Table of Contents - The Atmospheric Studies Group at TRC

Section 5: PMERGE Precipitation Data Preprocessor
5. PMERGE PRECIPITATION DATA PREPROCESSOR
PMERGE reads, processes and reformats the precipitation data files created by the PXTRACT program,
and creates either a formatted or an unformatted data file for input into the CALMET meteorological
model. The output file (e.g., PRECIP.DAT) contains the precipitation data sorted by hour, as required by
CALMET, rather than by station. The program can also read an existing unformatted output file and add
stations to it, creating a new output file. PMERGE also resolves "accumulation periods" and flags
missing or suspicious data.
Accumulation periods are intervals during which only the total amount of precipitation is known. The
time history of precipitation within the accumulation period is not available. For example, it may be
known that within a six-hour accumulation period, a total of a half inch of precipitation fell, but
information on the hourly precipitation rates within the period is unavailable. PMERGE resolves
accumulation periods such as this by assuming a constant precipitation rate during the accumulation
period. For modeling purposes, this assumption is suitable as long as the accumulation time period is
short (e.g., a few hours). However, for longer accumulation periods, the use of precipitation data with
poor temporal resolution is not recommended. PMERGE will eliminate and flag as missing any
accumulation periods longer than a user-defined maximum length.
PMERGE provides an option to "pack" the precipitation data in the unformatted output in order to reduce
the size of the file. A "zero packing" method is used to pack the precipitation data. Because many of the
precipitation values are zero, strings of zeros are replaced with a coded integer identifying the number of
consecutive zeros that are being represented. For example, the following record with data from 20
stations requires 20 unpacked “words”:
0.0, 0.0, 0.0, 0.0, 0.0, 1.2, 3.5, 0.0, 0.0, 0.0,
0.0, 0.0, 0.0, 0.7, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,
These data in packed form would be represented in six words:
-5., 1.2, 3.5, -6., 0.7, -6.
where five zero values are replaced by -5., six zero values are replaced by -6., etc. With many stations
and a high frequency of zeros, very high packing ratios can be obtained with this simple method. All of
the packing and unpacking operations are performed internally by PMERGE and CALMET, and are
transparent to the user. The header records of the data file contain information flagging the file to
CALMET as a packed or unpacked file. If the user selects the unpacked format, each precipitation value
is assigned one full word.
The input files used by PMERGE include a control file (PMERGE.INP), an optional unformatted data file
created in a previous run of PMERGE, and up to 150 TD-3240 precipitation station files (e.g., as created
by PXTRACT). The output file consists of a list file and a new unformatted or formatted data file in
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