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The "medium-scale" grid used in many simulation runs of the GISS Model II' atmospheric general circulation model subdivides the earth into 3312 horizontal boxes of dimension 4° NS × 5° EW. At this scale, it takes about 45 grid boxs to cover the United States. The center of the box designated (I,J) would be located at:
longitude: -177.5+(I-1)*5 (i.e., I=1 177.5W, I=2 172.5W ... I=72 177.5E)
latitude: -90 +(J-1)*4 (i.e., J=1 90S , J=2 86S ... J=46 90N)
The air column over each box starts at the mean elevation for that box and extends to the height where the pressure is 10 mb. The most frequently used grid scheme subdivides each column vertically into nine layers; hence the total number of 3D air boxes in the model is almost 30,000. For each of these boxes, the model keeps track of temperature, humidity, pressure, momentum (U and V wind), and other climate diagnostics.
A Model II' run on the grid scheme described above would be said to be on "the nine-layer medium grid", and so its run ID would end in "M9". Depending on the needs of the particular experiment, other grid schemes may be used. Thus, an experiment with greater vertical resolution might have a run ID which includes the designation "M23" or "M31". A simulation run on a nine-layer, 2°×2.5° model would be designated F9 (where F means "fine"), whereas a nine-layer, 8°×10° scheme would be designated C9 (where C means "coarse").
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