verified mathematically
> that the difference between the lower and middle time series is the
> same as that between
> the upper and middle time series - this is at odds with what the
> plotted image suggests.
> This leads me to conclude that some error is occurring at the plotting
> stage.
> Do you have an idea what's going wrong here? Or am i making some
> stupid mistake?
> I've attached a copy of my original script (rac_prob_slp_ts.ncl), and
> also two ascii
> files in which i've written the data that goes into the time series'
> in the upper plot.
> These can be used to reproduce the problem with the simplified script
> i've also
> attached (simple_ts.ncl).
> Any help/guidance you can offer would be immensely appreciated.
> Best wishes,
> Dave.
> PS I'm using NCL 5.2.1 on a SUSE system.
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Adam Phillips                      
NCAR/Climate and Global Dynamics Division       (303) 497-1726
P.O. Box 3000				
Boulder, CO 80307-3000

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Received on Mon Aug 12 16:33:59 2013

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