4.3. MODIS L1B Reader

The MODIS Reader operates on HDF4 Level 1B files from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on the Aqua and Terra satellites. The reader is designed to work with files created by the IMAPP direct broadcast processing system (file naming conventions such as a1.17006.1855.1000m.hdf), but can support other types of L1B files, including the NASA archived files (file naming conventions such as MOD021KM.A2017004.1732.005.2017023210017.hdf). The reader can be specified to the polar2grid.sh script by using the reader name modis or modis_l1b.

This reader’s default remapping algorithm is ewa for Elliptical Weighted Averaging resampling. The --weight-delta-max parameter set to 10 and the --weight-distance-max parameter set to 1.

It provides the following products:

Product Name

Description

vis01

Visible 1 Band

vis02

Visible 2 Band

vis03

Visible 3 Band

vis04

Visible 4 Band

vis05

Visible 5 Band

vis06

Visible 6 Band

vis07

Visible 7 Band

vis26

Visible 26 Band

bt20

Brightness Temperature Band 20

bt21

Brightness Temperature Band 21

bt22

Brightness Temperature Band 22

bt23

Brightness Temperature Band 23

bt24

Brightness Temperature Band 24

bt25

Brightness Temperature Band 25

bt27

Brightness Temperature Band 27

bt28

Brightness Temperature Band 28

bt29

Brightness Temperature Band 29

bt30

Brightness Temperature Band 30

bt31

Brightness Temperature Band 31

bt32

Brightness Temperature Band 32

bt33

Brightness Temperature Band 33

bt34

Brightness Temperature Band 34

bt35

Brightness Temperature Band 35

bt36

Brightness Temperature Band 36

ir20

Radiance Band 20

ir21

Radiance Band 21

ir22

Radiance Band 22

ir23

Radiance Band 23

ir24

Radiance Band 24

ir25

Radiance Band 25

ir27

Radiance Band 27

ir28

Radiance Band 28

ir29

Radiance Band 29

ir30

Radiance Band 30

ir31

Radiance Band 31

ir32

Radiance Band 32

ir33

Radiance Band 33

ir34

Radiance Band 34

ir35

Radiance Band 35

ir36

Radiance Band 36

fog

Temperature Difference between BT31 and BT20

true_color

Ratio sharpened rayleigh corrected true color

false_color

Ratio sharpened rayleigh corrected false color

For reflectance/visible products a check is done to make sure that at least 10% of the swath is day time. Data is considered day time where solar zenith angle is less than 90 degrees.

4.3.1. Command Line Arguments

usage: polar2grid.sh -r modis_l1b -w <writer> [-h] [--ir-products] [--bt-products] [--vis-products] [--awips-true-color] [--awips-false-color]

4.3.1.1. MODIS L1B Reader

--ir-products

Add IR products to list of products

--bt-products

Add BT products to list of products

--vis-products

Add Visible products to list of products

--awips-true-color

Add individual CREFL corrected products to create the ‘true_color’ composite in AWIPS.

--awips-false-color

Add individual CREFL corrected products to create the ‘false_color’ composite in AWIPS.

Examples:

polar2grid.sh -r modis_l1b -w geotiff -f <path to files>/<list of files>

polar2grid.sh -r modis_l1b -w geotiff -h

polar2grid.sh -r modis_l1b -w geotiff --list-products-all -f /data

polar2grid.sh -r modis_l1b -w geotiff -p true_color false_color -f ../l1b/a1.22261.1857.250m.hdf ../l1b/a1.22261.1857.500m.hdf  ../l1b/a1.22261.1857.1000m.hdf ../l1b/a1.22261.1857.geo.hdf

polar2grid.sh -r modis_l1b -w geotiff -p vis01 -f terra/t1.22061.1654.250m.hdf terra/t1.22061.1654.geo.hdf

polar2grid.sh -r modis_l1b -w geotiff --grid-configs ${POLAR2GRID_HOME}/grid_configs/grid_example.yaml -g my_latlon -f ../l1b/a1.17006.1855.1000m.hdf ../l1b/a1.17006.1855.geo.hdf

polar2grid.sh -r modis_l1b -w awips_tiled --awips-true-color --awips-false-color --sector-id LCC -g lcc_conus_300 --letters --compress --grid-coverage=.05 -f MOD021KM.A2017004.1732*.hdf MOD02HKM.A2017004.1732*.hdf MOD02QKM.A2017004.1732*.hdf MOD03.A2017004.1732*.hdf

polar2grid.sh -r modis_l1b -w hdf5 --bt-products --add-geolocation  -f /data/MOD*.hdf

polar2grid.sh -r modis_l1b -w hdf5 -g wgs84_fit_250 -p vis01 vis02 -f /data/rad/MOD02QKM.*.hdf /data/geo/MOD03.*.hdf

4.3.2. Product Explanation

4.3.2.1. True Color

The MODIS Level 1B “true_color” composite produced by Polar2Grid provides a view of the Earth as the human eye would see it; or as close as we can come to with satellite data and the channels we have available. This means things like trees and grass are green, water is blue, deserts are red/brown, and clouds are white. The True Color GeoTIFF 24 bit image is an RGB (Red, Green, Blue) image consisting of a combination of Red: MODIS Band 1, Green: MODIS Band 4, and Blue: MODIS Band 3 reflectance channels. Each channel goes through a series of adjustments to produce the final high quality image output by Polar2Grid.

Creation of true color RGBs includes the following steps:

See the Creating MODIS AWIPS Compatible Files example to see how Polar2Grid can be used to make this product as a set of AWIPS compatible NetCDF files.

4.3.2.2. False Color

A false color image is any combination of 3 bands outside of those used to create a “true color” image (see above). These combinations can be used to highlight features in the observations that may not be easily identified in individual band imagery. Polar2Grid can readily create a preconfigured legacy false color (product false_color) GeoTIFF 24 bit image that consists of a combination RGB (Red, Green, Blue) image using Red: MODIS Band 7 (2.21 μm), Green: MODIS Band 2 (.86 μm) and Blue: MODIS Band 1 (.65 μm). This band combination is very effective at distinguishing land/water boundaries as well as burn scars.

Creation of MODIS legacy false color RGBs includes the following steps:

See the Creating MODIS AWIPS Compatible Files example to see how Polar2Grid can be used to make this product as AWIPS compatible NetCDF files.

polar2grid.sh -r modis_l1b -w geotiff -p true_color false_color -f /aqua/a1.17006.1855.250m.hdf /aqua/a1.17006.1855.500m.hdf /aqua/a1.17006.1855.geo.hdf