Section 2.0 contains a detailed description of the instrumentation, siting, and algorithms used by the source network to collect the data. Section 2.1 contains a detailed description of the format of the composite dataset. See Section 2.2 for information on data processing, and Section 3.0 below for the quality control processing performed by the University Corporation for Atmospheric Research/Joint Office for Science Support (UCAR/JOSS) on this dataset. Section 4.0 contains references.
The NCEP/EMC daily precipitation dataset was formed by extracting data with NCEP/EMC element codes of PC and PP that fall on the hour. The NCEP/EMC observations of daily precipitation can include multiple measurements for a given day. When this occurs, the precipitation for that day is set to missing in this UCAR/JOSS IHOP 2002 Daily Precipitation Composite. The value reported for any daily observation represents data collected during the previous 24 hours. All raw precipitation data can be found on codiac in the GCIP/EOP Surface: Precipitation NCEP/EMC Gage Only Daily Dataset. NCEP/EMC excludes any precipitation value greater than 20 inches from the NCEP/EMC daily precipitation dataset. The information above was gathered from personal correspondence with NCEP/EMC during 1997 and 1998.
Data from cooperators networks such as NCEP/EMC has hundreds of reporting locations which change from incremental to accumulator and back to incremental. Others changed a couple of times and have now settled on accumulators.
Anyone using this data must be aware of the dynamic changes in the network of sites that are processed by HADS and be aware of the potential lag time from when a change is implemted to when NCEP/EMC becomes aware of the change. This is far, far from a stable environment.
To help users in identifying these changes, UCAR/JOSS checks the NCEP/EMC daily precipitation data for possible accumulated values and resets the UCAR/JOSS Quality Control flags for all "unchecked", "good", and "trace" accumulated precipitation values in the NCEP/EMC network to "questionable". For this purpose, UCAR/JOSS defines accumulated precipitation to be six identical, sequential daily precipitation values potentially followed by additional identical values or values that increase over time.
Parameters Units
---------------------- -------------------------------------
Date of Observation UTC
Network Identifier Abbreviation of platform name
Station Identifier Network Dependent
Latitude Decimal degrees, South is negative
Longitude Decimal degrees, West is negative
Station Occurrence Unitless
Daily Precip Value millimeters
Quality Flag See list of Definitions
QC Code See list of Definitions
Observation hour Beginning UTC hour for 24 hour period
for which value is valid.
The NCDC Summary of Day Co-operative data have observation hours that vary by station. Data values are valid for the 24 hours preceding the observation hour given for the day.
UCAR/JOSS also checks the NCEP/EMC daily precipitation data for possible accumulated values and resets the UCAR/JOSS Quality Control flags for all "unchecked", "good", and "trace" accumulated precipitation values in the NCEP/EMC network to "questionable".
Gross limit checks were used to determine the quality of the data in this composite. Data values less than 101.6 millimeters (4 inches) were flagged as good. Any data value greater than or equal to 101.6 millimeters but less than 127 millimeters (5 inches) was flagged as questionable. Any data value greater than or equal to 127 millimeters was flagged as unlikely. Several questionable and unlikely data values were also manually inspected. After inspection, the quality control code and flag may have been manually updated to better reflect the likelihood of the actual occurrence of the precipitation value based on an isohyet analysis. UCAR/JOSS also checks the NCEP/EMC daily precipitation data for possible accumulated values and resets the UCAR/JOSS Quality Control flags for all "unchecked", "good", and "trace" accumulated precipitation values in the NCEP/EMC network to "questionable". For this purpose, UCAR/JOSS defines accumulated precipitation to be six identical, sequential daily precipitation values potentially followed by additional identical values or values that increase over time. Data were never modified, unless the precipitation value was negative or too large to be represented in the output format. In these cases the data value was flagged with a QC code of 'C' and changed to a missing value (i.e., '-999.99'). Table 1 and 2 list the quality control flags and codes plus their meanings. After the datasets were merged to form the IHOP 2002 Daily Precipitation Composite, a statistics program was executed to ensure that the quality of the individual datasets had been retained.
Table 1
Quality Control Flag Description
--------------------------- ---------------------------------------------
0 No qualifier required
1 Accumulation period
2 End of Accumulation period
3 Deleted value. Original data was unreadable.
4 Trace of precipitation.
5 Probable amount as a result of melting frozen
precipitation.
6 Suspect amount relative to time or period of
occurrence.
7 Missing value
Table 2
QC Code Description
----------------- ------------------------------------------
U Unchecked
G Good
M Normally recorded but missing
D Questionable
B Unlikely
N Not available or Not observed.
X Glitch
E Estimated
C Reported value exceeds output format field
size or was negative precipitation.
T Trace
I Derived parameter can not be computed due
to insufficient data.