Puzzled by the streamfunction output

[See Hai Ooi]

Dear Mr Arlindo,

     Thank you.

     By examining the sample from the article with
that generated from GrADS, I calculated and added a
constant term of 44 [e.g. -36 + x = 8 in contour
values] to my script.  The signs and magnitudes of the
contours are now similar ! [see the revised
attachment].  Nevertheless, I hope someone can
enlighten me about adding this magic number of 44 [any
mathematical equation or relation involved ?].  In
this way, I am able to compare the GrADS-generated
outputs with those available in the standard
texts/published articles, particularly in terms of the
high(ridges) and lows(troughs) parallely.

     I still failed to resolve the wrap around problem
even by setting the lon as -180 180.  Can you or
someone help me on this ?

     Again, thank you and best regards.

Ooi See Hai




--- Arlindo da Silva <dasilva at alum.mit.edu> wrote:

> On Sat, Feb 23, 2008 at 2:27 AM, See Hai Ooi
> <axl419 at yahoo.com> wrote:
>
> > Dear Mr Arlindo da Silva,
> >
> >     For days, I have been puzzled by the output of
streamfunction using your fish() in the GrADS
v1.9.0-rc1 (windows).  By chance, I happened to see a
700 hPa streamfunction example in an article on Afican
Easterly Wave.  I used the NCEP/NCAR Reanalysis data
to reproduce the example shown.  As shown in my
attachment, you can notice the followings:-
> >     (1) Both patterns look similar.
> >     (2) The signs and magnitudes of streamfunction
generated from GrADS are vastly different from those
in the example figure.
>
>
>
> Remember that you can add any constant to a
streamfunction and still obtain an equally valid
streamfunction, in the sense that the rotational wind
you derive from it is exactly the same.. So, when
comparing to other studies, if you want to match the
streamfunction value-by-value you have to make sure
you know  exactly which constant the other person
used, something usually not possible when comparing to
published work. What is important is that the contour
lines are in general  parallel to each other. Also,
tropical winds are not very well observed, so unless
you are using exactly the same dataset chances are the
details of the vorticity fields may not agree too
well.
>
>
> >     (3) In the GrADS generated example, values
west of 0 degree longitude are absent.
> >
>
> From your scripts appear that your longitudes are
going from 0 to 360. Try selecting the longitudinal
range as [-180,180) before calculating your
streamfunction. This "missing half of the plot" is a
common issue with GrADS when it can no longer infer
that the longitudes wrap around.
>
> I'm cc'ing gradsusr. In the future send these kind
of questions there as the answers may benefit other
users as well.
>
>    Good Luck,
>
>       Arlindo
>
> --
> Arlindo da Silva
> dasilva at alum.mit.edu

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