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While addressing sub-optimal performance issues for interpolating from the octahedral reduced Gaussian grid introduced with IFS cycle 41r2 to regular latitude-longitude grids, an issue was discovered with the method used to calculate the longitudinal points in the source grid. In some specific cases, this issue leads to an incorrect computation of interpolation weights and hence to incorrect interpolated values at some points of the output grid. The issue affects all versions of EMOSLIB prior to cycle 000430. The problem is also present in fields retrieved with versions of MARS (including the WebAPI) that use and EMOSLIB prior to cycle 000430, Metview versions prior to 4.6.1 and for disseminated products from IFS cycle 41r1 and earlier. This page provides information about the problem and the specific cases where differences can occur. |
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Description of the problem
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- The error occurs whenever the grid spacing Dj=360/Nj at a specific latitude line, j, with Nj longitude points has a remainder smaller than 10 microdegrees (1/100000 of a degree).
- The error accumulates linearly from 0° to 360° along the line of latitude.
- The error is minimal at grid points to the east and maximal at those to the west of the 0° meridian.
- The error is larger the greater the number of longitude points (Nj) along the line of latitude.
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- Interpolations to rotated latitude-longitude grids are unaffected
- Transformations from spherical harmonic components to grid point fields are unaffected.
- Wave ?
Examples