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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, version 4.3.0, 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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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 at specific latitude lines, 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.
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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 4.3.67.
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 gridsglobal "staggered" regular lat-lon grids (i.e., global grids that are not centred on 0° latitude and 0° longitude).
In which software versions is the problem fixed ?
The problem is fixed in EMOSLIB cycle version 4.3.6 7 and newer. This is used by:
- MARS client to be updated in on 9 February 2016 (until then it is available with "mars -t")
- Metview version 4.6.3 4 and newer
- Disseminated products for IFS cycle 41r2 (implementation planned for March 2016)implemented 8 March 2016)
Can results obtained with EMOSLIB 4.3.x be compared with the old version ?
Users can check results with the previous versions of MARS by using 'mars -t -p'. This version of MARS uses EMOSLIB 4.2.2 which does not include the bug fix.
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The 'mars -t -p' version is provided for testing purposes only. It should not be used as a long term replacement for the default 'mars' version. Please let ECMWF know if you need to use 'mars -t -p'. |
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 O1280 octahedral reduced Gaussian grid to a 0.5°x0.5° regular latitude-longitude grid
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