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20020626: McIDAS ETA ABV vs calculated ABV question
- Subject: 20020626: McIDAS ETA ABV vs calculated ABV question
- Date: Thu, 27 Jun 2002 07:44:49 -0600
>From: "Jennie L. Moody" <address@hidden>
>Organization: UVa
>Keywords: 200206270315.g5R3F6u03265 McIDAS GRDCOPY XCD GRID
Jennie,
>I am still trying to track down what might be an error in some of
>the PV fields that we calculated previously, based on a
>correction factor (might turn out to be a misnomer!) that Owen
>applied. Anyway, in my process I am relearning mcidas (good),
>and confusing myself (bad). First, I was trying to see if the
>quantity ABV (absolute vorticity) in some of the ETA files was
>already there, or if it was placed there by Owen.
ABV is a parameter that is available from the ETA runs. Here
is an example that shows this:
DATALOC ADD RTGRIDS ADDE.UCAR.EDU
GRDLIST RTGRIDS/ETA FORM=FILE ALL
<pick out the 0Z run for forecast hours 0 to 24; in dataset position 8>
GRDLIST RTGRIDS/ETA.8 PAR=ABV NUM=ALL
Dataset position 8 Directory Title= ALL 00Z ETA 0 HR<=VT<=24 HR
PAR LEVEL DAY TIME SRC FHOUR FDAY FTIME GRID PRO
---- --------- ------------ -------- ---- ----- ------------ -------- ----- ----
ABV 1000 MB 27 JUN 02178 00:00:00 ETA 0 27 JUN 02178 00:00:00 230 LAMB
ABV 850 MB 27 JUN 02178 00:00:00 ETA 0 27 JUN 02178 00:00:00 231 LAMB
ABV 700 MB 27 JUN 02178 00:00:00 ETA 0 27 JUN 02178 00:00:00 232 LAMB
ABV 500 MB 27 JUN 02178 00:00:00 ETA 0 27 JUN 02178 00:00:00 233 LAMB
ABV 250 MB 27 JUN 02178 00:00:00 ETA 0 27 JUN 02178 00:00:00 234 LAMB
ABV 1000 MB 27 JUN 02178 00:00:00 ETA 6 27 JUN 02178 06:00:00 387 LAMB
ABV 850 MB 27 JUN 02178 00:00:00 ETA 6 27 JUN 02178 06:00:00 388 LAMB
ABV 700 MB 27 JUN 02178 00:00:00 ETA 6 27 JUN 02178 06:00:00 389 LAMB
ABV 500 MB 27 JUN 02178 00:00:00 ETA 6 27 JUN 02178 06:00:00 390 LAMB
ABV 250 MB 27 JUN 02178 00:00:00 ETA 6 27 JUN 02178 06:00:00 391 LAMB
ABV 1000 MB 27 JUN 02178 00:00:00 ETA 12 27 JUN 02178 12:00:00 740 LAMB
ABV 850 MB 27 JUN 02178 00:00:00 ETA 12 27 JUN 02178 12:00:00 741 LAMB
ABV 700 MB 27 JUN 02178 00:00:00 ETA 12 27 JUN 02178 12:00:00 742 LAMB
ABV 500 MB 27 JUN 02178 00:00:00 ETA 12 27 JUN 02178 12:00:00 743 LAMB
ABV 250 MB 27 JUN 02178 00:00:00 ETA 12 27 JUN 02178 12:00:00 748 LAMB
ABV 1000 MB 27 JUN 02178 00:00:00 ETA 18 27 JUN 02178 18:00:00 1738 LAMB
ABV 500 MB 27 JUN 02178 00:00:00 ETA 18 27 JUN 02178 18:00:00 1739 LAMB
ABV 250 MB 27 JUN 02178 00:00:00 ETA 18 27 JUN 02178 18:00:00 1740 LAMB
ABV 700 MB 27 JUN 02178 00:00:00 ETA 18 27 JUN 02178 18:00:00 1744 LAMB
ABV 850 MB 27 JUN 02178 00:00:00 ETA 18 27 JUN 02178 18:00:00 1746 LAMB
ABV 1000 MB 27 JUN 02178 00:00:00 ETA 24 28 JUN 02179 00:00:00 1890 LAMB
ABV 850 MB 27 JUN 02178 00:00:00 ETA 24 28 JUN 02179 00:00:00 1894 LAMB
ABV 700 MB 27 JUN 02178 00:00:00 ETA 24 28 JUN 02179 00:00:00 1896 LAMB
ABV 500 MB 27 JUN 02178 00:00:00 ETA 24 28 JUN 02179 00:00:00 1898 LAMB
ABV 250 MB 27 JUN 02178 00:00:00 ETA 24 28 JUN 02179 00:00:00 1899 LAMB
Number of grids listed = 25
GRDLIST - done
To get some stats on one of these grids, run:
GRDINFO RTGRIDS/ETA.8 STAT GRID=230 <- for 1000 MB ABV
Statistics of
Parameter: ABV
Units: PS
Level of data: 1000 MB
Time of data: 0 UTC
Day of data: 2002178
Data came from: ETA
0-hour forecast
Dataset: RTGRIDS/ETA
Minimum: -0.4100000E+01 occurred at [row,col]: [ 62, 83]
[lat,lon]: [ 17.88, 72.03]
Maximum: 0.3564000E+02 occurred at [row,col]: [ 1, 93]
[lat,lon]: [ 57.31, 49.37]
Mean: 0.8647677E+01
SD : 0.3556637E+01
Number of points analyzed: 6045
Number of points missing: 0
Row range: 1 to 65
Col range: 1 to 93
GRDINFO Done, Number of grids statistically analyzed=1
GRDINFO - done
The question is what the unit PS represents; per second?
>I made a new
>grid with the derived quantity ABV, and compared it with the old
>grid. The grids look the same (place contours in the same
>location), but they have different units. This matters to me,
>we are doing MATH on grids, and we have to be sure that the
>units are working out!
Absolutely. The first thing to check in any calculation is units.
>Anyway, this raises one question...the
>units on these two grids (one that I made with the DERIVE=ABV
>command, the other one that was in my ETA grid directory, but
>could have been made by Owen in some step I haven't seen) the
>units of these two grids are different by 6 orders of magnitude
>(one has a scale factor of 2, the other has a scale factor of
>8!)....but, they list units of PS and 1/S respectively.
So, I think that the discrepency may be cleared up by correct
interpretation of exactly what PS means. The GRIB table used
by XCD decoders defines absolute vorticity as follows:
041 | Absolute vorticity | ABV | 1/s | PS | 7
The units are 'per second' and the scale factor is 10**7. This tells
me that the values listed above for ABV range from -4.15 * 10**-7
to 35.64 * 10**-7.
>Their
>is no listing for frequencies in the units table (cu.k ), but
>this looks like its trying to say these are both per second???
>Does this make sense to you Tom?
Yes, but the values in the XCD decoded grid have a scale factor applied
to them. This seems to be what is causing the interpretation difference.
>I have appended the long
>form listing of these two grids, from two different grid files,
>so you might see whats up:
>
>windfall: /home/jlm8h/mcidas/data $ grdlist.k MYDATA/GRIDS.9001
>PAR=ABV FOR=ALL
>Dataset position 9001 Directory Title=
>PAR LEVEL DAY TIME SRC FHOUR FDAY
>FTIME GRID PRO
>---- --------- ------------ -------- ---- ----- ------------
>-------- ----- ----
>ABV 1000 MB 08 MAR 00068 12:00:00 ETA 0 08 MAR 00068
>12:00:00 119 LAMB
>Total pts= 6045 Num rows= 65 Num columns= 93 received:
>2000097 160201Z
>Absolute vorticity
>GRIB ID numbers: Geographic =211; PAR = 41; Model ID = 89; Level
>type =100
>Units of gridded variable are PS Scale of variable is: 2
>Lambert Conformal Tangent Cone Projection
>Row num of pole= -113.34 Col num of pole= 53.00 Col spacing
>(m)= 81270.0
>Standard Latitudes= 25.00 25.00 Standard Longitude=
>95.00
>Number of grids listed = 1
>GRDLIST - done
OK, this is ETA output equivalent to what I listed above.
>windfall: /home/jlm8h/mcidas/data $ grdlist.k MYDATA/GRIDS.9004
>PAR=ABV FOR=ALL
>Dataset position 9004 Directory Title=
>PAR LEVEL DAY TIME SRC FHOUR FDAY
>FTIME GRID PRO
>---- --------- ------------ -------- ---- ----- ------------
>-------- ----- ----
>ABV 400 MB 08 MAR 00068 12:00:00 ETA 0 08 MAR 00068
>12:00:00 1 LAMB
>Total pts= 6045 Num rows= 65 Num columns= 93 received:
>2000097 160200Z
>Absolute vorticity
>GRIB ID numbers: Geographic =211; PAR = 33; Model ID = 89; Level
>type =100
>Units of gridded variable are 1/S Scale of variable is: 8
>Lambert Conformal Tangent Cone Projection
>Row num of pole= -113.34 Col num of pole= 53.00 Col spacing
>(m)= 81270.0
>Standard Latitudes= 25.00 25.00 Standard Longitude=
>95.00
>Number of grids listed = 1
>GRDLIST - done
This must be the one you calculated since the PAR = 33 indicates that
the grid is the U component of the wind. The ABV that comes with the
ETA run output specifies the parameter with PAR = 41. For reference,
the GRIB tables used for the XCD grib decoder are gbtbpds001.2v[123].
You can find these in the ~mcidas/workdata directory (~mcidas/uvaworkdata
on windfall).
-- deleted cu.k output --
>If you will bear with me a minute, this is only the first step
>that I was trying to follow. One reason I decided to look more
>carefully at even a simple step like the ABV is that I had looked
>up the way ABV is defined in GRDCOPY and I found what I hope is
>just a typo in the definition table (surely this cannot really
>be an error in the mcidas code!).....ABV=VOR+COR, but you will
>see that this is not true in the table in the users guide (I am
>looking at Version 7.7, which is what I am running). The
>derivatives are switched. VOR is right, you get vorticity
>(circulation) by taking the variation of the u wind-component
>in the y direction, and the v wind-component in the x direction.
>The ABV in the table must be written incorrectly. Anyway,
>this sidetracked me so I thought I would just look at ABV,
>and then derive it myself by taking the derivatives of the
>U and V winds respectively and building ABV. But, I didn't
>get this far and decided I won't waste too much time with this
>until I know how the units work.
OK.
>What is PS versus 1/S?
PS is the unit assigned to several parameters decoded by XCD. It
apparently represents 'per second'. The missing piece is the scaling
by 10**7.
>I am trying to say this clearly, but reading over what
>I wrote, is might not strike you that way.
Not to worry.
>Like I said
>at the beginning, this might work better if we had a
>conversation. Let me know what you think, and if its
>better explained over the phone, maybe I can give you a
>call tomorrow sometime.
I think I understand your explanation. Does the scaling of the XCD
grids rectify the situation for you?
>Just to state this one more time, this is just the first part
>of my effort to sort out what Owen did (and where there
>might be a mistake).
I understand.
>Ultimately, to get isentropic PV from
>data on constant pressure surfaces, you need to make a
>correction which involves the vertical derivatives
>of wind and horizontal derivatives of Temperature. I
>think he might have done this incorrectly, but I need
>to recalculate things, and I want to do it one step
>at a time. So.....thanks for any light you can shine
>here at the beginning of this tunnel regarding making
>sense of SCALE and UNIT.
If the scaling of ABV in the ETA output doesn't jibe with what
you are seeing, we should definitely chat about this.
>I sure this must be more straight foward than it appears
>at the moment. I am also pretty sure that there must just
>be a typo in the definition table of the derived parameters
>available to GRDCOPY.
A typo wouldn't suprise me at all.
>(I am sorry, because I know this is all ground I covered
>in one way or another previously....sigh)
>
>
>One last by the way....I know I failed to list the exact same
>grids above...that doesn't make any difference...but if you want
>them the same, here they are:
The grids being different didn't change the point you were making.
I believe that I understood what you are trying to reconcile.
Again, if what I wrote does not clear things up/make sense, let's talk
about the situation.
Tom
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