Table of Contents - The Atmospheric Studies Group at TRC

Section 4: PXTRACT Precipitation Data Extract Program
4. PXTRACT PRECIPITATION DATA EXTRACT PROGRAM
PXTRACT is a preprocessor program which extracts precipitation data for stations and time periods of
interest from a formatted precipitation data file in NCDC TD-3240 format. The TD-3240 data used by
PXTRACT can be in either the fixed record length format or the variable record length format. The fixed
record length format reports each hourly precipitation event in a separate record, whereas the variable
record length format reports all hourly precipitation events that occur on a single day in a single record,
and also includes the daily total. The hourly precipitation data usually come in large blocks of data sorted
by station. For example, a typical TD-3240 file for California may contain data from over 100 stations
statewide in blocks of time of 30 years or more. Modeling applications require the data sorted by time
rather than station, and involve specific spatial domains and time periods from less than one year up to
five years. PXTRACT allows data for a particular model run to be extracted from the larger data file and
creates a set of station files that are used as input files by the second-stage precipitation preprocessor,
PMERGE (see Section 5.0).
NOTE: If wet removal is not to be considered by the CALPUFF or MESOPUFF II dispersion models, no
precipitation processing needs to be done. PXTRACT (and PMERGE) are required only if wet removal
is an important removal mechanism for the modeling application of interest. In addition, if wet removal
is a factor, the user has the option of creating a free-formatted precipitation data file that can be read by
CALMET. This option eliminates the need to run the precipitation preprocessing programs for short
CALMET runs (e.g., screening runs) for which the data can easily be input manually.
The input files used by PXTRACT include a control file (PXTRACT.INP) containing user inputs, and a
data file (TD3240.DAT) containing the NCDC data in TD-3240 format. The precipitation data for
stations selected by the user are extracted from the TD3240.DAT file and stored in separate output files
(one file per station) called xxxxxx.DAT, where xxxxxx is the station identification code. PXTRACT
also creates an output list file (PXTRACT.LST) which contains the user options and summarizes the
station data extracted. Table 4-1 contains a summary of PXTRACT's input and output files.
The PXTRACT control file contains the user-specified variables which determine the method used to
extract precipitation data from the input data file (i.e., by state, by station, or all stations), the appropriate
state or station codes, and the time period to be extracted. A sample PXTRACT control file is shown in
Table 4-2. The format and contents of the file are described in Table 4-3.
The PXTRACT output list file (PXTRACT.LST) contains a listing of the control file inputs and options.
It also summarizes the station data extracted from the input TD-3240 data file, including the starting and
ending date of the data for each station and the number of data records found. Since the TD-3240 data are
not hourly, PXTRACT will extract the records that cover the period requested by the user. Therefore, the
dates of the data extracted from different stations may be different although the same time period was
requested by the user. If the starting (or ending) record has a data flag, the previous (or next) record will
also be extracted to complete the information necessary for PMERGE to interpret the data correctly. A
4-1