Table 4. Values of relevant PRISM interpolation parameters.
Regression slope bounds are sometimes different among regions; values separated by forward slashes are for west, central, and east regions, respectively. See [7] and [8] for details on PRISM parameters and Table 3 for citations to details on weighting function formulations.
| Name | Description | T dmean | VPD min | VPD max |
|---|---|---|---|---|
| Regression Function | ||||
| R | Radius of influence | 20 km* | 20 km* | 20 km* |
| s t | Minimum number of total stations desired in regression | 25 stations | 25 stations | 25 stations |
| β 1ma | Min valid regression slope, layer 1 | -0.5 + | -10/-1/-1.5 # | 0.99/0/0 # |
| β 1xa | Max valid regression slope, layer 1 | 2.5 + | 10/2/1 # | 1.01/1.5/1.5 |
| β 1da | Default regression slope, layer 1 | 0.5 + | 1/0.5/0.5 # | 1/1/1 # |
| β 1mb | Min valid regression slope, layer 2 | -0.5 + | -10/-1/-1 # | 0.99/0/0 # |
| β 1xb | Max valid regression slope, layer 2 | 2.5 + | 10/2/1 # | 1.01/1.5/1.5 # |
| β 1db | Default regression slope, layer 2 | 0.5 + | 1/1/0.5 # | 1/1/1 # |
| Distance Weighting | ||||
| A | Weighting exponent | 1.5 | 1.5 | 1.5 |
| F d | Importance factor | 0.5 | 0.5 | 0.5 |
| D m | Minimum allowable distance | 0 km | 0 km | 0 km |
| Elevation Weighting | ||||
| B | Weighting exponent | 1.5 | 1.5 | 0.5 |
| F z | Importance factor | 0.5 | 0.5 | 0.5 |
| Δz m | Station-grid cell elev difference below which weight is maximum | 100 m | 100 m | 100 m |
| Δz x | Station-grid cell elev difference above which weight is zero | 5000 m | 5000 m | 5000 m |
| Coastal Proximity Weighting | ||||
| v | Weighting exponent | 1.0 | 1.0 | 1.0 |
| Δp m | Station-grid cell proximity difference below which weight is maximum | 100 $ | 100 $ | 100 $ |
| Δp x | Station-grid cell proximity difference above which weight is zero | 300 $ | 300 $ | 300 $ |
| Vertical Layer Weighting | ||||
| y | Weighting exponent | 0.5 | 0.5 | 0.5 |
| Δl m | Station-grid cell distance to adjacent layer below which weight is maximum | 100 m | 100 m | 100 m |
| Δl x | Station-grid cell distance to adjacent layer above which weight is zero | N/A | N/A | N/A |
| Topographic Position Weighting | ||||
| μ | Weighting exponent | 1.5 | 1.5 | 1.5 |
| Δt m | Station-grid cell topo difference below which weight is maximum | 50 m | 50 m | 50 m |
| Δt x | Station-grid cell topo difference above which weight is zero | 900 m | 900 m | 900 m |
* Expands as needed to encompass minimum number of total stations desired in regression (s t).
+ Slope is expressed as the change in dew point depression (DPD) per unit Tmin from the predictor grid
# Slope is expressed as the change in VPD per unit first-guess VPD from the predictor grid
$ Units are effective distance in grid cells from the coast, incorporating terrain barriers