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While addressing a sub-optimal performance issues issue for interpolating interpolation from the octahedral reduced Gaussian grid introduced with IFS cycle 41r2 to regular latitude-longitude grids, an issue a problem was discovered with the method used to calculate the longitudinal points in the source grid. In some specific cases, this issue problem leads to an incorrect computation of interpolation weights and hence to incorrect interpolated values at some points of the output grid. The issue problem 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 any EMOSLIB version 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
The problem originates in the calculation of the longitude values of the source grid in EMOSLIB routines IRDIWE and IGDIWE. These routines use an integer value of the longitudinal grid increment (the 'stride') which, in some cases, results in a truncated value of the increment. This can result in an error in the computation of the longitude values for points in the source grid.
- The error occurs whenever the grid spacing Dj=360/Nj at a specific latitude linelines, j, with Nj longitude points whenever the grid spacing Dj=360/Nj 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.
When the input grid is an original reduced Gaussian ("N-grids"):
- the error occurs in at bands of consecutive lines of latitude that have the same Nj.
- generally, the error is largest away from the equator (at about 20° N and 20° S for the N640 grid).
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For which cases is the problem fixed ?
The issue problem is fixed in EMOSLIB cycle 000430 and newer for the following cases:
- interpolations from original reduced or octahedral reduced Gaussian grids to global, unrotated regular Gaussian grids (gridType=regular_gg);
- interpolations from original reduced or octahedral reduced Gaussian grids to global, unrotated latitude-longitude grids (gridType=regular_ll).
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The issue is fixed in EMOSLIB cycle 000430 and newer. This is used by:
- MARS client to be updated in February 2016 (until then it is available with "mars -t")
- Metview version 4.6.1 and newer
- Disseminated products for IFS cycle 41r2 (implementation planned for March 2016)
For the Disseminated products from IFS cycle 41r2, the problem is also fixed for interpolations to sub-areas of regular latitude-longitude grids.
For which cases does the problem still exist ?
The issue problem has not been fixed for the following cases
- interpolations (with MARS and Metview) from original reduced or octahedral reduced Gaussian grids to sub-areas of regular Gaussian grids (gridType=regular_gg);
- interpolations (with MARS and Metview) from original reduced or octahedral reduced Gaussian grids to sub-areas of unrotated regular latitude-longitude grids (gridType=regular_ll).
- interpolations from sub-areas of regular Gaussian grids or unrotated regular latitude-longitude grids.
In which software versions is the problem fixed ?
The problem is fixed in EMOSLIB cycle 000430 and newer. This is used by:
- MARS client to be updated in February 2016 (until then it is available with "mars -t")
- Metview version 4.6.1 and newer
- Disseminated products for IFS cycle 41r2 (implementation planned for March 2016)
Examples
Mean sea-level pressure
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- between -49.26 Pa and 37.76 Pa for interpolation from an the N640 original reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid
- between -169.26 Pa and 146.26 Pa for interpolation from an the O12800 octahedral reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid
The plots in the upper panels show differences at the global level while those in the lower panels show the same fields plotted over the Europe area in a polar stereographic projection.
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2 metre temperature
The plots show the differences between the new interpolation method implemented in EMOSLIB cycle 000430 and the old method for the interpolation of the 2m temperature from the reduced Gaussian grids to a 0.5°x0.5° regular latitude-longitude grid for the period 1-14 December 2015.
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- between -8.21 K and 4.79 K for interpolation from an the N640 original reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid
- between -3.62 K and 4.44 K for interpolation from an the O1280 octahedral reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid
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