Impact tests on the assimilation of PAOBs
BUTTERWORTH, P. ; Dalby, T.D.
PAOBs are bogus measurements of mean sea-level pressure for the southern hemisphere produced by the Bureau of Meteorology, Melbourne. They are currently used in operational assimilation at ECMWF. Two low-resolution impact trials of the assimilation of PAOBs into the global UM were run to cover periods in March and June 1999. Assimilation of PAOBs, over sea only, produced changes in total weighted mean skill against analyses of +0.09% and -0.02% for March and June, respectively. Against observations the changes were +0.07% and +0.00%. Close inspection of the RMS forecast and persistence scores show that the SH forecast scores are generally improved. However, bigger changes in mean weighted skill were seen in tropical wind forecasts and northern hemisphere PMSL fields than expected. Plots of mean field differences (control minus test) show the main changes happening in the southern oceans around the edge of Antarctica, yet the signal is not large. The importance of a single observation (a drifting buoy) in a data-void area was exposed by the impact trial in June, as was a discrepancy between on-line and off-line MetDB retrievals. Since the overall changes between control and test skill scores and mean fields were very small, the result of the trials was considered to be neutral, and therefore PAOBs were not accepted for inclusion in the autumn package for 1999.
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