WRF - Developmental Testbed Center

1. four_pt : Four-point bi-linear interpolationThe four-point bi-linear interpolation method requires four valid source points a ij ,1 ≤ i, j ≤ 2 , surrounding the point (x,y), to which geogrid or metgrid must interpolate, asillustrated in the figure above. Intuitively, the method works by linearly interpolating tothe x-coordinate of the point (x,y) between a 11 and a 12 , and between a 21 and a 22 , and thenlinearly interpolating to the y-coordinate using these two interpolated values.2. sixteen_pt : Sixteen-point overlapping parabolic interpolationThe sixteen_pt overlapping parabolic interpolation method requires sixteen valid sourcepoints surrounding the point (x,y), as illustrated in the figure above. The method works byfitting one parabola to the points a i1 , a i2 , and a i3 , and another parabola to the points a i2 ,a i3 , and a i4 , for row i, 1≤ i ≤ 4; then, an intermediate interpolated value p i within row i atthe x-coordinate of the point is computed by taking an average of the values of the twoparabolas evaluated at x, with the average being weighted linearly by the distance of xfrom a i2 and a i3 . Finally, the interpolated value at (x,y) is found by performing the sameoperations as for a row of points, but for the column of interpolated values p i to the y-coordinate of (x,y).WRF-NMM V3: User’s Guide 3-47