Description - Source Data
GHCN-Daily source data have been obtained via personal contacts with representatives from National Meteorological and Hydrological Centers(NMHCs) around the world, through official bi-lateral agreements, and from a number a data archives at NCDC. These sources fall into four broad categories: (1) the U.S. Collection; (2) the International Collection; (3) Government Exchange Data; and (4) the Global Summary of the Day. A brief synopsis of each of these categories follows. Note that the source for each datum is indicated by a source flag as described in the readme.txt file.
The U.S. Collection contains daily data from a dozen separate datasets archived at NOAA NCDC. These archives provide some of the earliest observations available for the United States (from the U.S. Forts and Voluntary Observer Program covering much of the 19th century) to the latest measurements from the state-of-the-art climate monitoring stations that make up the U.S. Climate Reference Network. GHCN-Daily thus contains the most complete collection of U.S. daily climate summaries available. Currently active U.S. stations in GHCN-Daily are updated both via real-time data feeds and via time-delay, archive-quality data feeds. U.S. data users formerly accustomed to receiving “final” archived data (e.g. for DSI-3200) should confirm that a real-time data source for a particular station/day has been replaced by an archive-quality data source. Real-time data sources include the High Plains Regional Climate Center data feed (source flag=”H”) and real-time ASOS summaries (source flag=”A”), which are subject to change upon receipt of archive-quality sources. Archive quality data sources are normally available 45-60 days following the close of a data month and include DSI-3207 (WxCoder3 data; source flag=”7”); keyed paper Cooperative Observer forms (source flag=”K”); and DSI 3210 (“First Order Summary of the Day”; source flag=”X”).
Since GHCN-Daily is continually reprocessed and because late changes/additions can be received even for archive-quality data sources, a data value and/or quality flag is subject to change even after all archive quality sources are integrated. However, changes to data values for U.S. data should be rare 60 days after the close of a particular data month.
The International Collection contains historical records for approximately 20000 locations outside the United States (from over 100 different countries) and largely reflects the data collection efforts that led to the release of Global Daily Climatology Network dataset (the predecessor to GHCN-Daily). International Collection records were generally obtained through personal contacts in various countries. Contributions to the International Collection have resulted in particularly dense station networks with daily precipitation totals in Brazil, South Africa, and India, although the summaries from this collection are purely historical and are not updated. Precipitation records end generally in the late 1990s for Brazil and South Africa, and in 1970 for India.
Government Exchange Data refers to data collected through official GCOS or bilateral agreements. In the best case scenario, an NMHC may offer its complete digital, daily climate database for inclusion in GHCN-Daily, which is the case for Canada (with over 7500 station records provided) and Australia (with over 17000 station records). In other cases, NMHC’s have provided daily data only for the GCOS Surface Network stations under their jurisdiction. To date, 76 different countries have officially provided daily data for just over half of the 1025 GCOS Surface Network stations that make up the GSN network implementation for the year 2009 (GCOS, 2009). Apart from Canada and Australia, Government Exchange Data are regularly updated for only a handful of stations, but new historical contributions are periodically added to this collection.
The Global Summary of the Day contains 24-hour summaries encoded in the special “climatological code” group transmitted with SYNOP reports on the GTS. These reports are archived in NCDC's Integrated Surface Dataset (DSI-3505) and the 24-hour summary period purportedly ends at midnight UTC (i.e., 24 Z). Daily maximum and minimum temperatures from this source are included in GHCN-Daily only when provided as a nominal 24-hour climatological summary as indicated in the SYNOP messages, whereas daily precipitation totals are also included when they must be summed from two 12-hour or four 6-hour sub-totals (as provided in standard SYNOP code groups). Sub-daily summations are identified by associated “measurement” flag codes in the GHCN-Daily data format. In any case, daily summaries from the Global Summary of the Day may differ significantly from climate summaries with 24-hour periods ending at local midnight, particularly in the case of precipitation. Nevertheless, data from this GTS source are available for a number of locations that are not contained in any other data archive available to NCDC, and they provide the only source of updates for many stations.