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Under construction. This combines information from:
The information presented here will be made available to the public under the History of EMOSLIB changes. |
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While addressing a sub-optimal performance issue for interpolation from the octahedral reduced Gaussian grid introduced with IFS cycle 41r2 to regular latitude-longitude grids, a problem was discovered with the method used to calculate the longitudinal points in the source grid. In some specific cases, this problem leads to an incorrect computation of interpolation weights and hence to incorrect interpolated values at some points of the output grid. The problem affects all versions of EMOSLIB prior to version 4.3.0. The problem is also present in fields retrieved with versions of MARS (including the WebAPI) that use 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.
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The error also affects the identification of points used for the land-sea mask processing of surface fields.
Which interpolations are affected ?
The problem affects interpolations from:
- input original reduced Gaussian grid point fields to output regular Gaussian or regular latitude-longitude grids (without rotation)
- input octahedral reduced Gaussian grid point fields to output regular Gaussian or regular latitude-longitude grids (without rotation).
Which interpolations are not affected ?
- Interpolations to rotated latitude-longitude grids are unaffected
- Transformations from spherical harmonic components to grid point fields are unaffected.
- Interpolations of wave (WAM, ENS-WAM etc) fields are unaffected.
For which cases is the problem fixed ?
The problem is fixed in EMOSLIB version 4.3.0 and newer for the following cases:
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In addition, the problem is fixed for interpolations from original reduced or octahedral reduced Gaussian grids to sub-areas of unrotated regular Gaussian grids or regular lat-lon grids at EMOSLIB 4.3.6.
For which cases does the problem still exist ?
The 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 4.3.6 and newer. This is used by:
- MARS client to be updated in February 2016 (until then it is available with "mars -t")
- Metview version 4.6.3 and newer
- Disseminated products for IFS cycle 41r2 (implementation planned for March 2016)
Examples
Mean sea-level pressure
The plots show the differences between the new interpolation method implemented in EMOSLIB version 4.3.0 and the old method for the interpolation of the mean sea-level pressure 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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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 version 4.3.0 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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