Description and assessment of the OSTIA reanalysis. - Met Office

Quality level 3,4 and 5 Quality level 5
Regions Mean (RMS) No. of obs Mean (RMS) No. of obs
Global -0.24(0.84) 1579712 -0.06(0.86) 414936
North Atlantic -0.08(0.83) 245197 -0.05(0.93) 61849
North Pacific -0.24(0.86) 361846 -0.04(0.91) 102795
South Pacific -0.29(0.76) 344913 -0.04(0.70) 74871
South Atlantic -0.24(0.77) 100920 -0.02(0.71) 20272
Mediterranean 0.08(0.78) 36333 0.14(0.77) 22344
Arctic -0.15(1.21) 31508 -0.15(1.12) 20037
Southern Ocean -0.34(0.88) 160925 -0.07(0.78) 36732
Indian -0.30(0.82) 271524 -0.18(0.73) 63824
Tropics -0.32(0.78) 847599 -0.16(0.73) 95589
Table 3.1: Average regional ATSR-1 observation minus background bias(RMS) in K and no. of
observations for experimental runs using ATSR-1 quality level 3,4 and 5 and 5 data
It is worth noting that in the tropics mean bias is decreased from 0.12K to 0.07K and the RMS
decreases from 0.88K to 0.87K. As the in-situ observation statistics are used to quantify bias and
accuracy for validation purposes and these showed improvement in the run with the lower quality
data both globally and in the Tropics where the additional ATSR-1 data was located, the decision
was made to use data of quality level 3,4 and 5 in the reanalysis.
A further set of experiments was carried out to ascertain whether the ATSR-1 data (levels?)
should be bias corrected to the in-situ observations or assimilated without any bias correction. Two
experimantal runs were carried out for August and September 1991. Tables 3.4 and 3.4 show the
observation minus background statistics globally and regionally for the ATSR-1 and in-situ observations
averaged over August and September for runs with and without bias correcting the ATSR-1
data. The tables show the expected statistical outcome of the bias correction to the insitu data with
the ATSR-1 observational bias increasing globally from -0.11K to -0.24K when the bias correction
is carried out, whilst as expected the in-situ bias decreases from 0.05K to 0K. The global ATSR-1
observation minus background RMS increases from 0.76K to 0.84K whilst the in-situ observation
minus background RMS remains unchanged when the bias correction is carried out. The same
pattern is seen across almost all the regions. Taking into account the in-situ results and the desire
to make the different sources of observations as consistent as possible the decision was made to
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