...

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.

...

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 version 4.3.7 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 4.6.4 and newer
• Disseminated products for IFS cycle 41r2 (implementation planned for 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.

Examples

Mean sea-level pressure

...

• 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

...