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Let's start up the program:
prompt% gdinfo GDFILE Grid file $GEMPAK/data/hrcbob.grd LSTALL Full list flag YES OUTPUT Output device (TNF) T GDATTIM Grid date/time LAST GLEVEL Grid level 500 GVCORD Grid vertical coordinate PRES GFUNC Scalar grid TMPC Parameters requested: GDFILE,LSTALL,OUTPUT. GEMPAK-GDINFO>When we were running GDLIST just a minute ago, we already set GDFILE to point at the Hurricane Bob data file. LSTALL and OUTPUT are usually set to
YES and T, respectively. Now use the RESTORE
command to load the gdinfo.nts defaults:
GEMPAK-GDINFO>restore defaults/gdinfo.nts GEMPAK-GDINFO>l GDFILE = LSTALL = yes OUTPUT = t GDATTIM = all GLEVEL = all GVCORD = all GFUNC = all GEMPAK-GDINFO>
The default configuration is to display the information about all grids in the file. The GDATTIM, GLEVEL, GVCORD and GFUNC parameters can be used to restrict the output to certain types of grids in the file. We ned to reset the GDFILE since it was cleared when we restored the defaults:
GEMPAK-GDINFO>gdfile = $GEMPAK/data/hrcbob.grd
If we weren't sure how to run GDINFO, we could look it up using the on-line
help (phelp gdinfo at any GEMPAK prompt). For this program, it's
pretty self-explanatory.
Let's run the program and see what we get:
GEMPAK-GDINFO>run
GRID FILE: $GEMPAK/data/hrcbob.grd
GRID NAVIGATION:
PROJECTION: STR
ANGLES: 90.0 -105.0 0.0
GRID SIZE: 53 45
LL CORNER: 7.65 -133.44
UR CORNER: 44.29 -23.75
GRID ANALYSIS BLOCK:
ANALYSIS TYPE: BARNES
DELTAN: 3.750
DELTAX: 2.110
DELTAY: 0.833
GRID AREA: 7.65 -133.44 44.29 -23.75
EXTEND AREA: 7.65 -133.44 44.29 -23.75
DATA AREA: 7.65 -133.44 44.29 -23.75
Number of grids in file: 432
Maximum number of grids in file: 500
NUM TIME1 TIME2 LEVL1 LEVL2 VCORD PARM
1 910819/0000F00 0 PRES PRES
2 910819/0000F00 0 PRES PMSL
3 910819/0000F00 1000 PRES TMPK
4 910819/0000F00 1000 PRES RELH
5 910819/0000F00 1000 PRES HGHT
6 910819/0000F00 1000 PRES OMEG
7 910819/0000F00 1000 PRES UREL
8 910819/0000F00 1000 PRES VREL
9 910819/0000F00 950 PRES TMPK
10 910819/0000F00 950 PRES RELH
11 910819/0000F00 950 PRES HGHT
12 910819/0000F00 950 PRES OMEG
13 910819/0000F00 950 PRES UREL
14 910819/0000F00 950 PRES VREL
15 910819/0000F00 900 PRES TMPK
16 910819/0000F00 900 PRES RELH
17 910819/0000F00 850 PRES TMPK
18 910819/0000F00 850 PRES RELH
19 910819/0000F00 850 PRES HGHT
20 910819/0000F00 850 PRES OMEG
21 910819/0000F00 850 PRES UREL
22 910819/0000F00 850 PRES VREL
23 910819/0000F00 800 PRES TMPK
24 910819/0000F00 800 PRES RELH
25 910819/0000F00 750 PRES TMPK
26 910819/0000F00 750 PRES RELH
27 910819/0000F00 700 PRES TMPK
28 910819/0000F00 700 PRES RELH
29 910819/0000F00 700 PRES HGHT
30 910819/0000F00 700 PRES OMEG
31 910819/0000F00 700 PRES UREL
32 910819/0000F00 700 PRES VREL
33 910819/0000F00 500 PRES TMPK
34 910819/0000F00 500 PRES RELH
35 910819/0000F00 500 0 PRES LIFT
Enter <CR> to page or type EXIT:
(If you're working along with the tutorial, use the scroll bar on your GEMPAK
window to scroll back up so you can see the start of the output.)The first thing we see is information about the grid file, including the filename, the grid navigation, the grid analysis block, the number of grids in the file, and the maximum number of grids allowed in the file. This is called the header information. It tells you common information that applies to all the grids in the file. For now, don't worry to much about this stuff. Later it will become important when we are creating our own grid files.
The second section contains a numbered list of all the grids stored in the file. We know from the header section that there are 432 grids in this file. Only 35 grids show on the first page. If you hit enter (return), you'll get another page of grids.
Let's take a closer look at the entry for the first grid:
NUM TIME1 TIME2 LEVL1 LEVL2 VCORD PARM
1 910819/0000F00 0 PRES PRES
Each grid is given a number. You can refer to this grid by the number in
most grid programs. Usually, however, this is inconvenient because there
is no easy way to remember which number corresponds to which grid. Most often, GEMPAK users refer to specific grids by specifying the time, level, vertical coordinate, and parameter name. As you can see in the grid description, there is room for two times and two levels. This is useful if you have a grid that is an accumulation or average (two times) or a layer difference or average (two levels). Most grids, however, use only one time and one level to describe them. The vertical coordinate and parameter name are given in the last two columns.
These variables completely specify this particular grid. Later, if I want to use this grid in a contour plot, or to compute a diagnostic, I must refer to it using all these quantities:
GDATTIM = 910819/0000F00
GLEVEL = 0
GVCORD = PRES
GFUNC = PRES
This is how the GDINFO program can be so useful. Let's say that you know there
is a surface pressure grid in the Hurricane Bob data set, but you don't remember
exactly how to ask for it. All you need to do is run GDINFO, and look at the
list of grids.
Finally, a word about the date and time. You may have noticed that the date/time
specification for these grids is slightly different that what we've seen in the
surface and upper air data. The HHMM (hour-minute) field is followed by an
F00. The F means "forecast", and the
00 is the forecast hour. So, the grids we've been looking at in
the GDINFO list are all the "zero hour forecast". These are also know as the
initialization grids, or the grids that are used as input to the model.
When these models are run, data is written out at preset intervals corresponding to forecast hours. For example, later in the list of grids from GDINFO we see:
... 81 910819/0000F06 0 PRES PRES 82 910819/0000F06 0 PRES PMSL 83 910819/0000F06 1000 PRES TMPK 84 910819/0000F06 1000 PRES RELH 85 910819/0000F06 1000 PRES HGHT 86 910819/0000F06 1000 PRES OMEG 87 910819/0000F06 1000 PRES UREL 88 910819/0000F06 1000 PRES VREL 89 910819/0000F06 950 PRES TMPK 90 910819/0000F06 950 PRES RELH ...These grids are the 6-hour forecast grids. They correspond to what the model anticipates each field will be 6 hours after 910819/0000. Depending on which model is used to generate the forecasts, they can go quite far out in time. The Hurricane Bob data set has forecasts that go out to 24 hours:
... 354 910819/0000F24 0 PRES PRES 355 910819/0000F24 0 PRES PMSL 356 910819/0000F24 1000 PRES TMPK 357 910819/0000F24 1000 PRES RELH 358 910819/0000F24 1000 PRES HGHT 359 910819/0000F24 1000 PRES OMEG 360 910819/0000F24 1000 PRES UREL 361 910819/0000F24 1000 PRES VREL 362 910819/0000F24 950 PRES TMPK 363 910819/0000F24 950 PRES RELH ...
After working through this chapter, you should now be familiar with:
Before moving on to the next chapter, test your GEMPAK skill by working
through the Listing Program Exercises.
After each question, there will be a answer window 
.
Work through the exercise, and then check your results against the answers.
The next chapter discusses mapping programs. These programs are used to draw maps and plot data. They are undoubtably more fun than the listing programs, but they are also more complicated, so its a good thing that we have such a good understanding of basic GEMPAK workings.
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