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FG departures for 2D-OI (2m temperature, relative humidity and snow depth analysis) are calculated at in fg2obs.F90. At first, observations are sorted by OBS_RAWTABLE(jpraw_refcoord, jtotal) based on latitude and longitude (prepared at in scan_cma_odb.F90).

            izlat = int( zlat * 100.0_JPRB)
            izlon = int( zlon * 100.0_JPRB)
            i_refcoord = sign(1,izlat) * (abs(izlat) * 100000 + izlon)
            OBS_RAWTABLE(jpraw_refcoord, jtotal) =&
              &sign(1,i_refcoord) * (abs(i_refcoord) +&
              &1 * 0.01_JPRB)

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The observations should be sorted in ascending order by latitude because FG departures are calculated in turn from the south pole to the north pole by the following code.

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However, latitude is rounded down to the second decimal place when OBS_RAWTABLE(jpraw_refcoord, jtotal) is calculated. As a resultsresult, the indexes indices are sometimes not in ascending order precisely and the FG departures are not calculated from surrounded 4 model grids. The following table shows the an example. ZOBSLAT should be at the latitude between ALAT(jlatN) and ALAT(jlatS) but it is not there (at the south side of ALAT(jlatS)).

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I found there are about 200 of snow depth observations and 400 of 2m temperature observations affected by this bug (at 0 UTC on 3 Dec 2020). You can see the list of affected observations in affected_points.xlsx (note that this list comes from a 49r1 experiment including snow DA changes). In order to modify the bug, it seems to be better to sort observations in ascending order by latitude directlywithout rounding down.

!**   Used to compute averages (always in the same order)

!call KEYSORT(irc, POBS, KLEN,key=jpraw_refcoord,transposed=.TRUE.)
call KEYSORT(irc, POBS, KLEN,key=kplat,transposed=.TRUE.)

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