McIDAS Learning Guide
Version 2006

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Real-time Data Access - Gridded Data

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 RTGRIDS dataset.
    Type:  DSINFO GRID RTGRIDS
  2. List all of the 850 mb relative humidity (RH) grids from today's 00 UTC NAM grid.
    Type:  GRDLIST RTGRIDS/NAM-USLC LEV=850 PAR=RH DAY=#Y TIME=00 NUM=ALL
    All of the relative humidity grids available (usually from 0 to 60 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 RTGRIDS/NAM-USLC 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            
 Grid is at      0 UTC on 2005208 from dataset: RTGRIDS/NAM-USLC                   
     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;ERASE;MAP USA
    Type:  GRDDISP RTGRIDS/ETA-USLC 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 NGM model run.
    Type:  ERASE F 1 5
    Type:  GRDDISP RTGRIDS/NGM-USLC DAY=#Y TIME=00 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
  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

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