I haven't yet looked at daily data, so this is work in progress.
I was unaware daily GHCN data was available to download online, but it appears that is the case:
www1.ncdc.noaa.gov/pub/data/ghcn/daily/readme.txtA 1.7gb data file contains minimum and maximum daily temperature values for a couple thousand stations.
It looks like no adjustments are made to daily values as part of GHCN:
At present, GHCN-Daily does not contain adjustments for biases resulting from historical changes in instrumentation and observing practices. However, there is ongoing work at NCDC to develop adjustments that can be applied to daily maximum and minimum temperatures, and a GHCN-daily-derived product containing adjusted daily temperatures may become available in the future.www.ncdc.noaa.gov/oa/climate/ghcn-daily/index.php?name=biasQuality Control tests are applied. It looks like if any of the daily data fail quality control tests the data has a flag added alongside it. The quality control tests are:
Naught check - Checks for days on which maximum and minimum temperature are both equal to 0°C at stations not operated by the United States or are both equal to -17.8ºC (0ºF) at United States stations.
Duplicate check - Checks for duplication of the data between entire years, different years in the same calendar month, different months within the same year, and maximum and minimum temperature within the same month.
Bounds check - Identifies maximum and minimum temperature that fall outside the world extremes for the highest and lowest temperatures ever observed.
Streak check - Checks for sequences of 15 or more identical values in time series of nonmissing daily maximum or minimum temperature.
Gap check - Identifies maximum/minimum temperatures that are at least 10ºC warmer or colder than all other maximum/minimum temperatures for a given station and calendar month.
Climatological outlier check - Checks for daily maximum and minimum temperatures that exceed the respective 15-day climatological means by at least six standard deviations.
Internal consistency check - Checks for days on which the minimum temperature is greater than the maximum temperature.
Interday consistency check - Checks for daily maximum temperatures that are less than the minimum temperatures on the preceding, current, and following days as well as for minimum temperatures that are greater than the maximum temperatures during the relevant three-day window.
Lagged range check - Identifies maximum temperatures that are at least 40ºC warmer than the minimum temperatures on the preceding, current, and following days as well as minimum temperatures that are at least 40ºC colder than the maximum temperatures within the three-day window.
Temporal consistency check - Checks whether a daily maximum (minimum) temperature exceeds the maximum (minimum) temperatures on the preceding and following days by more than 25ºC.
Spatial consistency check - checks for temperatures whose anomalies differ by more than 10ºC from the anomalies at neighboring stations on the preceding, current, and following days.
Megaconsistency check - Looks for daily maximum temperatures that are less than the lowest minimum temperature and for daily minimum temperatures that are greater than the highest maximum temperature for a given station and calendar month.
The GCHN site also lists the sources of the station data. There are about a dozen sources. Eg:
www.ncdc.noaa.gov/oa/climate/ghcn-daily/index.php?name=dataI will see how far they go back to figure out the chain, but already it should be possible to take the GHCN daily max and mins for a month and calculate the monthly mean and then compare that to the GHCN monthly mean database.