Methods - Quality Control

GHCN-Daily data have been subjected to a variety of internal consistency, frequent-value, outlier, and spatial consistency checks. The 15 temperature, 12 precipitation, 14 snowfall, and 12 snow depth checks are performed in a deliberate sequence, with each procedure operating on only those values that did not fail any of the preceding tests. The performance of each check has been rigorously evaluated using the methodology of Durre et al. (2008) in order to minimize the numbers of false positives and misses.

A list of all checks performed is provided below. Checks are divided into four groups: temperature checks, precipitation checks, snowfall checks, and snow depth checks. Within each group, procedures are listed in the order in which they are applied. Further details about the procedures will be contained in a journal article which is currently in preparation.

Temperature Checks

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 -17.8°C (0°F) at United States stations.
Duplicate checks - Check 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 to 20 identical values in time series of nonmissing daily maximum or minimum temperature.
Gap check - Identifies maximum/minimum temperatures that are at least 10°C 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 40°C warmer than the minimum temperatures on the preceding, current, and following days as well as minimum temperatures that are at least 40°C 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 25°C.
Spatial consistency check - checks for temperatures whose anomalies differ by more than 10°C 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.

Precipitation Checks

Trace flag consistency check - Checks for days on which the data measurement flag indicates a trace of precipitation yet the precipitation amount is nonzero.
Duplicate checks - Check for duplication of the data between entire years, different years within the same calendar months, and different months within the same year, using only years and months with at least three nonzero totals.
Bounds check - Identifies precipitation totals that exceed the world extreme for the highest 24-hour total ever observed.
Streak check - Checks for sequences of 10 or more consecutive identical values in Time series of nonzero precipitation observations.
Frequent-value check - Checks for clusters of 5-9 identical moderate to heavy totals in time series of nonzero precipitation observations.
Gap check - Identifies values that are at least 300 mm larger than all other precipitation totals for a given station and calendar month.
Climatological outlier check - Checks for daily precipitation totals that exceed the respective 29-day climatological 95th percentiles by at least a certain factor (9 when the day's mean temperature is above freezing, 5 when it is below freezing).
Snowfall to precipitation ratio check - Checks for cases in which snowfall is excessively large compared to precipitation. I.e., if the current day's snowfall is more than 100 times larger than both the current+previous and current+following days' precipitation sums, then the current day's precipitation and snowfall totals fail the check.
Snow depth increase to precipitation ratio check - Checks for cases in which a day-to-day increase in snowed depth is excessively large compared to precipitation. I.e., if the snow depth increase between the previous and current days is more than 100 times larger than both the current+previous and current+following days' precipitation sums, the current day's precipitation total and the current and previous day's snow depths fail the check.
Spatial consistency check - Checks for precipitation totals that differ significantly from totals reported at neighboring stations on the preceding, current, and following days.

Snowfall

Trace flag consistency check - Checks for days on which the data measurement flag indicates a trace of snowfall yet the snowfall amount is nonzero.
Duplicate checks - Check for duplication of the data between entire years, different years within the same calendar months, and different months within the same year, using only years and months with at least three nonzero totals.
Bounds check - Identifies snowfall totals that exceed the world extreme for the highest 24-hour total ever observed.
Streak check - Checks for runs of 10 or more consecutive days on which the same nonzero snowfall total is reported.
Warm check - Checks for nonzero snowfall totals that occur when daily minimum temperatures at the same station are equal to or warmer than 7°C on the preceding, current, and following days.
Snowfall to snow depth increase consistency check - Checks for days on which the increase in snow depth from the previous day to the current day exceeds the current+previous and current+following days' snowfall sums by more than 25 mm. In such cases, the current day's snowfall and snow depth as well as the previous day's snow depth fail the check.
Snowfall to precipitation ratio check - Checks for cases in which snowfall is excessively large compared to precipitation. See the same entry under the Precipitation Checks above.
Spatial warm check - Checks for nonzero snowfall totals that occur when daily minimum temperatures at neighboring stations are equal to or warmer than 7°C on the preceding, current, and following days.
Megaconsistency checks - Look for (1) nonzero snowfall totals for stations and calendar months whose lowest reported minimum temperature is 7°C or warmer and (2) warm season nonzero snowfall totals at stations where no valid cold season snowfall was ever reported.
Climatological outlier check - Identifies daily nonzero snowfall totals on dates outside the likely snow season as calculated from the snowfall records at the station and its suitable neighbors.

Snow Depth

Trace flag consistency check - Checks for days on which the data measurement flag indicates a trace of snow depth yet the snow depth amount is nonzero.
Bounds check - Identifies snow depths that exceed the world extreme for the highest snow depth ever observed.
Temporal consistency check - Checks for day-to-day increases in snow depths that exceed the world record 24-hour snowfall total.
Streak check - Checks for runs of 90 or more consecutive days on which the same nonzero snow depth is reported.
Gap check - Identifies values that are at least 35 cm larger than all other reported snow deaths for a given station and calendar month.
Warm check - Checks for day-to-day increases in snow depth that occur when daily minimum temperatures at the same station are equal to or warmer than 7°C on the preceding, current, and following days.
Snowfall to snow depth increase consistency check - Checks for days on which the increase in snow depth from the previous day to the current day exceeds the current+previous and current+following days' snowfall total by more than 25 mm. See the same entry under Snowfall consistency check.
Snow depth increase to precipitation ratio check - Checks for cases in which a day-to-day increase in snow depth is excessively large compared to precipitation. See the same entry under Precipitation Checks above.
Spatial warm check - Checks for day-to-day increases in snow depth that occur when daily minimum temperatures at neighboring stations are equal to or warmer than 7°C on the preceding, current, and following days.
Megaconsistency checks - Look for (1) nonzero snow depths for stations and calendar months whose lowest reported minimum temperature is 7°C or warmer and (2) warm season nonzero snow depths at stations where no valid cold season snow depth was ever reported.
Climatological outlier check - Checks for day-to-day increases in snow depth on dates that fall outside the likely snow season as calculated from the snow depth records at the station and its suitable neighbors.