pygac-fdr

Python package for creating a Fundamental Data Record (FDR) of AVHRR GAC data using pygac

Installation

To install the latest release:

pip install pygac-fdr

To install the latest development version:

pip install git+https://github.com/pytroll/pygac-fdr

Usage

To read and calibrate AVHRR GAC level 1b data, adapt the config template in etc/pygac-fdr.yaml, then run:

pygac-fdr-run --cfg=my_config.yaml /data/avhrr_gac/NSS.GHRR.M1.D20021.S0*

Results are written into the specified output directory in netCDF format. Afterwards, collect and complement metadata of the generated netCDF files:

pygac-fdr-mda-collect --dbfile=test.sqlite3 /data/avhrr_gac/output/*

This might take some time, so the results are saved into a database. You can specify files from multiple platforms; the metadata are analyzed for each platform separately. With a large number of files you might run into limitations on the size of the command line argument ("Argument list too long"). In this case use the following command to read the list of filenames from a file (one per line):

pygac-fdr-mda-collect --dbfile=test.sqlite3 @myfiles.txt

Finally, update the netCDF metadata inplace:

pygac-fdr-mda-update --dbfile=test.sqlite3

Tips for AVHRR GAC FDR Users

Checking Global Quality Flag

The global quality flag can be checked from the command line as follows:

ncks -CH -v global_quality_flag -s "%d" myfile.nc

Cropping Overlap

Due to the data reception mechanism consecutive AVHRR GAC files often partly contain the same information. This is what we call overlap. For example some scanlines in the end of file A also occur in the beginning of file B. The overlap_free_start and overlap_free_end attributes in pygac-fdr output files indicate that overlap. There are two ways to remove it:

• Cut overlap with subsequent file: Select scanlines 0:overlap_free_end
• Cut overlap with preceding file: Select scanlines overlap_free_start:-1

If, in addition, users want to create daily composites, a file containing observations from two days has to be used twice: Once only the part before UTC 00:00, and once only the part after UTC 00:00. Cropping overlap and day together is a little bit more complex, because the overlap might cover UTC 00:00. That is why the pygac-fdr-crop utility is provided:

$ pygac-fdr-crop AVHRR-GAC_FDR_1C_N06_19810330T225108Z_19810331T003506Z_...nc --date 19810330
0 8260
$ pygac-fdr-crop AVHRR-GAC_FDR_1C_N06_19810330T225108Z_19810331T003506Z_...nc --date 19810331
8261 12472

The returned numbers are start- and end-scanline (0-based).