Description and assessment of the OSTIA reanalysis. - Met Office

A background of the previous day’s bias field, scaled by 0.9, is used together with a longer correlation
length scale of 700km. This bias field is then interpolated to the satellite observation locations
and the bias value is removed from the measurements prior to assimilation.
(a) AATSR observations
(b) AVHRR observations
(c) All in-situ observations
Figure 3.4: Global observation minus background RMS (red) and Mean (black) statistics for preliminary
run of the OSTIA reanalysis
The decision not to use ATSR-1 data as a reference data set was taken following results from
a preliminary version of the reanalysis in which all (A)ATSR missions were used as reference.
Figure 3.4 shows the observation minus background statistics for (A)ATSR, AVHRR Pathfinder and
in-situ observation (see section 4.1 for description of how the statistics where calculated) for the
preliminary version of the reanalysis for the full reanalysis time period. It is evident that the use of
the ATSR-1 data as a reference data set in the bias correction has had a detrimental impact on both
the mean and the RMS of the observation minus background values for both the in-situ and AVHRR
Pathfinder observations. This is clearly seen in the disruption to the seasonal cycle in both. The
statistics recover towards the end of the ATSR-1 period and once the ATSR-2 data comes online in
June 1995.
Figure 3.5 is the similar to figure 3.4 but for 1990-1992 only. On the left are statistics from the
© c Crown Copyright 2011 29