**McIDAS Learning GuideVersion 2015**

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- Generate a local grid dataset in position 2 of the TEST-GRID dataset with
a maximum of 100 grids, and the description ADVECTION AND DIVERGENCE GRIDS.
Copy in the 12 hour 1000 mb forecast grids from the first BLIZZARD grid position.
- Type:
**GRDCOPY BLIZZARD/GRIDS.1 TG.2 DEL=YES LEVEL=1000 FHOUR=12 TITLE='ADVECTION AND DIVERGENCE GRIDS' MAXGRD=100 NUM=ALL**

- Type:
- List the new dataset.
- Type:
**GRDLIST TG.2 NUM=ALL**

- Type:
- Check to see that TG.2 contains the grids required to calculate divergence.
- Type:
**GRDLIST TG.2 DER=DVG**

GRDLIST TG.2 DER=DVG PARAM LEVEL DAY TIME SRC FHOUR FDAY FTIME GRID PRO ----- ------- ------------ -------- ---- ----- ------------ -------- ----- ---- DVG 1000 MB 11 MAR 93070 12:00:00 NGM 12 12 MAR 93071 00:00:00 MATH MERC Number of grids listed = 1 GRDLIST - done

Notice the GRID number is listed as "MATH" or "N/A". This indicates that there is no actual divergence grid but that the component grids required to create it all exist within the TG.2 grid file. See the help sections of the individual commands for more information on the common meteorological parameters you can calculate from grids by using the DERIVE keyword.

If the grid cannot be created, an error will be produced.

- Type:
- Create a divergence grid from from the grids in the dataset.
- Type:
**GRDCOPY TG.2 TG.2 DER=DVG**

- Type:
- List the contents of the dataset to verify that a divergence grid was added.
- Type:
**GRDLIST TG.2 NUM=ALL**

Dataset position 2 Directory Title= ADVECTION AND DIVERGENCE GRIDS PARAM LEVEL DAY TIME SRC FHOUR FDAY FTIME GRID PRO ----- ------- ------------ -------- ---- ----- ------------ -------- ------ ---- Z 1000 MB 11 MAR 93070 12:00:00 NGM 12 12 MAR 93071 00:00:00 1 MERC T 1000 MB 11 MAR 93070 12:00:00 NGM 12 12 MAR 93071 00:00:00 2 MERC U 1000 MB 11 MAR 93070 12:00:00 NGM 12 12 MAR 93071 00:00:00 3 MERC V 1000 MB 11 MAR 93070 12:00:00 NGM 12 12 MAR 93071 00:00:00 4 MERC DVG 1000 MB 11 MAR 93070 12:00:00 NGM 12 12 MAR 93071 00:00:00 5 MERC Number of grids listed = 5

Notice that the the DVG grid has now been created, has a grid number, and is listed as a PARAMETER.

- Type:
- List information about the divergence grid:
- Type:
**GRDINFO TG.2 PARAM=DVG**

GRDINFO TG.2 PARAM=DVG Statistics of Parameter: DVG Units: 1/S Level of data: 1000 MB Initial time of data: 120000 UTC Initial day of data: 1993070 Data came from: NGM 12-hour forecast Dataset: TG Minimum: -0.6531000E-04 occurred at [row,col]: [ 33, 26] [lat,lon]: [ 20.00, 77.50] Maximum: 0.3372000E-04 occurred at [row,col]: [ 33, 24] [lat,lon]: [ 20.00, 82.50] Mean: 0.2849890E-07 SD : 0.1168576E-04 Number of points analyzed: 1188 Number of points missing: 282 Row range: 1 to 33 Col range: 1 to 36 GRDINFO Done, Number of grids statistically analyzed=1

- Type:
- Create a temperature advection grid using the U, V and T grids. Use the
formula TADV = -(u*DDX(T) + v*DDY(T)). Label the new
grid TADV and set the units to K/sec.
- Type:
**GRDCOPY TG.2 TG.2 G1='PARAM T' G2='PARAM U' G3='PARAM V' MATH='-(G2*(DDX(G1))+G3*(DDY(G1)))' NEWPAR=TADV K/S**

*Gn*keywords. See the help sections of the individual commands for more information on the use of these keywords. - Type:
- List information about the temperature advection grid:
- Type:
**GRDINFO TG.2 PARAM=TADV**

- Type:
- OR, if you would like to get statistical information on a grid created
with a mathematical equation, but won't need to actually display the grid,
you can run the GRDINFO command with the MATH keywords included.
- Type:
**GRDINFO TG.2 G1='PARAM T' G2='PARAM U' G3='PARAM V' MATH='-(G2*(DDX(G1))+G3*(DDY(G1)))' NEWPAR=TTAD K/S**

- Type:

In the next lesson, you will display the grids you created using the GRDDISP command.

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