Data Access

TOVS Pathfinder Atmospheric Sounding Data

Mean temperature at four pressure layers

Total effective cloud fraction

Cloud fractions at seven pressure layers

Longwave cloud radiative forcing

Outgoing long-wave radiation

Cloud top pressure

Precipitation estimate

Precipitable water vapor above the surface and four pressure levels

Surface pressure

Cloud top temperature

Surface skin temperature

Readme Contents

Data Set Overview

Sponsor

Original Archive

Future Updates

The Data

Characteristics

Source

The Files

Format

Name and Directory Information

Companion Software

The Science

Theoretical Basis of Data

Processing Sequence and Algorithms

Scientific Potential of Data

Validation of Data

Contacts

Points of Contact

References

Data Set Overview

The TOVS (TIROS Operational Vertical Sounder) data set is a collection of monthly means of global coverage during the years 1985 - 1992 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, 10, 11, 12, and 14. The fields are gridded on a 1 X 1 degree latitude-longitude grid. The monthly means are generated from each satellite separately. 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.

Sponsor
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 Sounder Research Team (Code 910.4) and the Distributed Active Archive Center (Code 902) 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.

Original Archive
The atmospheric data from which this data set is derived were produced by the Sounder Research Team of NASA's 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's Atmospheric Dynamics site. The derived data set contains a subset of the most important geophysical parameters contained in the original archive.

Future Updates
It is expected that additional three years of data will become available in Fall 1997 for the satellite coverage by NOAA-11, 12, & 14 of the years 1993, 1994 and 1995. 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.

The Data

Characteristics

• Parameters, Units, Range
  

PARAMETER	DESCRIPTION	UNITS	DATA RANGE
CLTEMP	Mean temperature in the layers: surface-500 mb, 500-300 mb, 300-100 mb, and 100-30 mb	K	180 - 295
PRWAT	Precipitable water (integrated water vapor) above the levels: surface, 850 mb, 700 mb, 500 mb, 300 mb	cm	0 - 8
TSURF	Surface skin temperature	K	200 - 320
FCLD	Total effective cloud fraction	fraction	0 - 1
FCLD7	Cloud fractions for the layers <180 mb, 180-310 mb, 310-440 mb, 440-560 mb, 560-680 mb, 680-800 mb, >800 mb	fraction	0 - 1
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 of the atmosphere	W/m^2	80 - 350
LWF	Longwave cloud radiative forcing, or the difference between the cloudy and clear sky OLR	W/m^2	-70 to +160
PRC	Precipitation estimate	mm/day	0 - 40
SPRC	Surface pressure derived from forecast model	mb	525 - 1050

• Temporal Coverage: January 1985 - December 1992
• Temporal Resolution: Monthly Means

  NOAA-9     Jan'85-Dec'86 
  NOAA-10    Dec'86-Dec'90
  NOAA-11    Jan'89-Dec'92

• Spatial Coverage: Global
• Spatial Resolution: 1 degree x 1 degree

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

Satellites:

The HIRS/2 and MSU instruments are carried aboard National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental Satellites NOAA-9,10,11,12 and 14. .

Nominal orbit parameters for the NOAA satellites are:

Launch date: Dec 12, 1984 (NOAA-9); Sept 17, 1986 (NOAA-10);

Sept 24, 1988 (NOAA-11); May 14, 1991 (NOAA-12);

Dec 30, 1994 (NOAA-14)

Orbit: Sun synchronous, near polar

Nominal altitude: 833 km

Inclination: 98.8 degrees

Orbital period: 102 minutes

Nominal Equatorial crossing times for ascending Node, LST:
14:30 (NOAA-9); 13:30 (NOAA-11);
19:30 (NOAA 10 & 12); 13:30 (NOAA 14)
Nominal Equatorial crossing times for descending Node, LST:
02:30 (NOAA-9); 01:30 (NOAA-11);
07:30 (NOAA 10 & 12); 01:30 (NOAA-14)

Nodal Increment: 25.3 degrees

The orbital period of about 102 minutes produces 14.1 orbits per day. Because the daily number of orbits is not an integer, the suborbital tracks do not repeat daily. The crossing times are nominal because satellite orbits drift over time. As of March 95, NOAA-9 north bound equator crossing time has moved from 14:30 launch time to 21:30, NOAA-10 from 19:30 to 17:53, and NOAA-11 from 13:30 to 17:30 LST.

Instruments:

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 nominal values of central wave numbers and wavelengths of these channels are

chan	wavenumber (cm-1)	wavelength (microns)	chan	wavenumber (cm-1)	wavelength (microns)
1)	667.70	14.9768	11)	1363.32	7.33504
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

*For better retrieval of atmospheric water vapor the spectral location of channel 10 was changed (to avoid silicate absorption feature) from 1224 cm-1 to be centered near 900 cm-1 on NOAA-11,14 and future HIRS2 instruments.

These channels are chosen to sample

• atmospheric emission in seven CO2 (temperature) channels near 15.3 micron
• atmospheric emission in five O2 (temperature) channels near 4.3 micron
• surface and H2O emission in a channel near 11.0 micron
• surface and O3 emission in a channel near 9.6 micron
• atmospheric emission in three H2O channels near 6.7 micron
• surface emission and reflected solar radiation in two window channels near 3.7 micron.

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.

Microwave Sounding Unit (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 O2 (temperature) channels near 50, 54, and 55 GHz
• surface emission in one window channel near 57 GHz.

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
Ground IFOV (km) - 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.

Format

Compressed:

The data files have been compressed using Lempel-Ziv coding. Files with a .gz ending are compressed versions of the .bin file. When decompressing the files use the -N option so that the original .bin file name ending is restored. For additional information on decompression see aareadme file in the directory:

software/decompression/

Uncompressed:

• File Size: There are eleven data files for each monthly 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,89.5N) and proceeding west to east and then from north to south as in

    
     (179.5W,89.5N), (178.5W,89.5N), ... ,(179.5E,89.5N),
     (179.5W,88,5N), (178.5W,88.5N), ... ,(179.5E,88.5N),
           ...           ...         ...       ...      
     (179.5W,88,5S), (178.5W,88.5S), ... ,(179.5E,88.5S)
  

  The TOVS monthly means files in this data set (the Interdisciplinary data set) have been reordered in north to south orientation, while the TOVS files available through the DAAC Information Management System (IMS) from which they derive have been produced in south to north 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 seventh 259200 bytes, which represent the cloud fraction for the layer > 800 mb.

Name and Directory Information

Naming Convention:

The file naming convention for the TOVS data files is

xxxxxxxx.pppppp.lpmegg.yymm.ddd

xxxxxxxx instrument and satellite code where

tovsnf for NOAA-9

tovsng for NOAA-10

tovsnh for NOAA-11

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

m= monthly 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

ddd= File type, (gz=compressed, bin=binary uncompressed, ctl=GrADS control file)

NOTE: When decompressing the data files be sure to use the -N option. This will restore the original .bin filename. For additional information on decompression see the format section of this readme and the aareadme file in the directory:

software/decompression/

Directory Path:

/data/inter_disc/tovs_atmo_sound/pppppp/yyyy

where pppppp is parameter and yyyy is year

Companion Software
Several software packages have been made available on the CIDC CD-ROM set. The Grid Analysis and Display System (GrADS) is an interactive desktop tool that is currently in use worldwide for the analysis and display of earth science data. GrADS meta-data files (.ctl) have been supplied for each of the data sets. A GrADS gui interface has been created for use with the CIDC data. See the GrADS document for information on how to use the gui interface.

Decompression software for PC and Macintosh platforms have been supplied for datasets which are compressed on the CIDC CD-ROM set. For additional information on the decompression software see the aareadme file in the directory:

software/decompression/

Sample programs in FORTRAN, C and IDL languages have also been made available to read these data. You may also acquire this software by accessing the software/read_cidc_sftwr directory on each of the CIDC CD-ROMs

Theoretical Basis of Data
The radiation fluxes at specific infrared and microwave frequencies 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 pressure for a maximum varies 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.

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 parameters measured by the TOVS all undergo a diurnal cycle. To obtain good diurnal averages, several measurements should be made each 24 hour day. A Sun synchronous satellite obtains one day and one night time measurement per 24 hours over most of the earth. The mean of these two observations yield an estimate of the daily averages for the parameters. When both morning and afternoon satellites (e.g. NOAA-10 and NOAA-11) are operating four daily measurements are available and the mean of these four will yield an improved daily average. However only one satellite is frequently available; sometimes this will be a morning satellite and sometimes an afternoon satellite. Thus care must be taken in comparing one year to another year in which the observing times are different. In 1989 and 1990 both NOAA-10 and NOAA-11 are operating. We have chosen not to combine their measurements into an improved diurnal average. This allows the user to estimate the types of regional biases that may be present when measurement from only one satellite are available.

Processing Sequence and Algorithms
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 30 MB 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 monthly AM and PM files are averaged together. The cloud top pressure and temperature are weighted by the cloud fraction when their means are calculated. The output is in the form of flat binary files where the order of the latitude bands is flipped to north to south. In the HDF files, the order was from south to north. A more complete description of processing is available in TOVS Pathfinder Path A Guide: Data Processing Sequence.

Scientific Potential of Data
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

• Assimilation of TOVS data into large scale models to improve forecast skill (Baker et al. 1984, Schubert et al. 1993).
• Intercomparison and validation of global parameters derived from other satellite instrumentation such as AVHRR SST (Susskind and Reuter 1985, Susskind et al. 1997) .
• Provide global moisture profile estimates for use in atmospherically correcting AVHRR radiances for the determination of vegetation indices (Justice at al. 1991).

Validation of Data
The level 3 Path A parameters were validated against independently measured data from both in situ and satellite sources (Susskind et al. 1997).

• 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.

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

EOS Distributed Active Archive Center (DAAC)

Code 902

NASA Goddard Space Flight Center

Greenbelt, Maryland 20771

Internet: daacuso@daac.gsfc.nasa.gov

301-614-5224 (voice)

301-614-5268 (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, 12: 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.

Susskind, J., P. Piraino, L. Rokke, L. Iredell, and A. Mehta, 1997: Characteristics of the TOVS Pathfinder A dataset, Bull Amer. Meteor. Soc., 78: 1449-1472.

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.

NASA	Goddard	GDAAC	CIDC