Under construction. This combines information from: The information presented here will be made available to the public under the History of EMOSLIB changes. |
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. |
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 in the source grid.
- 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.
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).
When the input grid is an octahedral reduced Gaussian ("O-grids"):
- the error can occur at almost all lines of latitude because the Nj changes continuously
- the error is largest close to the equator where the resolution is highest.
In situations where the error occurs, the incorrect computation of the longitude values leads to two issues:
- The longitude points in the source grid are computed incorrectly. This leads to an incorrect computation of interpolation weights and hence to incorrect values at some points of the output grid.
- The nearest grid points used for the interpolation may be incorrectly identified due to a numerical 'shift' of the input grid cell.
The error is most evident for parameters where the gradient of the field is large and where a change in the nearest grid points or the interpolation weights used thus has a larger effect.
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 issue 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).
In which software versions is the problem fixed ?
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 latitude-longitude grids.
For which cases does the problem still exist ?
The issue has not been fixed for the following cases
- interpolations from original reduced or octahedral reduced Gaussian grids to sub-areas of regular Gaussian grids (gridType=regular_gg);
- interpolations from original reduced or octahedral reduced Gaussian grids to sub-areas of unrotated regular latitude-longitude grids (gridType=regular_ll).
Examples
Mean sea-level pressure
The plots show the differences between the new interpolation method implemented in EMOSLIB cycle 000430 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.
For this field the differences are:
- 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
Global  Global differences between old and new methods for interpolation of mean sea-level pressure from the O1280 octahedral reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid for the period 1-14 December 2015. |
Global differences between old and new methods for interpolation of mean sea-level pressure from the N640 original reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid for the period 1-14 December 2015. |
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Sub-area
Differences between old and new methods for interpolation of mean sea-level pressure from the O1280 octahedral reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid over Europe for the period 1-14 December 2015. |
Differences between old and new methods for interpolation of mean sea-level pressure from the N640 original reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid over Europe for the period 1-14 December 2015. |
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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.
For this field the differences are:
- 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
Global 
Global differences between old and new methods for interpolation of 2m temperature from the O1280 original reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid for the period 1-14 December 2015. |
Global differences between old and new methods for interpolation of 2m temperature from the N640 original reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid for the period 1-14 December 2015. |
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Sub-area
Differences between old and new methods for interpolation of 2m temperature from the O1280 octahedral reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid over Europe for the period 1-14 December 2015. |
Differences between old and new methods for interpolation of 2m temperature from the N640 original reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid over Europe for the period 1-14 December 2015. |
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