The GHCNM v2 contained several thousand stations that had multiple time series of monthly mean temperature data. The 12th digit of each data record, indicated the time series number, and thus there was a potential maximum of 10 time series (e.g. 0 through 9). These same stations in v3 have undergone a merge process, to reduce the station time series to one single series, based upon these original and at most 10 time series.
A simple algorithm was applied to perform the merge. The algorithm consisted of first finding the length (based upon number of non missing observations) for each of the time series and then combining all of the series into one based upon a priority scheme that would "write" data to the series for the longest series last.
Therefore, if station A, had 3 time series of TAVG data, as follows:
1900 to 1978 (79 years of data) [series 1]
1950 to 1985 (36 years of data) [series 2]
1990 to 2007 (18 years of data) [series 3]
The final series would consist of:
1900 to 1978 [series 1]
1979 to 1985 [series 2]
1990 to 2007 [series 3]
The original series number in GHCNM v2, is retained in the GHCNM v3 data source flag.
One caveat to this merge process, is that in the final GHCNM v3 processing there is still a master level construction process performed daily, where the entire dataset is construction according to a source order overwrite hiearchy (section 2.3), and it is possible that higher order data sources may be interspersed within the 3 series listed above.
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