Under construction.

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 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 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 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 incorrect computation of the longitude values leads to two potential 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. Generally, the area to the west of the Andes and close to the equator suffers the greatest from this error. This is because of the high gradients of many of the fields and its latitudinal proximity to the equator and to the west of the 0° meridian where the error is generally largest.

Which interpolations are affected ?

The problem affects interpolations from input octahedral original reduced Gaussian grid point fields to output regular Gaussian or regular latitude-longitude grids.

Which interpolations are not affected ?