Description and assessment of the OSTIA reanalysis. - Met Office

(b) AVHRR observations minus background merid­
ional Hovmuller plot
(a) ATSR-1 observations minus background meridional
Hovmuller plot
Figure 4.4: Meridionally averaged observation minus background Hovmuller plot for ATSR-1 observations
(Left) and AVHRR Pathfinder observations (right).
the ATSR-1 data than in 4.4(b) for the AVHRR data is a greater cold bias at high latitudes which is
discussed shortly.
During this time period the OSTIA reanalysis system attempts to correct the biases in both the
ATSR-1 and the AVHRR Pathfinder data using the in-situ data as a reference data set as described
in section 3.4. Figures 4.3(c) and 4.3(d) show the average bias correction applied to the satellite
observations from both sources during the OSTIA reanalysis assimilation during August 1991.
Comparison of these figures with figures 4.3(a) and 4.3(b) illustrates that the correction applied
to the data resembles the spatial pattern of the bias observed and discussed previously. This is
consistent throughout the period of the ATSR-1 data. The spatial coverage of the in-situ reference
data set is particularly poor at high southern latituides during this time period. The bias correction
methodology is dependent on a significant number of match-ups being found betwen the observations
and the reference data set which cannot be achieved in the Southern Ocean during this time
period, thus the validity of the bias corrections applied at this time in the Southern Ocean and in
other undersampled regions may be in doubt. This problem may be exacerbated by possible cold
biases in the ATSR-1 data due to cloud contamination (Merchant, 2010, pers. comm.).
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