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Regarding the generation of the monitoring performance reports for land surface stations, there are four NWP monitoring centres to choose from: DWD, ECMWF, NCEP and JMA. For the upper-air land stations only ECMWF, JMA and DWD are available. Note that the four 6-hourly reports provided by the participating WIGOS monitoring Centres are based on the observations that were made available to the assimilation system at each Monitoring Centre, which is used as a proxy to for the observations available on the GTS. The way the four different NWP data assimilation systems deal with observations varies, which explains some of the differences seen on the performance reports across Centres. Below are highlighted some of the procedures used in the different NWP centres that should be taken into consideration when comparing the results between NWP centres. NCEP monitoring reports show only the data that pass a quality control step prior to DA, therefore part of the data available that is deemed to be of poor quality or duplicate is filtered out and not available to the DA. Another important aspect of its DA system is the assimilation of surface pressure observations provided by METAR reports - mainly over US - rather than from SYNOP reports when both reports are available in the same location, therefore some SYNOPs observations over this region may not show up in NCEP maps.
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