The atmospheric model uses a Reduced Gaussian Octahedral grid.  This is triangular in nature. NWP model values are interpolated both onto and from the grid by MIR (Meteorological Interpolation and Regridding).  For output purposes, MIR uses a triangular interpolation technique.  This uses model grid point values (black points) to interpolate a value for an off-grid location (red point).


Fig3.3-1: The Reduced Gaussian grid is triangular in nature.  The interpolation uses the three corner points (black points) closest to the selected location (red point).  It then takes a weighted average based upon the proximity of the point to the to vertices to arrive at the interpolated value.  

A value for point P is derived using a weighting factor for each point. The weighting for:

  • Point A is equal to the area of triangle PBC (pink) divided by area of triangle ABC. (Weighting  = WA).
  • Point B is equal to the area of triangle PCA (cyan) divided by area of triangle ABC. (Weighting  = WB).
  • Point C is equal to the area of triangle PAB (green) divided by area of triangle ABC. (Weighting  = WC).
  • The value at point P is then the sum of these three contributions: i.e. P = (A x WA) + (B x WB) + (C  x WC)


A special case then arises when Point P lies on the line directly between two points. (see Fig3.3-2).

Fig3.3-2: Sometimes a point lies directly between two grid points. The interpolation uses the two points (black points) each side of selected location (red point).  It then takes a weighted average based upon the ratio of distances from the two end points to arrive at the interpolated value.

A value for point P is derived using a weighting factor for each end point. The weighting for:

  • Point A is equal to the distance PB divided by the length of AB. (Weighting  = WA).
  • Point B is equal to the distance AP divided by the length of AB. (Weighting  = WB).
  • The value at Point P is then the sum of these two contributions: i.e. P = (A x AW) + (B x BW)