Puzzled by the streamfunction output

[Arlindo da Silva]

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.
>