McIDAS-X Learning Guide
Version 2020

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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 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 12 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 LIST DAY=#Y TIME=0 LEV=850 FHOUR=0 PARAM=RH LAT=56 58 LON=132 135 FORMAT=F4.1
    Row, column, latitude, longitude, and values are listed for each grid point falling in this geographical region (the southeast corner of Alaska). The amount of points listed depends on the resolution of the first matching grid: 
     Listing data from  12-hour forecast RH   Grid at 850 MB   from NAM
 Grid is at      0 UTC on 2021075 from dataset: RTGRIDS/NAM
    Row    Col    Latitude    Longitude    Value         Units  Level  Param
     2      18      57.669     134.944    74.0            %    850  MB   RH
     3      18      57.091     134.620    65.0            %    850  MB   RH
     4      18      56.508     134.301    87.0            %    850  MB   RH
     2      19      57.842     133.865    98.0            %    850  MB   RH
     3      19      57.263     133.549    97.0            %    850  MB   RH
     4      19      56.680     133.238    90.0            %    850  MB   RH
     5      19      56.092     132.931    92.0            %    850  MB   RH
     3      20      57.431     132.473    92.0            %    850  MB   RH
     4      20      56.848     132.169    98.0            %    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/GFS G1='PARAM T;LEV 850' G2='PARAM U;LEV 850' G3='PARAM V;LEV 850' MATH='-(G2*(DDX(G1))+G3*(DDY(G1)))' NEWPAR=TADV K/S NAV=C FHOUR=12 TIME=00 DAY=#Y
    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 1-4. Create a sequence of the 6, 12, 18, and 24 hour 500 mb height field today's 00 UTC UKMT model run. Note: Grid availability may change on the server.
    Type:  ERASE F 1 4
    Type:  GRDDISP RTGRIDS/UKMT DAY=#Y TIME=00 FHOUR=6 12 18 24 LEV=500 PARAM=Z CINT=60 GRA=1 4 LSIZE=6 LINT=2 SF=YES
  6. Set the loop bounds to frames 1-4. Change the dwell rate so it pauses longer on the last frame. Then, start the loop.
    Type:  LS 1-4;DR 3*3 10
    Press:  Alt L
  7. Stop the loop.
    Press:  Alt L
  8. Exit McIDAS.
    Type:  EXIT

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