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Atmospheric Infrared Sounder (AIRS) FAQ

General Questions

 

Documentation Questions

 

Data Access Questions

 

AIRS Subsetting Questions

 

AIRS Imagery Questions

 

Science

 

Answers to General Questions

     
  • What is AIRS?
  • The Atmospheric Infrared Sounder (AIRS) is a facility instrument onboard the polar-orbiting Earth Observing System (EOS) Aqua satellite which was successfully launched on May 4, 2002. In combination with the Advanced Microwave Sounding Unit (AMSU-A) and the Humidity Sounder for Brazil (HSB), the AIRS instrument suite constitutes an innovative atmospheric sounding group of visible, infrared, and microwave sensors.

     
  • What does the AIRS instrument suite measure?
  • The AIRS instrument is a high resolution spectrometer with 2378 bands in the thermal infrared spectral region (3.74 - 15.4 micrometers) and 4 bands in the visible spectral region (0.4 - 1.0 micrometers). These ranges have been specifically selected to determine atmospheric temperature with an accuracy of of 1 K per 1 km thick layer in the troposphere and moisture profiles at an accuracy of 20% per 2 km thick layer in the lower troposphere (20% 60% in the upper troposphere). See the AIRS instrument guide for details.

    AMSU-A is a multi-channel microwave temperature/humidity sounder that measures global atmospheric temperature profiles and provides information on atmospheric water in all of its forms (with the exception of small ice particles, which are transparent at microwave frequencies). Information from AMSU-A in the presence of clouds is used to correct the AIRS infrared measurements for the effects of clouds. See the AMSU-A instrument guide for details.

    HSB is primarily a humidity sounder providing supplementary water vapor and liquid data to be used in the cloud clearing process. Due to its higher spatial resolution (which equals that of AIRS) and a higher scan rate, the measurement density is 2.4 times that of AMSU-A. See the HSB instrument guide for details. The HSB instrument failed on February 5th, 2003.

     
  • What AIRS data products are available from Goddard GES DISC? Does the data cost anything?
  • See AIRS Data Product page for the complete list of publicly available products. AIRS data is available to thepublic at NO COST.

  • What is AIRS Algorithm Version versus Data Collection?

    Version 2.7 Algorithm produced Collection 1
    Version 3 Algorithm produced Collection 2
    Version 4 Algorithm produced Collection 3
    Version 5 Algorithm produces Collection 5

    Starting with the delivery of Algorithm Version 4 in 2005, AIRS Instrument Team began to produce two types of retrievals due to the failure of HSB on February 5, 2003. Data types that encompass retrievals using HSB data will have "H" letter in the short name of the data type, e.g. AIRH2RET.  Retrievals without HSB have the familiar names, e.g. AIRX2RET. 

    Documentation is associated with the Version of the Algorithm, which is embedded in the data filenames.

    Data are organized and stored as Collections at GES DISC. Collections are different from the Algorithm Version until Version 5 where data version became the same as the collection. Our search engines will naturally show Collections.

    Because reprocessing earlier data takes time and multiple months are scheduled concurrently, Collections generated by the previous Algorithm Version will necessarily coexist for a period of time along with Collections generated by the current Algorithm Version. Users are cautioned to double-check the Collection before downloading to ensure the data are those desired.

  • How do I apply QA filtering on Level-1B IR and Level-2 standard products?
AIRS products contain a comprehensive set of QA flags that allow users to select the best quality data in their research.  AIRS documentation package contains 3 documents that address this topic. For convenience, GES DISC has produced a summary document on how to apply QA filtering. It has links to the full AIRS QA documents.
     
  • What is the format of AIRS data products? Are conversion tools available?
  • All AIRS data products are in HDF-EOS format. See our Tools page for details.

     
  • What is the daily volume of AIRS L1B and L2 data products, and the size of each granule?
  • The daily volume of AIRS L1B products is:

    • L1B-AIRS-IR-Rad: 13,440 Mb
    • L1B-Vis/NIR-Rad: 2,640 Mb
    • L1B-AMSU-Rad: 144 Mb
    • L1B-HSB-Rad: 408 Mb
    • L2-Standard-Product: 552 Mb
    • L2-Support-Product: 2,496 Mb
    • L2-Cloud-Cleared-Rad: 2,280 Mb

     

    The average size of each granule:

    • L1B-AIRS-IR-Rad: 56 Mb
    • L1B-Vis/NIR-Rad: 11 Mb
    • L1B-AMSU-Rad: 0.6 Mb
    • L1B-HSB-Rad: 1.7 Mb
    • L2-Standard-Product: 2.3 Mb
    • L2-Support-Product: 10.4 Mb
    • L2-Cloud-Cleared-Rad: 9.5 Mb

     

     
  • May I order data in smaller units than your granules (subsets of the granules)?
  • Yes. When you order data from GES DISC, you can add on channel or variable subsetting options to be applied on L1B and L2 products.

     
  • What is the length of time between satellite measurement and data availability?
  • There will typically be a 30-hour to 36-hour lag for AIRS L1B processed data.

     
  • Where do I go online to search and order AIRS data?
  • There are several options available for obtaining AIRS data, see our Data Access page for a list of options. Because all the AIRS data are online, these data access methods will converge on the FTP step at the end. Pick one method that suits your preference.


  • Why AIRS L1B HSB radiation data (AISRHBRAD brightness temperature) always display -9999.0 for channel 1?
  • HSB channel 1 was never implemented and so always -9999.0. There are four channels (channel 2-5) in HSB and their characteristics are summarized below.

Channel Central Frequency (GHz) Bandwidth (MHz)
1 Deleted (89Ghz)  
2 150.0 4000
3 183.31 ± 1.0 2x500
4 183.31 ± 3.0 2x1000
5 183.31 ± 7.0 2x2000

     

     

     

    What do the time values in AIRS L1 and L2 data products represent?

     

    The time values (TAI93: International Atomic Time 1993) are elapsed time in seconds since Jan 1, 1993, Coordinated Universal Time (UTC). For example, a time value of 410227204.00 corresponds to 12/31/2005 23:59:59.00 UTC. Note that a leap second was added at the end of 2005 (i.e. December 31, 2005). Therefore, TAI93 value of 410227206.00 (2 seconds after the above example) is 01/01/2006 00:00:00.00 UTC (not 01/01/2006 00:00:01.00 UTC) due to the leap second. Leap seconds are introduced on an irregular basis (see http://maia.usno.navy.mil/eo/leapsec.html for additional information on leap seconds and ftp://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat for announcements on the introduction of new leap seconds).

 

Answers to Documentation Questions

     
  • Where can I find basic Readmes for AIRS data products?
  • Our readmes contain brief information about data product characteristics, data ordering, read software and contact information. The readmes for AIRS data products are available at:
    http://disc.sci.gsfc.nasa.gov/AIRS/documentation

     

     
  • Where can I find complete users guides for AIRS data products and instruments?
  • These guides contain detailed information about data product characteristics, ordering, distribution, usage and instruments. The users guides for AIRS data products and instruments are available at: http://disc.sci.gsfc.nasa.gov/AIRS/documentation

     

     
  • Where can I find information about the AIRS algorithms?

    The AIRS Algorithm Theoretical Basis Documents are available from the EOS Project Science Office, and links to those documents are available at: http://disc.sci.gsfc.nasa.gov/AIRS/documentation

     

     
  • Where can I find information about the 2378 AIRS IR channel properties?
  • A channel-by-channel list which describes all 2378 of the AIRS IR channels can be found at: http://disc.sci.gsfc.nasa.gov/AIRS/documentation

     

     
  • What document gives a description of the AIRS data field attributes (geolocation, swath)?
  • The document called AIRS Processing Files Description provides a detailed description of the dimensions, geolocation fields, full swath data fields and other attributes.

     
  • What are the AIRS Earth Science Data Type (EDST) data collection descriptors?
  • AIRS ESDT collection descriptors contain all AIRS HDF-EOS metadata attributes (headers) such as version number, data date, spatial coordinates, etc. AIRS ESDTs as well as other platform instrument ESDTs can be found at: The EOS Core System (ECS) Information page: http://ecsinfo.gsfc.nasa.gov/ECSInfo/ecsmetadata/ESDT/ESDT_descriptors.html

     

     
  • Where can I access disclaimers about AIRS data?
  • Disclaimers document is prepared by the AIRS science team and it can be found at:
    http://disc.sci.gsfc.nasa.gov/AIRS/documentation

     

     
  • From where can I get the current version of AIRS Level 2 retrieval algorithms?
  • The source code has not been cleared for public release at this time. The plan is to release the source code in the future, via Direct Broadcast contacts, but no schedule has been established for the release of Level 2 code. If you wish to compare retrieval products, the AIRS project is happy to cooperate and work with you as long as you choose to use actual AIRS data instead of simulation data.

 

Answers to Data Access Questions

     
  • Who can order AIRS data products?
  • All NASA Earth Observing System (EOS) mission data including AIRS data is freely available to the public. Anyone can order these data products free of charge.

     
  • Do I need to register to become a new user?
  • Registration is not required to obtain AIRS data from GES DISC. On the other hand, the AIRS science team at NASA JPL encourages you to register with them so that you will receive announcements of discovered features and caveats which are directly applicable to your area of research. You will also receive the AIRS Newsletter.

     

  • Is AIRS data available via FTP to/from any site?
  • See our Data Access page for the server names and directory paths. You may also subscribe to the AIRS data so that the data products can be pushed to your site. Please contact User Services to set up FTP push.

     

     
  • Why would there be missing data for certain days?
  • There are several possible reasons for missing data:

    • The launch of the Aqua satellite that carries the AIRS instrument occurred on May 4, 2002. Processing on AIRS data to be distributed to the public began on data date March 12, 2003.
    • If you are seeking most recent data, it is possible the data is still in the processing pipeline and will soon be available. There is typically a lag from the time instrument observations are made on the satellite until the data is processed and made available to the public.
    • Sometimes satellite maneuvers, other necessary onboard procedures or technical problems will cause gaps in the data collection. Those instances will be documented and found at the NASA JPL AIRS website when appropriate.

     

     
  • Who can I contact for help with ordering AIRS data?
  • If you need assistance with ordering via GES DISC or Reverb, please contact the GES DISC User Service group at 301-614-5224 or send an email to gsfc-help-disc@lists.nasa.gov

    GES DISC User Services
    NASA GSFC Code 610.2
    Greenbelt, MD 20771
    FAX (301) 614-5268

     

     

     
  • How can I find Level 1 and Level 2 granules corresponding to various combinations of time/location?

  • For many different search combination, below is the command line search against Mirador using Python script.

    Usage:  python mirador_cmd.py < input.txt > urls.txt

    Sample input file (start, end, lat, long, lat, long) :
    2007-01-01T00:00:00 2007-01-01T23:59:59 (-15.8,-69.4),(-15.8,-69.4)

    where Input file format is start and end are CCSDS format time, yyyy-mm-ddThh:mm:ss
    start end (lat,long),(lat,long).

    Once getting urls.txt, type wget -i urls.txt to get all the necessary files

    "mirador_cmd.py" contains the following. Depending on the data set, one has to change shortname (AIRX2RET below for AIRS Level 2 standard product).

    #!/usr/local/bin/python

    import os, sys, urllib, re, string

    def search_mirador(shortname, begin, end, bbox):
        url_root = 'http://mirador.gsfc.nasa.gov/cgi-bin/mirador/granlist.pl?'

        # Maxgranules set low for testing.  For ops, set high (e.g. 1000).
        maxgranules = 1000

        params = (url_root, maxgranules, shortname, bbox, begin, end)
        url='%sformat=rss&maxgranules=%d&dataSet=%s&version=005&location=%s&searchType=Location&startTime=%s&endTime=%s' % params
        # print url
        #Sample url = 'http://mirador.gsfc.nasa.gov/cgi-bin/mirador/granlist.pl?format=rss&maxgranules=100&dataSet=AIRIBRAD&version=005&location=(-15.8,-69.4),(-15.8,-69.4)&searchType=Location&event=&startTime=2007-01-01%2000:00:00&endTime=2007-12-31%2023:59:59'

        # Make HTTP request
        file=urllib.urlopen(url)
        lines=file.readlines()
        file.close()

        # Parse lines for data URLs
        pattern = re.compile('<item>.*?<link>(.*?)</link>')
        for line in lines:
            mobj = pattern.search(line)
            # print line
            if (mobj):
                print mobj.group(1)

    shortname='AIRX2RET'
    for line in sys.stdin.readlines():
        (start, end, bbox) = string.split(line)
        search_mirador(shortname, start, end, bbox)

     

     
  • Why can't I get monthly AIRS data the day after the month is over?

  • AIRS monthly data require a complete set of the daily Level 3 global products, which in turn require a complete-as-possible set of of the Level 2 orbital products (given that orbital maneuvers, data dropouts, etc. affect the completeness of the data). It sometimes takes several days to complete the processing of the daily data ("closing out" the data processing stream) to the point where a monthly Level 3 data product can be created.

 

Answers to AIRS Subsetting Questions

     

  • What are the AIRS subset services at the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC)?

    When you order data through WHOM or Mirador search engine, you can choose variable or channel subsetting on AIRS L1B and L2 products. L3 subsetting can be done in Giovanni.
  •  

  • What is "Golden Set of Channels" offered for AIRS channel subsetting services ?

    These are the groups of channels recommended by the AIRS Science Team for channel subsetting of AIRS L1B and L2 products. See table below. The grouping is generally based on hardware module in the AIRS instrument. The channel number range and the frequency range in addition to module name for each set are also listed to give a better description of each set. These are meant to make it easy for users to select all channels in various frequency ranges. The "comments" in this table are only meant as comments to let users know typical uses for data in this range. For example M-06 has the comment "Ozone". Not all channels in this set are necessarily good for ozone and not all ozone channels are in this set. It is the set of all channels on module 6, and the most prominent feature of this section of the spectrum is ozone sensitivity. One may also note that frequency ranges overlap somewhat between the modules, so frequency ranges can be deceptive.
    Hardware
    Module
    Frequency
    Range
    Channel Number
    Range
    Number of
    Channels
    Comments
    All 649.61 - 2665.24cm-1 1 - 2378 323 The Numerical Weather Prediction sets
    M-12 649.61 - 681.99cm-1 1 - 130 130 Long-wave temperature sounding
    M-11 687.60 - 728.44cm-1 131-274 143 Long-wave temperature sounding
    M-10 728.06 - 781.88cm-1 275 - 441 166 Long-wave surface
    M-09 789.26 - 852.43cm-1 442 - 608 166 Long-wave surface
    M-08 851.49 - 903.78cm-1 609 - 769 160 Long-wave surface
    M-07 911.24 - 974.29cm-1 770 - 936 166 Long-wave surface
    M-06 973.82 - 1046.20cm-1 937 - 1103 166 Ozone
    M-05 1056.10 - 1136.66cm-1 1104 - 1262 158 Long-wave surface, Ozone
    M-04d 1216.97 - 1272.59cm-1 1263 - 1368 105 Water vapor
    M-04c 1284.35 - 1338.86cm-1 1369 - 1462 93 Water vapor, methane
    M-03 1338.16 - 1443.07cm-1 1463 - 1654 191 Water vapor
    M-04b 1460.27 - 1527.00cm-1 1655 - 1760 105 Water vapor
    M-04a 1541.10 - 1613.86cm-1 1761 - 1864 103 Water vapor
    M-02b 2181.50 - 2325.06cm-1 1865 - 2014 149 Shortwave temperature sounding, CO
    M-01b 2299.80 - 2422.85cm-1 2015 - 2144 129 Shortwave temperature sounding
    M-02a 2446.20 - 2569.75cm-1 2145 - 2260 115 Shortwave Surface
    M-01a 2541.90 - 2665.24cm-1 2261 - 2378 117 Shortwave Surface

 

 

  • What are the channels contained in the "Numerical Weather Prediction (NWP) Set"?

    The NWP set is the set of channels used by Numerical Weather Prediction centers. These channels were selected as key well-behaved channels (as of 2003) that collectively have almost as much information as the full 2378 channel set.
    Index Channel Number Frequency
    (wavenumber)
    1 1 649.612976
    2 6 650.807007
    3 7 651.046021
    4 10 651.765015
    5 11 652.005005
    6 15 652.968018
    7 16 653.208984
    8 17 653.450012
    9 20 654.174988
    10 21 654.416992
    11 22 654.658997
    12 24 655.143982
    13 27 655.872986
    14 28 656.116028
    15 30 656.604004
    16 36 658.070007
    17 39 658.80603
    18 40 659.052002
    19 42 659.544006
    20 51 661.767029
    21 52 662.015015
    22 54 662.512024
    23 55 662.760986
    24 56 663.008972
    25 59 663.757019
    26 62 664.507019
    27 63 664.757019
    28 68 666.010986
    29 69 666.262024
    30 71 666.765991
    31 72 667.018005
    32 73 667.27002
    33 74 667.521973
    34 75 667.775024
    35 76 668.028015
    36 77 668.281006
    37 78 668.533997
    38 79 668.786987
    39 80 669.041016
    40 82 669.549011
    41 83 669.802979
    42 84 670.057983
    43 86 670.567017
    44 92 672.099976
    45 93 672.356995
    46 98 673.640991
    47 99 673.898987
    48 101 674.414001
    49 104 675.189026
    50 105 675.447998
    51 108 676.226013
    52 110 676.744995
    53 111 677.005005
    54 113 677.526001
    55 116 678.307983
    56 117 678.570007
    57 123 680.142029
    58 124 680.403992
    59 128 681.45697
    60 129 681.721008
    61 138 689.491028
    62 139 689.762024
    63 144 691.119019
    64 145 691.390991
    65 150 692.755005
    66 151 693.028992
    67 156 694.400024
    68 157 694.674011
    69 159 695.224976
    70 162 696.052002
    71 165 696.880981
    72 168 697.711975
    73 169 697.98999
    74 170 698.267029
    75 172 698.823975
    76 173 699.10199
    77 174 699.380981
    78 175 699.659973
    79 177 700.218018
    80 179 700.776978
    81 180 701.057007
    82 182 701.617981
    83 185 702.460999
    84 186 702.742004
    85 190 703.869995
    86 192 704.435974
    87 193 704.718994
    88 198 706.137024
    89 201 706.991028
    90 204 707.846985
    91 207 708.705994
    92 210 709.565979
    93 213 710.429016
    94 215 711.005005
    95 216 711.29303
    96 218 711.870972
    97 221 712.739014
    98 224 713.609009
    99 226 714.190979
    100 227 714.481995
    101 232 715.940979
    102 239 717.994019
    103 248 720.651001
    104 250 721.244019
    105 251 721.541016
    106 252 721.838013
    107 253 722.13501
    108 256 723.028992
    109 257 723.328003
    110 261 724.523987
    111 262 724.823975
    112 267 726.325989
    113 272 727.833984
    114 295 734.150024
    115 299 735.382019
    116 300 735.690002
    117 305 737.236023
    118 308 738.166992
    119 309 738.478027
    120 310 738.789001
    121 318 741.286011
    122 321 742.22699
    123 325 743.484985
    124 333 746.013977
    125 338 747.603027
    126 355 753.057007
    127 362 755.325012
    128 375 759.573975
    129 453 793.171021
    130 475 801.098999
    131 484 804.385986
    132 497 809.179993
    133 528 820.833984
    134 587 843.913025
    135 672 871.289001
    136 787 917.30603
    137 791 918.747009
    138 843 937.90802
    139 870 948.184021
    140 914 965.43103
    141 950 979.127991
    142 1003 1001.38397
    143 1012 1005.263
    144 1019 1008.29999
    145 1024 1010.48102
    146 1030 1013.10901
    147 1038 1016.63501
    148 1048 1021.07703
    149 1069 1030.52795
    150 1079 1035.08899
    151 1082 1036.46497
    152 1083 1036.92395
    153 1088 1039.22705
    154 1090 1040.151
    155 1092 1041.07703
    156 1095 1042.46802
    157 1104 1056.104
    158 1111 1059.44397
    159 1115 1061.36096
    160 1116 1061.84094
    161 1119 1063.28503
    162 1120 1063.76697
    163 1123 1065.21594
    164 1130 1068.60999
    165 1138 1072.51501
    166 1142 1074.47803
    167 1178 1092.45105
    168 1199 1103.19897
    169 1206 1106.82605
    170 1221 1114.67505
    171 1237 1123.16199
    172 1252 1131.229
    173 1260 1135.57495
    174 1263 1216.974
    175 1266 1218.49597
    176 1278 1224.62305
    177 1285 1228.22498
    178 1290 1230.81104
    179 1301 1236.53894
    180 1304 1238.10999
    181 1329 1251.35706
    182 1371 1285.47498
    183 1382 1291.70898
    184 1400 1302.03699
    185 1401 1302.61499
    186 1402 1303.19397
    187 1403 1303.77295
    188 1415 1310.76599
    189 1424 1316.05798
    190 1449 1330.97595
    191 1455 1334.60498
    192 1466 1339.68701
    193 1471 1342.23804
    194 1477 1345.31201
    195 1479 1346.34094
    196 1488 1350.98804
    197 1500 1357.23596
    198 1519 1367.25305
    199 1520 1367.78503
    200 1538 1377.42603
    201 1545 1381.21301
    202 1565 1392.15295
    203 1574 1397.13501
    204 1583 1402.15295
    205 1593 1407.77295
    206 1614 1419.72595
    207 1627 1427.229
    208 1636 1432.47095
    209 1644 1437.16394
    210 1652 1441.88794
    211 1669 1468.82703
    212 1674 1471.91003
    213 1681 1476.24695
    214 1694 1484.36902
    215 1708 1493.21497
    216 1717 1498.95801
    217 1723 1502.81006
    218 1740 1513.83203
    219 1748 1519.07397
    220 1751 1521.04895
    221 1756 1524.35205
    222 1763 1542.44897
    223 1766 1544.48206
    224 1771 1547.88196
    225 1777 1551.98096
    226 1780 1554.03796
    227 1783 1556.10095
    228 1794 1563.70898
    229 1800 1567.89001
    230 1803 1569.98901
    231 1806 1572.09204
    232 1812 1576.31702
    233 1826 1586.25903
    234 1843 1598.495
    235 1852 1605.04602
    236 1865 2181.49512
    237 1866 2182.3999
    238 1867 2183.30688
    239 1868 2184.21411
    240 1869 2185.12207
    241 1872 2187.8501
    242 1873 2188.76099
    243 1875 2190.58496
    244 1876 2191.49805
    245 1877 2192.41211
    246 1881 2196.07495
    247 1882 2196.99292
    248 1883 2197.91089
    249 1884 2198.83008
    250 1897 2210.8479
    251 1901 2214.57202
    252 1911 2223.93604
    253 1917 2229.59204
    254 1918 2230.53711
    255 1921 2233.37695
    256 1923 2235.2749
    257 1924 2236.2251
    258 1928 2240.03296
    259 1937 2248.64795
    260 1938 2249.60889
    261 1939 2250.57104
    262 1941 2252.49707
    263 1946 2257.3269
    264 1947 2258.29492
    265 1948 2259.26392
    266 1958 2269.00098
    267 1971 2281.78101
    268 1973 2283.75903
    269 1988 2298.70508
    270 1995 2305.7439
    271 2084 2363.98389
    272 2085 2364.94092
    273 2097 2376.48999
    274 2098 2377.45801
    275 2099 2378.42603
    276 2100 2379.39502
    277 2101 2380.36499
    278 2103 2382.30811
    279 2104 2383.28003
    280 2106 2385.22705
    281 2107 2386.2019
    282 2108 2387.17798
    283 2109 2388.15405
    284 2110 2389.13208
    285 2111 2390.11011
    286 2112 2391.08911
    287 2113 2392.06812
    288 2114 2393.04907
    289 2115 2394.03003
    290 2116 2395.01196
    291 2117 2395.99512
    292 2118 2396.979
    293 2119 2397.96411
    294 2120 2398.94897
    295 2121 2399.93604
    296 2122 2400.9231
    297 2123 2401.91089
    298 2128 2406.86304
    299 2134 2412.83203
    300 2141 2419.83496
    301 2145 2446.19507
    302 2149 2450.2981
    303 2153 2454.41406
    304 2164 2465.80396
    305 2189 2492.0791
    306 2197 2500.60205
    307 2209 2513.49292
    308 2226 2531.97803
    309 2234 2540.76904
    310 2280 2561.12891
    311 2318 2600.49902
    312 2321 2603.65991
    313 2325 2607.88696
    314 2328 2611.06592
    315 2333 2616.38306
    316 2339 2622.79199
    317 2348 2632.46606
    318 2353 2637.87207
    319 2355 2640.04102
    320 2363 2648.75195
    321 2370 2656.4231
    322 2371 2657.52197
    323 2377 2664.13794

 

Answers to AIRS Imagery Questions

     

  • Where can I get browse images of AIRS data? Do the images cost anything?
  • AIRS Daily summary browse images can be viewed through AIRS summary browse search option in GES DAAC Search and Order. The AIRS summary browse data is used as only ordering tool and cannot be distributed to public users as a data product. Starting from Version 5, the summary browse images are discontinued.

    All browse images SHOULD NOT be used as the basis of any science research and do not constitute a research-quality product. The images are available at NO COST.

     

  • What purpose do browse images serve?
  • Browse images are low-resolution visualizations used in search and order tools. The purpose of having browse images is to allow science data users to get an idea about what is in the data before ordering the data. For example, users should be able to tell if a location they have selected is sufficiently cloud-free or cloudy enough for cloud studies. In other words, the browse images have a role in helping users to select and organize the science data they have obtained.

     

  • Why should I take time to preview an image?
  • In the GES DISC Search and Order interface (Mirador), there is an option to preview a data granule before ordering the data granule. Previewing an image of the data can help to determine the extent and location of cloud cover, confirm its geographical location, or observe quality problems such as speckling, line drops, and sun glint. However, this is not to be confused with our AIRS summary browse image which is a daily product (i.e., some anomalies may not be visible on a summary browse image due to the sub-sampling process).

     

  • Can I use the browse image for research and analysis purposes?
  • No. Browse images represent only a fraction of the original data or have a low-resolution. They should not be used for analysis, evaluation or interpretation of data content.

     

  • What is the volume of a browse image?

    The size of the browse images for L1B products are less than 200 kilobytes with most ranging from 100 to 150 kilobytes.

  • Using ArcGIS, Why do I get different bounds for Level 3 data products depending upon whether they are V5 or V6, despite the fact that both are 1x1 deg grids covering the same spatial area?

    The V5 and V6 Level 3 gridded products share identical 1x1 degree grids.  The latitudes and longitudes of the grid box centers are provided in the data (LATITUDE, LONGITUDE).  The upper left box center location is (89.5,-179.5) and the lower right box center location is (-89.5, +179.5). The spatial extent of the 1x1 degree grid spans the upper left (+90, -180) to lower right (-90, +180).

    Many software packages, and ArcGIS is one such, look at the metadata to determine the spatial extent of the data sets rather than the grid resolution and spatial arrays themselves. The V6 metadata incorrectly specifies the spatial extent because they are set based on the upper left and lower right grid box centers.  The V5 metadata correctly provided the spatial extent values based on the upper left and lower right grid box outer edges.

    For example, if you peruse the V6 metadata, you will find:

    GROUP=GridStructure
        GROUP=GRID_1
            GridName="ascending"
            XDim=360
            YDim=180
            UpperLeftPointMtrs=(-179030000.000000,89030000.000000)
            LowerRightMtrs=(180030000.000000,-90030000.000000)


    Whereas, if you peruse the V5 metadata, you will find:

    GROUP=GridStructure
        GROUP=GRID_1
            GridName="ascending"
            XDim=360
            YDim=180
            UpperLeftPointMtrs=(-180000000.000000,90000000.000000)
            LowerRightMtrs=(180000000.000000,-90000000.000000)

    If your software analysis package uses the metadata rather than the grid resolution and latitude/longitude data arrays to determine spatial extent, you must override the grid bounds metadata of the V6 data set.

    The V6 metadata providing the corners, as required by ArcGIS, is incorrect and this will be fixed in a future release of AIRS products.  We recommend that ArcGIS users of the V6 L3 data products override the values of UpperLeftPointMtrs and LowerRightMtrs, setting them to those found in the V5 L3 data products metadata, i.e.:

            UpperLeftPointMtrs=(-180000000.000000,90000000.000000)
            LowerRightMtrs=(180000000.000000,-90000000.000000)

 

 

 

Answer to Science Questions

     

  • What is the repeat cycle of AIRS?
  • The AIRS swath repeat cycle is 16 days.

     

  • What is the vertical resolution of AIRS Level-2 products?
  • Standard product is on 28 pressure levels and Support product is on 100 pressure levels. Complete lists of those levels can be found at http://disc.sci.gsfc.nasa.gov/AIRS/documentation, under Level-2 Products section (Pressure Arrays). The documentation on AIRS Levels and Layers is also useful and can be found at the same web location.

     

  • What is cloud-clearing method?
  • The cloud-clearing approach is used to retrieve atmospheric and surface properties. It relies on the fact that cloud amount tends to vary appreciably among nearby 15 km AIRS footprints, but that other atmospheric and cloud properties (averaged within AIRS footprints) are more uniform. This means that radiance differences between adjacent AIRS footprints are primarily caused by changes in cloud amount. Adjacent, multi-spectral observations can then be solved in a least squares sense for the infrared radiance that is common to the clear portions of all field of view. Since the effect of clouds has been removed, atmospheric and surface variables can be retrieved from these radiances as if no clouds were present. Cloud properties can be also determined based on the difference between observed and cloud-cleared radiances.

    In this technique, any aerosols that interact with IR radiation can be treated as an unusual cloud layer. As with water clouds, there must be a clear region somewhere within each AMSU-A footprint. Thus, narrow smoke plumes are not a problem, but a uniform dust layer extending more than 45 km across would get folded into the "clear-column" radiances and could degrade the atmospheric retrieval. To assess the impact of such horizontally widespread aerosols, it should be noted that stratospheric sulfuric acid aerosols are optically thin in the IR, except after large volcanic eruptions. Tropospheric anthropogenic aerosols are also optically thin to AIRS because of their small size.

    Although this method has been used and proved by existing systems (TOVS), it should be noticed that the higher spectral resolution of AIRS allows this technique to be exploited to a higher degree than ever before.

     

  • Does AIRS Level-2 product include measurements from AMSU and HSB?
  • One flavor of the AIRS Level-2 Standard and Support Products does. Their product names are AIRH2RET and AIRH2SUP respectively. The end date for these products is 2003-Feb-05 due to HSB instrument failure. The Standard Product (AIRX2RET) and Support Product (AIRX2SUP) are created using only the AIRS and AMSU, without HSB.

     

  • Can radiance value be negative?
  • In general, AIRS can have negative radiances for shortwave channels. When the scene temperature is low, below 200K, shortwave AIRS channels can have negative radiances. This happens over very high cloud in tropics and over Antartica.

    In bad channels, radiance can be negative for several different reasons:

    1) -9999.0 is used as a flag value when input data is missing or out of range. Out of range data happens most frequently as a result of sun glint causing saturation.
    2) Radiances can go negative when the noise is greater than the scene. This will happen in very noisy detectors (so users should check the channel properties file and/or the ExcludedChans field in the data. Note ExcludedChans is not defined correctly in the docs. Any channel with a value 3 or higher should be ignored. NeN is also a good diagnostic of channels experiencing transient high noise episodes.)

    The users should be warned to use only the channels that are marked good ( BadFlag = 0 in the channel property file.).

     

  • Where can I find AIRS-RTA?
  • You can find AIRS-RTA in AIRS Level-2 ATBD,

    http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/docs/AIRS/atbd-airs-L2.pdf

    Section 4.2 Radiative Transfer of the Atmospheric in the Infrared.

     

  • Where can I find AIRS band chart?
  • AIRS band chart can be found in AIRS channel properties files: http://disc.sci.gsfc.nasa.gov/AIRS/documentation under Level-1B Products section.

     

  • How can I find when AIRS is over a particular area?
  • Various maps of the orbital tracks of Aqua (the satellite carrying AIRS) are available from University of Wisconsin's web site:

    http://www.ssec.wisc.edu/datacenter/aqua/

     

     

  • Where can I find time information for Level 3 data?
  • AIRS Level 3 data is bin-averaged (gridded) in 1x1 grid. Because of the averaging (gridding) process, time information is not meaningful, thus not provided. The only useful time information related to Level 3 data is that since Aqua (satellite on which AIRS in on-board) is sun-synchronous, the local time of satellite crossing depends only on the latitude. In other words, for a given latitude (except near the poles), the crossing time is fixed. The equator crossing time for Aqua is 13:30 (ascending) and 1:30 (descending). For other latitudes, the LT (in fact LMT; local mean time) crossing time of Aqua satellite is given as (plotted in image below),

    For Ascending mode;
    LMT(lamda)=13.5 +1/15* arcsin[tan(lamda)/tan(98.2)] ; given in hour
    where, lamda=latitude in degree,  98.2=inclination angle of Aqua satellite

    For descending mode;
    LMT(lamda)=13.5+12-1/15* arcsin[tan(lamda)/tan(98.2)]

    "Satellites, Orbits and Missions" by Michel Capderou (chapter 6) has an excellent explanation on the orbit and crossing time.

     

    airs_crosstime.jpg

     

  • How can I read AIRS Level 3 data into GrADS?
  •  

There are 2 methods which can be used to read AIRS Level 3 data into GrADS.  One method assumes the data are stored locally, and the other reads the data from an OPeNDAP server.

1) If the data are stored locally in an HDF file called AIRS.2011.02.21.L3.RetStd001.v5.2.2.0.G11054113301.hdf, and you want to read in the variable Temperature_A, create a control file with the following contents:

----------------------------------------------------------------------------
dset AIRS.2011.02.21.L3.RetStd001.v5.2.2.0.G11054113301.hdf
title AIRS
options yrev
undef -9999
dtype hdfsds
xdef 360 linear -179.5 1.0
ydef 180 linear -89.5 1.0
zdef 24 levels 1000 925 850 700 600 500 400 300 250 200 150 100 70 50 30 20 15
10 7 5 3 2 1.5 1
tdef 1 LINEAR 12z3sep2005 01dy
vars 1
Temperature_A=>Temperature_A 24 z,y,x Air Temperature (Asc.)
endvars
--------------------------------------------------------------------------------------------

The control file can be used as follows to read and plot the AIRS Temperature_A data:

ga-> open control_filename
ga-> set lon 50
ga-> set lev 1000 1
ga-> set zlog on
ga-> d Temperature_a

 

 

2) Alternatively, if you want to read in the variable Temperature_A from an OPeNDAP server without downloading the entire HDF file, you can create a control file with the following contents in which the control file points to the HDF file on the OPeNDAP server.

------------------------------------------------------------------------------------------------

dset
http://acdisc.gsfc.nasa.gov/opendap/Aqua_AIRS_Level3/AIRX3STD.005/2011/AIRS.2011.02.21.L3.RetStd001.v5.2.2.0.G11054113301.hdf
title AIRS
options yrev
undef -9999
xdef LongitudeU272 360 linear -179.5 1.0
ydef LatitudeU271 180 linear -89.5 1.0
zdef TempPrsLvlsU111 24 levels 1000 925 850 700 600 500 400 300 250 200 150 100
70 50 30 20 15 10 7 5 3 2 1.5 1
vars 1
Temperature_AU53=>Temperature_A 24 99 Air Temperature (Asc.)
endvars

------------------------------------------------------------------------------------------------


The control file can be used just as in the previous example to read and plot the AIRS Temperature_A data except you type “xdfopen control_filename” rather than “open control_filename.”

 

 

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Last updated: Nov 14, 2014 03:55 PM ET