McIDAS Learning Guide
Version 2015

## Grids and Grid Files - Displaying Gridded Data

In this exercise, you will use the GRDDISP command to display grids. GRDDISP can be used to draw contours, streamlines, or plots of grid data.
1. Show frame 4, and display the first GOES-7 4km Visible image centered on Washington DC. Add a high resolution map.
Type:  SF 4;IMGDISP GV4.1 4 STA=DCA;MAP H
2. Contour the divergence grid created in the previous lesson over the satellite image with a contour interval of 5 and graphics color level 3. Because the values of divergence are small, use the POWER keyword to scale the values. Make the convergence (negative divergence) areas dashed. Use NAV=C to use the navigation in the current frame.
Type:  GRDDISP TG.2 PARAM=DVG NAV=C CINT=5 COLOR=3 POWER=6 DASH=NEG
3. Contour the temperature advection grid over the image using a contour interval of five degrees per sec scaled by 10**5. Make negative values (cold air advection) dashed.
Type:  GRDDISP TG.2 PARAM=TADV NAV=C CINT=5 POWER=5 DASH=NEG
4. Create a plot on frame 1 of the 12 hour 500 mb height forecast over the U.S. from the 12 UTC/93071 NGM model run in the local TG.1 dataset. Use a label size of six pixels, a 60 m contour interval and label every other contour. Switch to the frame after plotting.
Type:  GRDDISP TG.1 DAY=12/MAR/93 TIME=12 FHOUR=12 LEV=500 PARAM=Z MAP=USA PRO=MERC GRA=1 LSIZE=6 CINT=60 LINT=2 SF=YES
5. Overlay the 500 mb temperatures in degrees C in yellow. Dash the contours and use a label size of six pixels and a contour interval of 5.
Type:  GRDDISP TG.1 DAY=12/MAR/93 TIME=12 FHOUR=12 LEV=500 PARAM=T UNIT=C NAV=C LSIZE=6 COLOR=3 CINT=5 DASH=ALL
6. Create a plot of wind barbs over the US at the 250 mb level using the 0 HR forecast from the TG.1 dataset (12 UTC/March 12, 1993). Draw the barbs in yellow with a size of 12 on frame 2 and show the frame after the plotting is complete.
Type:  GRDDISP TG.1 DAY=12/MAR/93 TIME=12 FHOUR=0 LEV=250 PARAM=WINDB MAP=USA LSIZE=12 COLOR=3 GRA=2 SF=YES
Notice that the density of the grid points and the size of the wind barbs makes them overlap.

7. Erase the frame and redo the previous plot, this time plotting only every other grid point.
Type:  ERASE G;GRDDISP TG.1 DAY=12/MAR/93 TIME=12 FHOUR=0 LEV=250 PARAM=WINDB MAP=USA LSIZE=12 COLOR=3 PINT=2 2
The PINT keyword can be used to declutter a dense grid plot.

8. Erase frames 1-5. Create a sequence of the 0, 12, 24, 36, and 48 hour 500 mb height field over the US from the 12 UTC March 12, 1993 NGM model run.
Type:  ERASE F 1 5
Type:  GRDDISP TG.1 DAY=12/MAR/93 TIME=12 FHOUR=0 12 24 36 48 LEV=500 PARAM=Z CINT=60 MAP=USA PRO=MERC GRA=1 5 LSIZE=6 LINT=2 SF=YES
9. Set the loop bounds to frames 1-5. Change the dwell rate so it pauses longer on the last frame. Then, start the loop.
Type:  LS 1-5;DR 4*3 10
Press:  Alt L
10. Stop the loop.
Press:  Alt L

In this exercise, you will display the forecasted sea level pressures for 12Z on March 14, 1993. Then, you will compute the 1000-500 mb thickness for that same time and display the thickness on top of the sea level pressure contours.

1. List the available sea level pressures from the March 12th, 1993 12 UTC NGM model run.
Type:  GRDLIST TG.1 NUM=ALL TIME=12 DAY=93071 PAR=P LEV=MSL
2. Erase the graphics in the first five frames.
Type:  ERASE G 1 5
3. Display the sea level pressure data from the 12 UTC NGM model run. Plot the contours over a map of the United States.
Type:  GRDDISP TG.1 TIME=12 DAY=93071 PAR=P LEV=MSL MAP=USA FHOUR=48
4. Calculate and display the 1000-500 mb thickness for the same forecast time.
Type:  GRDDISP TG.1 MAP=USA MATH='G2-G1' COLOR=3 NEWPAR=THCK GPM G1='LEV 1000;PAR Z;TIME 12;DAY 93071;FHOUR 48' G2='LEV 500'
5. Exit McIDAS.
Type:  EXIT