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20020627: McIDAS ETA ABV vs calculated ABV question (cont.)

>From:  "Jennie L. Moody" <address@hidden>
>Organization:  UVa
>Keywords:  200206270315.g5R3F6u03265 McIDAS GRDCOPY XCD GRID

Jennie,

I looked into the ETA ABV vs GRDCOPY ABV more after our staff meeting
yesterday.  You are absolutely correct, the scaling for ABV as decoded
by XCD decoders is WRONG by several orders of magnitude.

The bottom line is that the XCD decoder scales the ABV, VOR, SHUM, and
MIX parameters incorrectly.  The problem you are seeing is caused by
the exact same problem that Owen ran into some time ago with the SHUM
parameter.  The "fix" I came up for Owen worked for SHUM because I
changed the scaling factor in the XCD GRIB table from 2 to 5 and then
changed the output from kg/g to kg/g thus effectively scaling the
values by the three orders of magnitude that were in dispute.  The same
sort of thing can NOT be done for ABV and VOR _unless_ the user always
takes into account that the values need to be scaled by 5 orders of
magnitude.  Instead, I have to modify code to correctly scale those
parameters.

A comparison between the calculated ABV for ETA U,V fields and the ABV
that comes with the model output is, as I believe you already found,
pretty good.  For the most part, contour plots of the McIDAS calculated
vs ETA model fields are indistinguishable.  There are regions where
there is more detail in the calculated grids, but this is
understandable.

So, I will implement a fix for the XCD decoders as soon as I chat with
SSEC to see if they are in agreement.  This, however, does not help you
to use the ABV fields that you have in archived datasets -- unless you
saved the original GRIB messages.  The best approach if you do not have
the original GRIB messages is to use GRDCOPY calculated versions of the
ABV fields.  The other thing that could be done is to write a small
routine that would go in and change the scaling factor saved in each
GRID header for ABV and VOR.  This would fix-up your archive and
eliminate the need to calculate ABV/VOR.

Sorry I couldn't see the problem sooner...

Tom

>From address@hidden Fri Jun 28 22:56:56 2002
>Subject: Re: 20020626: McIDAS ETA ABV vs calculated ABV question

Tom,

Thanks so much, I was pretty swamped and didn't get back to this
was feeling harrassed by too many of lifes details.  I am glad to
be getting away for a couple weeks.  I won't be able to work on
McIDAS much while I am away, mayge a little when I am at UMBS
for a few days.  But, what you have told me is very helpful.
I will pursue this with great intent when I return.

Thanks so much for your help.

Jennie