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Temperature Cloud Fraction
September 3-8, 1988



Original Archive
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The TOVS (TIROS Operational Vertical Sounder) data set is a collection of 5-day (pentad) means of global coverage during the years 1985 - 1988 for eleven parameters that describe the thermodynamic and radiative state of Earth's atmosphere, including global profiles of temperature and moisture, cloudiness, and outgoing longwave radiation.It was generated from data obtained from the HIRS2 (High resolution Infrared Radiation Sounder) and MSU (Microwave Sounding Unit) instruments that are part of the TOVS suite of instruments flown on National Oceanic and Atmospheric Administration (NOAA) satellites NOAA-9 and NOAA-10.TOVS- derived data provide a means to investigate long-term climate change and interannual variability and study local and periodic phenomena such as El Nino and stratospheric warmings.


The production and distribution of this data set are being funded by NASA's Mission To Planet Earth program. The data are not copyrighted, however, we request that when you publish data or results using these data please acknowledge as follows:


The authors wish to thank the Satellite Data Utilization Office (Code 910.4) and the Distributed Active Archive Center (Code 610.2) at Goddard Space Flight Center, Greenbelt, MD, 20771 for the production and distribution of these data.These activities were sponsored by NASA's Mission to Planet Earth program.


The atmospheric data from which this data set is derived were produced by the Satellite Data Utilization Office of Goddard Space Flight Center and by the Goddard DAAC at Greenbelt, MD, using an algorithm developed by Joel Susskind and collaborators.The original data, which includes daily, 5-day, and monthly gridded products in Hierarchical Data Format (HDF), are currently available from the Goddard DAAC at NASA's Goddard Space Flight Center.The derived data set contains a subset of the most important geophysical parameters contained in the original archive.More information is provided in the TOVS Pathfinder Data Set online guide.


It is expected that an additional year of data will become available in Fall 1995 for the two-satellite coverage by NOAA-10 and NOAA-11 of the year 1989. Newly processed years of TOVS data will be made available through this data collection as they are released to the DAAC by the data producer.





  • Parameters, Units, Range
PARAMETER                DESCRIPTION                      UNITS     DATA RANGE

CLTEMP    Mean temperature in the layers: surface-500 mb,   K       180 - 295
          500-300 mb, 300-100 mb, and 100-30 mb

PRWAT     Precipitable water (integrated water vapor)       cm        0  - 8
          above the levels: surface, 850 mb, 700 mb,
          500 mb, 300 mb

TSURF     Surface skin temperature                          K       200 - 320

FCLD      Total effective cloud fraction                    -         0 - 1

FCLD7     Cloud fractions for the layers <180 mb,           -         0 - 1
          180-310 mb, 310-440 mb, 440-560 mb, 560-680 mb,
          680-800 mb, >800 mb

PCLD      Cloud top pressure for total cloud fraction       mb       50 - 1000

TCLD      Cloud top temperature for total cloud fraction    K       175 - 310

OLR       Outgoing long-wave radiation exiting the top      W/m^2    80 - 350
          of the atmosphere

LWF       Longwave cloud radiative forcing, or the          W/m^2   -70 - +160
          difference between the cloudy and clear sky OLR

PRC       Precipitation estimate                            mm/day    0 -   40

SPRC      Surface pressure derived from forecast model      mb      525 - 1050

  • Temporal Coverage: January 1985 - December 1988
  • Temporal Resolution: 5-day (pentad) Means
  • Spatial Coverage: Global
  • Spatial Resolution: 1 degree x 1 degree



These data are products of the Tiros Operational Vertical Sounder (TOVS) suite of instruments flown on NOAA-series satellites.


The HIRS/2 and MSU instruments are carried aboard National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting satellites NOAA-9 and NOAA-10.

                               NOAA9                        NOAA10
Launch date:               Dec. 12, 1984                Sept. 17,1986
Orbit:                     Circular, sun synchronous    Circular, sun 
Nominal altitude:          872 km                       833 km
Inclination:               98.8 degrees                 98.8 degrees
Nodal period:              102  min                     102 min 
Nodal increment            25.3                         25.3 degrees
Equatorial crossing time:  1420 Local Solar Time        1930 Local 
Solar Time
                           (Ascending Node)             (Ascending Node)


High Resolution Infrared Radiation Sounder 2 (HIRS/2)-- The HIRS/2 instrument measures radiation emitted by the Earth- atmosphere system in 19 regions of the infrared spectrum between 3.7 and 15 microns. A visible channel is also available to measure the albedo of Earth's surface. The central wave numbers and wavelengths of these channels are


   chan  wavenumber    wavelength      chan  wavenumber    wavelength 
    1)   667.70 cm-1   14.9768 microns  11)  1363.32 cm-1  7.33504 microns
    2)   680.23        14.7009          12)  1489.42        6.71402
    3)   691.15        14.4686          13)  2191.38        4.56333
    4)   704.33        14.1979          14)  2208.74        4.52747
    5)   716.30        13.9606          15)  2237.49        4.46929
    6)   733.13        13.6401          16)  2269.09        4.40705
    7)   750.72        13.3205          17)  2360.00        4.23729
    8)   899.50        11.1173          18)  2514.58        3.97681
    9)  1029.01        9.71808          19)  2665.38        3.75181
   10)  1224.07        8.16947          20) 14453.14        0.691891

These channels are chosen to sample

  • atmospheric emission in seven 15.3 micron CO2 (temperature) channels
  • atmospheric emission in five 4.3 micron CO2 (temperature) channels
  • surface and H2O emission in one 11.0 micron window channel
  • surface and O3 emission in one 9.6 micron window channel
  • atmospheric emission in three6.7 micron H2O channels
  • surface emission and reflected solar radiation in two 3.7 micron window channels.

A 15 cm diameter optical system is used to gather emitted energy from Earth's atmosphere and surface. The instantaneous field of view of all the channels is stepped across the satellite track by use of a rotating mirror. The energy received by the telescope is separated by a dichroic beam splitter into longwave (greater than 6.4 microns) and shortwave (less than 6.4 microns) energy, controlled by field stops, and passed through bandpass filters and relay optics to the detectors. There are 56 steps per scan, each requiring 100 milliseconds, for a total of 6.4 seconds per scan. The analog data output from the HIRS/2 sensor is digitalized onboard the satellite at a rate of 2880 bits per second, implying 288 bits per step. The data are digitized to 13 bit precision.

MSU--The MSU instrument is a four channel Dicke radiometer making passive microwave radiation measurements in four regions of the 50 GHz oxygen emission spectrum. The central frequencies of these channels are

         1)   50.30 GHz              3)   54.96 GHz
         2)   53.74 GHz              4)   57.95 GHz

These channels are chosen to sample

  • atmospheric emission in three 56 GHz O2 (temperature) channels
  • surface emission in one 56 GHz window channel.

The channel bandwidths are 200 MHz in each case, with a typical Noise Equivalent Differential Temperature (NEDT) of 0.3 degrees K. The instrument has two 4 inch scanning reflector antenna systems, orthomode transducers, four Dicke superheterodyne receivers, a data programmer, and power supplies. The antennas are step scanned through eleven individual 1.84 second Earth-viewing steps and require a total of 25.6 seconds to complete. The MSU data output represents an apparent brightness temperature after a 1.84 second integration period per step. The data are quantized to 12 bit precision and combined with telemetry and step position information to produce an effective output rate of 320 bits per second.

The TOVS methodology makes use of a combination of HIRS/2 and MSU channel radiances to infer information pertaining to the following groups of geophysical parameters from the associated channels.


     Parameter                Channels Used
     Temperature Profile      HIRS 1,2,4,13,14,15, MSU 3,4
     Moisture Profile         HIRS 8,10,11,12

     Clouds                   HIRS 4,5,6,7,8
     Surface Temperature      HIRS 8,18,19
     Cloud Cleared Radiances  HIRS 13,14, MSU 2
     Ozone                    HIRS 9

In particular, the combination of HIRS/2 channels and MSU channels (which can "see" through nonprecipitating clouds) is extremely useful in eliminating the effects of cloudiness on the satellite-observed infrared radiances, thus providing improved estimates of the temperature and moisture profiles.

Instrument Measurement Geometry--The instrument measurement geometry for the TOVS sensors are summarized in the following table.

     Instrument parameter         HIRS/2          MSU  
     Cross track scan angle
     (+/- degrees from nadir)      49.5           47.4 
     Number of steps                 56             11 
     Angular FOV (degrees)         1.25            7.5 
     Step Angle (degrees)          1.80            9.5 
     Ground IFOV (km)
       - at nadir                  17.4          109.3 
       - at end of scan         59 x 30      323 x 179 
     Swath width (+/- km)          1120           1174 

The NOAA Polar Orbiter Data User's Guide (Kidwell 1991) gives a more detailed description of the instruments and the NOAA series of satellites.





  • File Size: There are eleven data files for each 5-day average containing one or more horizontal fields of 360 x 180 = 64800 floating point numbers in IEEE 32-bit floating point notation.
       Parameter Name   File Size (Bytes)      Data Values
        CLTEMP           1036800               259200
        FCLD              259200               64800
        FCLD7            1814400               453600
        LWF               259200               64800
        OLR               259200               64800
        PCLD              259200               64800
        PRC               259200               64800
        PRWAT            1296000               324000
        SPRC              259200               64800
        TCLD              259200               64800
        TSURF             259200               64800
  • Data Format:IEEE floating point
  • Headers, trailers, and delimiters:none
  • Land or water mask: none
  • Fill value: -999.9
  • Data Ordering:
    Single fields: Starting at (179.5W,89.5S) and proceeding west to east and then from south to north as in
       (179.5W,88,5S), (178.5W,88.5S), ... ,(179.5E,88.5S),
       (179.5W,88,5S), (178.5W,88.5S), ... ,(179.5E,88.5S),
             ...           ...         ...       ...      
       (179.5W,89.5N), (178.5W,89.5N), ... ,(179.5E,89.5N)
    Note that the TOVS monthly mean files are ordered in north to south orientation.


    Multiple fields: each field in the order above and
    • For CLTEMP, the first 259200 bytes represent the mean layer temperature from the surface-500mb, the second 259200 bytes represent the mean layer temperature from 500-300 mb, and so on up to the fourth 259200 bytes, which represent the mean layer temperature from 100 - 30 mb.
    • For PRWAT, the first 259200 bytes represent the precipitable water (integrated water vapor) above the surface, the second 259200 bytes represent the precipitable water above 700 mb, and so on up to the fourth 259200 bytes, which represent the precipitable water above 300 mb.
    • For FCLD7, the first 259200 bytes represent the cloud fractions for the layer < 180 mb, the second 259200 bytes for the cloud fractions for the layer between 180-310 mb, and so on up to the seventh259200 bytes, which represent the cloud fraction for the layer > 800 mb.



Naming Convention
xxxxxxxx instrument and satellite code where
tovsnf for NOAA9
tovsng for NOAA10
pppppp parameter name
lptegg code for spatial/temporal resolution & coverage where
l= number of levels, 1, 4, 5, 7
p= pressure levels for vertical coordinate
p= 5-day (pentad) averages
e= 1 deg x 1 deg horizontal grid resolution
gg= global (land and ocean) coverage
yymm date of data where
yy= year in two digits
mm= month in two digits
dd= first day, in two digits, of five day period

Directory Path

where # is 9 or 10 (for NOAA9 and NOAA10 platforms), pppppp is parameter and yyyy is year

Ascii tables containing minimums, maximums, means and standard deviations of 20 degree latitudinal zones, as well as plots (displayed in gif format) of those statistics have been created. The directory paths for these are:

respectively. See the readmes in in each of the ascii_files and gif_files directories for additional information on these files.


Sample programs in FORTRAN and IDL languages are available to read and display these data files.





The radiation fluxes at specific infrared and microwave frequencies incident on a satellite looking down through the atmosphere most heavily sample temperature and density properties near particular atmospheric pressures. This maximum in transmission of radiation from a particular pressure in the atmosphere up to the satellite is due to the cumulative effects of the spectroscopic properties of the constituent atmospheric molecules and their dependence on temperature and density in the column above the particular pressure.(This presure for a maximum varys only slightly with the temperature and density of the gases in the column and is most dependent on the frequency.)

For CO2 and water molecules particular frequencies in the microwave and infrared ranges permit sampling of the atmosphere from the surface up to 20 mb.For O3 molecules the sampling effect is sufficiently broad that only the total ozone in the atmosphere can be measured.The total ozone estimates is not included in this subset but is in the original data set.

Using a retrieval algorithm the measured radiances at the satellite can be "inverted" to find the temperature and densities of the constituents in the atmosphere giving rise to those measured radiances.In some cases the maximum occurs at or near the surface and in these cases it is possible to measure surface temperatures and emissivities.Though clouds "contaminate" the retrieval process it is possible to allow for their contribution in a self-consistent manner and obtain temperature and moisture retrievals even below clouds.



The processing system steps through an interactive forecast- retrieval-analysis cycle. In each 6 hour synoptic period, the 6 hour forecast fields of temperature, humidity, and geopotential thickness generated by the Goddard Laboratory for Atmospheres (GLA) 2nd order General Circulation Model (GCM) (Takacs et al. 1994) are used as the first guess for all soundings occurring within a 3 hour time window centered on the forecast time.These retrievals are then assimilated with all available in situ measurements (such as radiosonde and ship reports) in the 6 hour interval using an Optimal Interpolation (OI) analysis scheme.This analysis is then used to specify the initial conditions for the next 6 hour forecast, thus completing the cycle.The GCM and the OI were developed by the Data Assimilation Office (DAO) at Goddard Space Flight Center.

The retrieval algorithm itself is a physical method based on the iterative relaxation technique originally proposed by Chahine (1968).The basic approach consists of modifying the temperature profile from the previous iteration by an amount proportional to the difference between the observed brightness temperatures and the brightness temperatures computed from the trial parameters using the full radiative transfer equation applied at the observed satellite zenith angle. For the case of the temperature profile, the updated layer mean temperatures are given as a linear combination of multichannel brightness temperature differences with the coefficients given by the channel weighting functions. Constraints are imposed on the solution to ensure stability and convergence of the iterative process. For more details see Susskind et al. (1984).

Two important procedures are necessary for accurate retrieval of the geophysical parameters using satellite-based radiance measurements. The first involves reconstruction of the clear sky radiances that would have been observed in the absence of cloud contamination. This is performed using a variation of the N* method applied to adjacent fields of view (over an area covering 2 along- track and 2 cross-track HIRS2 spots) using a combination of infrared and microwave channels. The second procedure involves the need for a bias correction stemming from a combination of instrument calibration errors and drifts and errors in the radiance computations. The systematic errors between computed and observed brightness temperatures are modeled as a function of latitude and satellite zenith angle, with the coefficients determined by a least squares fit to the radiance residuals resulting between the observed brightness temperatures and those obtained from the globally unbiased GLA forecast model. These coefficients are updated periodically throughout the day and the resulting radiance corrections are applied to all computed brightness temperatures used in the derivation of the geophysical parameters.

The output from the processing at this point consists of geophysical quantities that are located along the satellite track that are measured at approximately the same local time in two groups, the ascending orbital tracks designated as AM, and the descending orbital tracks that are designated PM (Level 2 data).These data are subsequently gridded in the AM and the PM groups separately into 1 degree x 1 degree gridboxes by averaging the satellite track measurements that fall in the same box (Level 3 data).The data are then converted to Hierarchical Data Format (HDF) and output as 30MB daily files.The data are also averaged into 5 day composites (pentads) and monthly averages in separate AM and PM groups.

To obtain the data set described by this document, the original 30 MB HDF 5-day AM and PM files are averaged together (using appropriate weighting in each gridbox for the case of cloud amounts) and then output as flat binary files. A more complete description of processing is available in TOVS Pathfinder Path A Guide: Data Processing Sequence.



TOVS is the only long-term source of high resolution global information pertaining to the temperature and moisture structure of the atmosphere. Because similar HIRS/2 and MSU instrumentation has flown on operational satellites from 1979 to the present, data from these instruments can make an important contribution to our understanding of the variability of atmospheric and surface parameters as well as the correlations between spatial variations of atmospheric and surface quantities. In addition, the data can potentially be used to identify and monitor trends in temperature, moisture, cloudiness, OLR, and precipitation, provided that quantitative results can be obtained that account for differences in instrumentation on different satellites, as well as sampling differences in local crossing time. A prerequisite for such studies is an algorithm that does not change during the course of the processing. This is required since algorithm changes can introduce spurious "climate changes." The TOVS data set satisfies this important criterion and as such will be useful for all of the applications listed above. Other possible applications of the data set include




The level 3 Path A parameters were validated against independently measured data from both in situ and satellite sources.

  • Temperature and humidity parameters were compared to collocated radiosonde data. The total precipitable water above oceanic areas was also compared to data derived from the Special Sensor Microwave/Imager (SSM/I).
  • The surface skin temperature over ocean was compared to values produced by the NOAA Climate Analysis Center (CAC) based on ship, buoy, and AVHRR data.
  • The total atmospheric column ozone burden was validated against Total Ozone Mapping Spectrometer (TOMS) data, which were also used in the zonal mean sense as part of the systematic error removal scheme for total ozone retrievals.
  • OLR was validated against OLR determined by the ERBE team using the ERBE instruments on NOAA-10 and ERBS.ERBS is a tropical orbiting satellite, and this adds a temporal sampling bias in the tropics. Longwave cloud radiative forcing has not been validated at this time.
  • The precipitation estimate was compared with rain gauges, which are primarily over land.

In addition to these direct correlative data comparisons, errors between interannual differences computed for the TOVS data and the interannual differences computed from the correlative data were provided based on the monthly gridded results from July 1987 and July 1988 and are available in TOVS Pathfinder Path A Guide: Data Validation.





The TOVS atmospheric Global Data Set resides on DAAC anonymous FTP.You may access the files from this document,

ftp icon TOVS 5-DAY ATMOSPHERIC DATA SET (Binary data files)


or directly via FTP at


password:< your internet address >



For information about or assistance in using any DAAC data, contact


EOS Distributed Active Archive Center (DAAC)
Code 610.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
301-614-5304 (fax)




Baker, W.E., R. Atlas, M. Halem, and J. Susskind. 1984. A case study of forecast sensitivity to data and data analysis techniques. Mon.Wea. Rev., 122:544-1561.

Chahine, M. T. 1968. Determination of the temperature profile in an atmosphere from its outgoing radiances. J. Opt. Soc. Am., 58:1634-1637.

Justice, C.O., T.F. Eck, D. Taure, and B.N. Holben. 1991. The effect of water vapor on normalized difference vegetation index derived for the Sahelian region from NOAA AVHRR data. Int. J. Remote Sensing, 1165-1187.

Kidwell, K. 1991. NOAA Polar Orbiter Data User's Guide. NCDC/SDSD. National Climatic Data Center, Washington, DC.

Schubert, S.D., R. Rood, and J. Pfaendtner. 1993. An assimilated data set for earth science applications. Bull. Amer. Meteor. Soc., 74:2331-2342.

Susskind, J., J. Rosenfield, D. Reuter, and M.T. Chahine. 1984. Remote sensing of weather and climate parameters from HIRS2/MSU on TIROS-N. J. Geophys. Res., 89:4677-4697.

Susskind, J., and D. Reuter. 1985. Retrieval of sea-surface temperatures from HIRS2/MSU. J. Geophys. Res., 90C:11602- 11608.

Takacs, L., A. Molod, and T. Wang. 1994. Documentation of the Goddard Earth Observing System (GEOS) General Circulation Model Version 1, NASA Technical Memorandum 104606 Volume I.

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