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.
2.0 Detailed Data Description
2.0.1 NCDC Hourly Precipitation Algorithms
The NCDC Hourly Precipitation dataset (TD 3240) is formed by
retaining both incremental and accumulative precipitation values from the
raw precipitation data. Both incremental and accumulative values are
contained in the IHOP 2002 Hourly Precipitation Composite. If an
accumulation period began prior to but continued into the beginning of the
IHOP 2002 time period, the precipitation values in that accumulation period
were reset to missing values. An accumulation period that began near the end
of the IHOP 2002 time period, but continued after the end of the IHOP 2002 time
period would also have its precipitation values reset to missing. For more
information on the NCDC Hourly Precipitation dataset, TD 3240, see
NCDC, 1986.
2.0.2 National Centers for Environmental Prediction/Environmental Modeling
Center (NCEP/EMC) Algorithms
The Environmental Modeling Center (EMC), previously the Climate Prediction
Center (CPC), is a component of the National Centers for Environmental
Prediction (NCEP). The NCEP/EMC Hourly Precipitation dataset was formed by
extracting incremental precipitation values that fell on the hour. The value
reported for any hourly observation represents data collected during the
previous 60 minutes. All raw precipitation data, including data that do not fall
on the hour, can be found on codiac in the
GCIP/EOP Surface:
Precipitation NCEP/EMC Gage Only Hourly Dataset.
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 implemented 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 hourly 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 hourly precipitation values potentially
followed by additional identical values or values that increase over time.
2.0.3 Hourly Precipitation Extract from IHOP 2002 Hourly Surface
Composite
This dataset is formed by extracting both incremental and accumulative
precipitation data from the
IHOP 2002 Mesonet:
Hourly Surface Meteorological Composite. Both incremental and accumulative
values are contained in this IHOP 2002 Hourly Precipitation Composite. Refer
to the
IHOP 2002 Mesonet: Hourly Surface meteorological Composite description document
for more information.
2.1 Detailed Format Description
The IHOP 2002 Hourly Precipitation Composite contains seven
metadata parameters and three data parameters. The metadata parameters
describe the date/time, network, station, and location at which the data
was collected. The three data parameters repeat once for each UTC hour from
0000 through UTC hour 2300. Data reported for a designated hour
represents data collected during the previous hour. All times are reported
in UTC, and precipitation data values are reported in millimeters. A
Quality Flag and a QC Code are assigned to each data value during the
University Corporation for Atmospheric Research/Joint Office for Science
Support (UCAR/JOSS) Quality Control (QC) processing. For a list of
possible Quality Flag and QC Code values see the Quality Control
Section 3.0. The table below details each
parameter.
Parameters Units
---------------------- ----------------------------------
Date of Observation UTC
Time 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
Hourly Precip Value Millimeters
Quality Flag See list of Definitions
QC Code See list of Definitions
UCAR/JOSS checks the NCEP/EMC hourly 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 more information on the quality
control performed on this network by UCAR/JOSS, please see
section 3.0 below.
Gross limit checks were used to determine the quality of the data
in the IHOP 2002 Hourly Precipitation Composite. Data values less
than 20 millimeters were flagged as good. Any data value greater than or
equal to 20 millimeters but less than 50 millimeters was flagged as
questionable. Any data value greater than or equal to 50 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 checks the NCEP/EMC hourly 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 hourly
precipitation values potentially followed by additional identical values or
values that increase over time. The data were never changed, only flagged.
Tables 1 and 2 contain the quality control codes and flags, respectively.
After the datasets were merged to form the IHOP 2002 Hourly
Precipitation Composite, a statistics program was executed to ensure that
the quality of the individual datasets had been retained.
3.0 Quality Control Processing
This precipitation composite was formed from several data
sources. Each of these datasets was Quality Controlled independently.
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 precipitation amount recorded.
I Derived parameter can not be computed due
to insufficient data.
4.0 References
National Climatic Data Center, 1986: Hourly Precipitation Data, TD 3240.
National Climatic Data Center, Asheville, NC, 20 pp.