NWP systems wait for observation data coming in a little amount of time after the time of observation. It is called cut-off time because the data coming after the time is virtually discarded as if it's on a cut wire of telegraph. How long should we wait is a question. The WMO Manual on GDPFS says the target of reporting is 3 hours after the observation time. How is the reality?
I found an article in HIRLAM newsletter:
Eerola, 2003: Statistics of the arrival times of conventional observations for HIRLAM at FMI
http://hirlam.org/index.php/component/docman/doc_view/634-hirlam-newsletter-no-44-article06-kalle-eerola?Itemid=70
The finding is
- Two hours is almost complete data for surface, aircraft and satob
- Upper-air observation namely TEMP and PILOT comes slower, and three hours needed for complete set
FMI seems to have shorten the cut-off from 2.5 hours to probably two hours. Three hours wait was considered way too much even with the additional upper-air data is taken into account.
One thing to note is the study was for european regional model, and may be a bit different in global model.
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