In this exercise, you will review the McIDAS grid commands by accessing real-time grids. If you need a review of grids and the grid file format, see the Basic Concepts section of the Grids and Grid Files Chapter.

1. List the grid files in the RTGRIBS dataset.

   Type: DSINFO GRID RTGRIBS

2. List all of the 850 mb relative humidity (RH) grids from today's 00 UTC NAM-ALL grid.

   Type: GRDLIST RTGRIBS/NAM-ALL LEV=850 PAR=RH DAY=#Y TIME=00 NUM=ALL

   All of the relative humidity grids available (usually from 0 to 48 hours) should be listed.

3. List the 24 hour forecasted 850 mb Relative Humidity grid point values between 37° and 39° N and 102° and 105° W from this dataset. Format the data as a real value with one decimal place.

   Type: GRDINFO RTGRIBS/NAM-ALL LIST DAY=#Y TIME=0 LEV=850 FHOUR=24 PARAM=RH LAT=37 39 LON=102 105 FORMAT=F4.1

   Row, column, latitude, longitude, and values are listed for each grid point falling in this geographical region (the southeast corner of Colorado):

   Listing data from  24-hour forecast RH   Grid at 850 MB   from NAM-ALL            
   Grid is at      0 UTC on 1999362 from dataset: RTGRIBS/NAM-ALL                   
       Row    Col    Latitude    Longitude    Value         Units  Level  Param
       36      42      38.308     104.968    30.0            %    850  MB   RH
       37      42      37.599     104.902    32.0            %    850  MB   RH   
       36      43      38.358     104.065    31.0            %    850  MB   RH    
       37      43      37.648     104.005    32.0            %    850  MB   RH  
       36      44      38.403     103.161    32.0            %    850  MB   RH 
       37      44      37.693     103.107    32.0            %    850  MB   RH
       36      45      38.444     102.256    30.0            %    850  MB   RH    
       37      45      37.734     102.208    30.0            %    850  MB   RH   
       38      45      37.021     102.160    28.0            %    850  MB   RH  
   GRDINFO Done, Number of grids listed=1                      
   

4. Plot a map of the United States on Frame 6. 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: SF 6;MAP USA

   Type: GRDDISP RTGRIBS/NAM-ALL G1='PARAM T' G2='PARAM U' G3='PARAM V' MATH='-(G2*(DDX(G1))+G3*(DDY(G1)))' NEWPAR=TADV K/S NAV=C FHOUR=12 TIME=00 DAY=#Y LEV=850

   The G1, G2, ...., Gn keywords are used to define the grids you want to use. The MATH keyword defines the mathematical operation to perform on the grids specified with the Gn keywords. See the GRDDISP help section for more information on the use of these keywords.

5. Erase frames 1-5. Create a sequence of the 0, 12, 24, 36, and 48 hour 500 mb height field over the United States from today's 00 UTC NAM model run.

   Type: ERASE F 1 5

   Type: TE GRIDLOOP "GRDDISP RTGRIBS/NAM-ALL DAY=##Y TIME=0 FHOUR=!1 LEV=500 PARAM=Z CINT=60 MAP=USA PRO=MERC GRA=!2 LSIZE=6 LINT=2 SF=YES

   
   

   Type: REPEAT GRIDLOOP 0 TO 48 BY 12 1 BY 1

6. 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

7. Stop the loop.

   Press: Alt L

8. Exit McIDAS.

   Type: EXIT