Program convert_amsu_L1

Overview

The following is an excerpt from the AIRS L1B AMSU-A documentation. The complete documentation provided by the Goddard Earth Sciences Data and Information Services Center (GES DISC) can be within the Documentation->README Document found here.

The Atmospheric Infrared Sounder (AIRS) Version 5 Level 1B Advanced Microwave Sounding Unit (AMSU)-A Products (AIRABRAD) contain calibrated and geolocated brightness temperatures in degrees Kelvin. AIRABRAD_NRT (Near Real Time) products are also available within ~3 hours of observations globally and stay for about 5 days from the time they are generated. This data set is generated from AMSU-A level 1A digital numbers (DN) and contains 15 microwave channels in the 50-90 GHz and 23-32 GHz regions of the spectrum. A day’s worth of data is divided into 240 scenes (granules), each of 6 minute duration. An AMSU-A scene contains 30 cross-track footprints in each of 45 along-track scanlines, for a total of 45 x 30 = 1350 footprints per scene. AMSU-A scans three times as slowly as AIRS (once per 8 seconds) and its footprints are approximately three times as large as those of AIRS (45 km at nadir). This results in three AIRS scans per AMSU-A scans and nine AIRS footprints per AMSU-A footprint.

For more details on the history of the AMSU/A satellite instrumentation see the following link.

To summarize, AMSU/A was flown on satellites NOAA 15-17. Versions of AMSU-A also fly on the Aqua satellite (that also houses AIRS) as well as the European MetOp satellite. It has been replaced by the Advance Technology Microwave Sounder (ATMS) on the satellite NOAA-20.

Instructions to download the AMSU-A L1B Version 5 (AIRABRAD) dataset

The AMSU-A data is located within the Goddard Earth Sciences Data and Information Services Center (GES DISC) located here. You need to create an Earthdata account before you can download data. To access the AMSU-A data, search on keyword AIRABRAD and locate the AIRS/Aqua L1B AMSU (A1/A2) geolocated and calibrated brightness temperatures V005 (AIRABRAD) heading within your search results.

Next, under the Data Access header, click on Subset/Get Data, then refine your search results by 1) data range (time) and 2) spatial region.

There are various options for downloading, however, the most straightforward approach for macOS and Linux users is to use the wget command. The download instructions provide the proper wget flags/options. The Download Links List provides the AMSU-A file list based on your search results.

Each granule is about 560K and has names like
AIRS.2019.06.22.236.L1B.AMSU_Rad.v5.0.0.0.G19174110442.hdf

Advanced Microwave Sounding Unit (AMSU-A) L1B Brightness Temperatures

Perform the following steps to convert the AMSU_L1 observations:

  1. Download the h4tonccf_nc4 tool provided from the hdf-eos website. Options are provided for Mac, Linux and Windows platforms. For example, the following command downloads the CentOS7 v1.3 executable that works for Derecho:

    wget  https://hdfeos.org/software/h4cflib/bin/linux/v1.3/CentOS7/h4tonccf_nc4

  2. Convert the AMSU data file from HDF-EOS to netCDF format using the h4tonccf_nc4 exectuable as shown below. Be sure to provide execute permission first:

    chmod +x h4tonccf_nc4
./h4tonccf_nc4 AMSU.hdf

Done with writing netcdf file AMSU.nc

  1. b. Optional: The netCDF files have two global attributes that are exceedingly large and uninformative. If needed you can remove these attributes, you can use the ncatted command from NCO through the following command:

    module load nco
ncatted -a coremetadata,global,d,,, -a StructMetadata_0,global,d,,, AMSU.nc AMSU_final.nc

  2. Run convert_amsu_L1 to convert the AMSU_final.nc file to the DART obs_seq format. Important: Be sure to configure your namelist settings (below) before running the converter. Also be sure you have compiled the convert_amsu_L1 executable using the proper ~/DART/build_templates/mkmf.template that includes both RTTOV and HDF-EOS2 libraries as described here: AIRS and AMSU

    ./convert_amsu_L1

Check the completed obs_seq. It should include brightness temperatures for the EOS_2_AMSUA_TB observation type. The converter should also produce the following metadata underneath the mw (microwave) header as shown in the table below. For more information on the metadata see the RTTOV documentation

Metadata variable Name

Description

Sat_az

Azimuth of satellite position (degrees)

Sat_ze

Aenith of satellite position (degrees)

Platform_id

EOS (9), RTTOV User Guide, Table 2

Sat_id

(2), RTTOV User Guide, Table 2

Sensor_id

AMSU-A (3), RTTOV User Guide, Table 2

Channel

Microwave frequency channel (1-15)

Mag_field

Earth magnetic field strength (Gauss)

cosbk

Cosine of angle between magnetic field and viewing direction

Fastem_p(1-5)

Land/sea-ice parameters 1-5 for FASTEM emissivity model Table 21, RTTOV User Guide

Namelist

The convert_amsu_L1 converter requires MODULE obs_def_rttov_mod. Only two &obs_def_rttov_nml options are required when converting the observations: use_zeeman and rttov_sensor_db_file.

Be aware that if the RTTOV namelist option use_zeeman = .true. certain metadata must be available in the observation. This is not fully implemented in the AMSU-A observation converter, so we recommend setting use_zeeman = .false.. For more information, please see GitHub Issue 99 “AIRS AMSUA observation converter … Zeeman coefficients and channels”

Namelists are read in a file called input.nml. We adhere to the F90 standard of starting a namelist with an ampersand ‘&’ and terminating with a slash ‘/’ for all our namelist input. Character strings that contain a ‘/’ must be enclosed in quotes to prevent them from prematurely terminating the namelist. The default values are shown below. More realistic values are provided in AIRS/work/input.nml

&convert_amsu_L1_nml
   l1_files           = ''
   l1_file_list       = ''
   outputfile         = ''
   append_output      = .false.
   channel_list       = 'null'
   along_track_thin   = 0
   cross_track_thin   = 0
   lon1               =   0.0
   lon2               = 360.0
   lat1               = -90.0
   lat2               =  90.0
   verbose            = 0
/

&obs_def_rttov_nml
 rttov_sensor_db_file   = '../../../forward_operators/rttov_sensor_db.csv'
 use_zeeman             = .false.
/

Contents

Type

Description

l1_files

character(len=256), dimension(512)

A list of one or more names of the netCDF file(s) to read.

l1_file_list

character(len=256)

The name of an ascii text file which contains one filename per line. Each file will be read and the observations converted into a single output file. Only one of ‘l1_files’ and ‘l1_file_list’ can be specified. The other must be ‘ ‘ (empty).

outputfile

character(len=256)

The name of the output observation sequence file.

append_output

logical

If the output observation sequence file exists it is possible to add to it. The observations are added consistent with the paradigm that the observation linked list will be traversed in temporally-ascending fashion, no matter the physical location of the observation in the file. .true. adds the new observations to the existing file, .false. will cause an existing output file to be overwritten.

channel_list

character(len=8), dimension(15)

The AMSU channels desired. See the table below for valid input.

along_track_thin

integer

Provides ability to thin the data by keeping every Nth data value in the along-track scan. [0,45] e.g. 4 == keep only every 4th row. 0 is no thinning.

cross_track_thin

integer

Provides ability to thin the data by keeping every Nth data value in the cross-track scan. [0,30] e.g. 3 == keep every third value. 0 is no thinning.

lon1

real(r8)

The West-most longitude of interest in degrees. [0.0, 360]

lon2

real(r8)

The East-most longitude of interest in degrees. [0.0, 360]

lat1

real(r8)

The South-most latitude of interest in degrees. [-90.0,90.0]

lat2

real(r8)

The North-most latitude of interest in degrees. [-90.0,90.0]

verbose

integer

Controls the amount of run-time output. 0 == bare minimum. 3 is very verbose. Only use 3 if converting one or two files for testing.

Channel Specification

The following channel description is excerpted from the Documentation->README Document found here.

“AMSU-A primarily provides temperature soundings. It is a 15-channel microwave temperature sounder implemented as two independently operated modules. Module 1 (AMSU-A1) has 12 channels in the 50-58 GHz oxygen absorption band which provide the primary temperature sounding capabilities and 1 channel at 89 GHz which provides surface and moisture information. Module 2 (AMSU-A2) has 2 channels: one at 23.8 GHz and one at 31.4 GHz which provide surface and moisture information (total precipitable water and cloud liquid water).”

To facilitate the selection of channels, either the Integer or String values may be used to specify channel_list within &convert_amsu_L1_nml. The Documentation and netCDF values are provided for reference only.

For example the following channel list settings are identical and specify the AMSU channels centered on 50.3 and 89 GHz:

channel_list       = 3,15
channel_list       = 'A1-1','A1-13'

Integer

String

Documentation Frequency

netCDF center_freq

Module 2 - surface and moisture information

1

‘A2-1’

23.8

23.8

2

‘A2-2’

31.4

31.4

Module 1 - primary temperature sounding capability

3

‘A1-1’

50.3

50.3

4

‘A1-2’

52.8

52.8

5

‘A1-3’

53.596

53.596

6

‘A1-4’

54.4

54.4

7

‘A1-5’

54.94

54.94

8

‘A1-6’

55.5

55.5

9

‘A1-7’

57.29034

57.29034

10

‘A1-8’

57.29034

11

‘A1-9’

57.29034

12

‘A1-10’

57.29034

13

‘A1-11’

57.29034

14

‘A1-12’

57.29034

15

‘A1-13’

89

89