Description and assessment of the OSTIA reanalysis. - Met Office

quality control procedures including a maximum and minimum value check and a background check
against the previous day’s analysis after relaxation to climatology (section 3.3), before inclusion in
the OSTIA reanalysis. The standard deviation provided for each cell with the Pathinder data was
used as the observation error in the OSTIA reanalysis. For instances where the value in the cell was
derived from few observations and hence had a standard deviation close to 0, an error of 0.33 K
was assumed, following Kearns et al. (2000).
Pathfinder v5.0 includes some information on sea ice. This is used in the Pathfinder data processing
procedure to exclude pixels from the internal 3 week reference SST check mentioned
above. The aim of this approach is to reduce the likelihood of mis-classified SST pixels, for example
those with high quality levels but falling on the ice mask (NODC, 2009). The ice data is not
used to specifically flag the data as poor quality, although there are plans to include this in future
versions of the Pathfinder dataset (NODC, 2009). Thus ice-covered pixels should only be assigned
quality flags of 3 or below. However, it was found during investigations of a preliminary run of the
OSTIA reanalysis that this processing was unsuccessful in the Arctic in the boreal summer. A considerable
number of pixels in the ice-covered region are flagged as high quality in the summertime,
see section 3.2 for a description of how the OSTIA system quality controlled these erroneous SST
observations.
In addition, NODC (2009) notes the presence of erroneously cold SST values in high latitudes in
the Pathfinder dataset and recommends masking them out. As mentioned in section 3.2, a minimum
SST of -2.0 o C was set for all datasets used in the OSTIA reanalysis which removes the spurious
values.
2.3 (A)ATSR multi-mission series
The (A)ATSR multi-mission archive is a ESA/NEODC project to produce a 15-year archive of sea
surface temperatures using the (A)ATSR series of instruments at levels of accuracy of 0.3 K or
better (NEODC, 2011). The dataset begins in 1991 with the launch of the ATSR-1 instrument,
onboard the ERS-1 satellite. This was followed in 1995 by the ATSR-2, onboard ERS-2 and AATSR
onboard Envisat in 2002.
The ATSR series of instruments are multichannel radiometers which have three wavebands in
the infra-red part of the spectrum (Robinson, 2004). All three ATSR instruments employ the same
onboard calibration procedure for the thermal channels, whereby two high precision blackbodies
are viewed every scan cycle (NEODC, 2011; Robinson, 2004). The temperatures of the blackbodies
approximately straddle the ground temperature range to precisely calibrate the brightness
temperatures for each channel (Robinson, 2004). A cooling procedure is used to lower the detector
temperature, thereby improving the stability of the calibration procedure (Robinson, 2004).
The instruments employ a unique conical scanning geometry, using a single detector to view the
same area of ground twice; once at a forward angle of around 55 o and once at nadir (around
© c Crown Copyright 2011 17