Table of Contents - The Atmospheric Studies Group at TRC

Section 8: CALMET Model Files
8.10 Terrain Weighting Factor Data File (WT.DAT)
CALMET contains several options for introducing MM4/MM5 winds into the calculation of the wind
fields. These include the use of the MM4/MM5 winds as:
• Step 1 field (IPROG = 3 or 13)
• initial guess field (IPROG = 4 or 14)
• "observation" (IPROG = 5 or 15)
If the MM4/MM5 fields are used as an initial guess field for CALMET, the MM4/MM5 winds are subject
to a full diagnostic adjustment for terrain effects on the fine-scale (CALMET) grid. But if the
MM4/MM5 winds are used as either a Step 1 field or as "observations," CALMET does not perform
additional terrain adjustment to the MM4/MM5 winds. When combining these MM4/MM5 winds with
observed winds, local near-surface effects captured in the observations may be lost due to the scale of the
terrain used in the MM4/MM5 simulations (e.g., 80 km resolution). To avoid this, CALMET accepts a
three-dimensional grid of terrain weighting factors. The weight W o is applied to the observation, and its
complement (1-W o ) is applied to the MM4/MM5 wind. The factors used to determine this weighting are
assumed to be a function of the fine-scale terrain unresolved by the MM4/MM5 grid, and height above
the surface.
The WT.DAT file contains the terrain-weighting factor. This file is required only if IPROG = 3, 13 or
IPROG = 5, 15 (i.e., MM4/MM5 data are used as the Step 1 field or as "observations").
Table 8-24 contains a sample WT.DAT file for a 25 23 18-km CALMET grid. A detailed description
of the contents of the WT.DAT file are contained in Table 8-25. The first three lines consist of
descriptive information on the development of the weighting factor. Records 4 and 5 describe the finescale
(CALMET) grid system and the coarse-scale (MM4/MM5) grid. These are followed by a set of NZ
groups of records, one for each CALMET layer, which contain the actual weighting factors.
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