NASA Climatology Interdisciplinary Data Collection/NASA Climatology Interdisciplinary Data Collection - Disc 1.iso/readmes/readme.toms_ozone

Text File | 1998-03-04 | 28KB | 617 lines

                              [CIDC FTP Data]
                        [TOMS OzoneIDC Data on FTP]

Data Access

     Total Column Ozone from Nimbus-7 Total Ozone Mapping Spectrometer
     (TOMS)

                                   [rule]

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

                                   [rule]

Data Set Overview

     The dataset consists of 1 x 1 degree gridded monthly averaged
     total column ozone. It is derived from the Nimbus-7 TOMS monthly
     averaged total column ozone data gridded at 1.25 x 1 degree
     (lon/lat). The Nimbus-7 TOMS data is the only source of high
     resolution global information about the total ozone content of the
     atmosphere for the period November 1, 1978 - May 6, 1993. The
     ozone dataset was produced by the Ozone Processing Team (OPT)
     using final Nimbus-7, Version 7, OPT data reduction algorithm and
     was released in the Spring of 1996. This ozone data set is
     important in studies involving atmospheric chemistry and upper air
     dynamics on both short (interannual) and long (decadal) time
     scales.

     Sponsor
     The production and distribution of this data set are being funded
     by NASA's Earth Science enterprise. 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 Ozone Processing Team
          (OPT) of the Atmospheric Chemistry & Dyamics Brach(Code
          916) 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 Earth Science
          enterprise.

     Original Archive
     The total ozone data from which this data set is derived were
     produced by the Ozone processing team under the direction of Dr.
     Richard McPeters (code 916), the Nimbus Project Scientist of
     NASA's Goddard Space Flight Center. The original data, which
     includes daily, and monthly gridded products, are currently
     available from the Goddard DAAC's Atmospheric Chemistry Site

     Future Updates
     In the Spring of 1996, the old Version 6 of the Nimbus-7 TOMS data
     was replaced with the new Version 7. No future updates of the
     Nimbus-7 product are planned. However the ozone data from follow-
     on TOMS instruments such as those on the Meteor-3, Earth Probe
     (EP), Advanced Earth Observation Satellite (ADEOS) will be added
     to this data set collection as they become available.

The Data

     Characteristics

        * Parameters: Total Ozone, defined as the vertically integrated
          ozone in a column from the surface to the top of the
          atmosphere

        * Units: Dobson Units (D.U), where 1 D.U. corresponds to the
          amount of gas at standard temperature and pressure (STP) that
          would form a layer .001 cm thick
        * Typical Range (year): global 100 - 500 D.U.

        * Temporal Coverage: November 1, 1978 - May 6, 1993
        * Temporal Resolution: Gridded monthly means

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

     Source
     The Total Ozone Mapping Spectrometer (TOMS) was designed and built
     in the mid-1970s as part of a comprehensive package of scientific
     instruments to be flown aboard NASA's Nimbus-7 spacecraft. It was
     one of eight instruments designed to provide continuous, long-term
     monitoring of atmospheric, ocean, and surface parameters on a
     global basis throughout most of the 1980s.

     Nominal orbit parameters for the Nimbus-7 spacecraft were

          Launch date: 10/24/78
          Orbit: Sun synchronous, near polar
          Nominal altitude: 955 km
          Inclination: 99.3 degrees
          Nodal period: 104 minutes
          Equatorial crossing time: 12:00 PM (ascending)
          Nodal Increment: 26.1 degrees

     The TOMS instrument is a single stage fixed-grating Ebert-Fastie
     monochromater with a rotating chopper wheel to resolve the
     incoming light into 6 wavelength bands with a 1 nanometer (nm)
     bandpass. These wavelengths are

             312.5 nm        317.5 nm        331.3 nm
             339.9 nm        360.0 nm        380.0 nm

     TOMS scans in the cross-track direction in 3 degree steps from 51
     degrees on one side of nadir to 51 degrees on the other, for a
     total of 35 samples. The instantaneous field of view (IFOV) of 3
     degrees by 3 degrees results in a footprint varying from a 50 km x
     50 km square at nadir to a 125 km x 280 km diamond at the scan
     extremes. The total swath width is 3000 km, implying that
     consecutive orbits overlap to create a contiguous mapping of ozone
     data. Approximately 200,000 measurements are made on a daily basis
     during the sunlit portions of the orbits.

     The ratios of backscattered to incident ultraviolet radiation at
     the four shortest TOMS wavelengths are used to infer total ozone,
     while the corresponding ratios at the two longer wavelengths are
     used for estimating the effective reflectivity caused by the
     combined influence of Earth's surface, clouds, and tropospheric
     aerosols. The new Version 7 User's Guide describes the algorithm
     in detail. It can be obtained from the Ozone Processing Team's
     Nimbus-7/TOMS page.

     The Nimbus-7 TOMS instrument ceased to function on May 7, 1993.

The Files

     Format

        * File Size: 259200 bytes, 64800 data values
        * Data Format: IEEE floating point notation
        * Headers, trailers, and delimiters: none
        * Fill value: -999.9
        * Image orientation: North to South
          Start position: (179.5W, 89.5N)
          End position: (179.5E, 89.5S)

     Name and Directory Information

     Naming Convention:

     The file naming convention for this data set is

          tomsn7.o3.1nmegg.[yymm].ddd

     where
          tomsn7 = data product designator
          o3 = parameter name
          1 = number of levels
          n = vertical coordinate, n = not applicable
          m = temporal period, m = monthly
          e = horizontal grid resolution, e = 1 x 1 degree
          gg = spatial coverage, gg = global (land and ocean)
          yy = year
          mm = month
          ddd = file type designation, (bin=binary, ctl=GrADS control
          file)

     Directory Path:

          /data/inter_disc/atmo_constituents/toms_ozone/yyyy/

     where 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

The Science

     Theoretical Basis of Data
     Incoming solar radiation undergoes absorption by gases such as
     ozone and Rayleigh scattering by molecules in the stratosphere.
     Radiation that penetrates to the troposphere is scattered by
     clouds and aerosols, with the radiation that reaches the ground
     being scattered by surfaces of widely different reflectivity. The
     two shortest wavelengths chosen for use in the TOMS ozone
     measurements were selected because of their high ozone absorption.
     Absorption by other atmospheric components, at these wavelengths,
     is negligible compared to that of ozone.

     The backscattered radiance at a given wavelength depends, in
     principle, upon the entire ozone profile from the top of the
     atmosphere to the surface. At wavelengths longer than 310 nm,
     however, the backscattered radiance consists primarily of solar
     radiation that penetrates the stratosphere and is reflected back
     by the dense tropospheric air, clouds, aerosols and the Earth's
     surface. Because most of the ozone (about 90%) is in the
     stratosphere, the principal effect of total atmospheric ozone is
     to attenuate both the solar flux reaching the troposphere and the
     component reflected back to the satellite. This nearly complete
     spatial separation of the absorber elements in the stratosphere
     (i.e. ozone) from the "reflector" elements in the troposphere
     (i.e. aerosols, clouds, and Earth's surface) causes backscattered
     radiances longer than 310 nm to depend only weakly on the vertical
     distribution of ozone in the atmosphere. In the simplest case,
     whereby tropospheric and surface characteristics remain unchanged
     from one measurement to the next, and with no aerosols present in
     the stratosphere, a decrease (increase) in the backscattered
     radiance at the shortest TOMS wavelengths would signify an
     increase (decrease) in the total ozone amount below the satellite.
     Further discussion concerning the theory behind backscattered
     ultraviolet radiation and its relationship to atmospheric ozone
     can be found in Liou (1980) and Klenk et al. (1982).

     Derivation of atmospheric ozone content from measurements of the
     backscattered radiances requires a treatment of reflection from
     the Earth's surface and of scattering by clouds and other
     aerosols. In general, the scattered or reflected light depends on
     both incidence angle of the sunlight and viewing angle of the
     satellite. Studies (e.g., Dave, 1978) and ( Bhartia et al. 1993).
     show that, in practice, the contribution of clouds and
     tropospheric aerosols to the backscattered intensity can be
     treated by assuming that the effective lower boundary of the
     atmosphere is located at an average pressure in the troposphere,
     representing a "mix" of the estimated surface and cloud top
     pressures. Furthermore, this lower boundary will be assigned an
     effective Lambertian reflectivity which accounts collectively for
     the backscattering effects of clouds, tropospheric aerosols and
     the surface of the earth. This process of deriving an effective
     "scene reflectivity" and an associated effective "scene pressure"
     is performed for every instantaneous field-of-view (IFOV) along
     the TOMS scanline. In the presence of stratospheric aerosols (such
     as those resulting from volcanic eruptions), the concept of an
     average scene reflectivity and scene pressure break down, and the
     data must be flagged for this contamination accordingly ( Bhartia
     et al. 1993).

     Processing Sequence and algorithms
     The intensity of solar radiation backscattered by the
     earth-atmosphere system and received by a sensor aboard an
     earth-orbiting satellite can be expressed as:

                     I(i) = Ia(i) + Ig(i)

     where

        * I(i) is the backscattered radiance at wavelength i
        * Ia(i) is the atmospheric contribution to the radiance at
          wavelength i
        * Ig(i) is the contribution due to multiple reflections from
          the surface at wavelength i.

     The ground contribution is given by:

                     Ig(i) = R*T(i) / (1-R*S(i))

     where

        * R is the Lambertian reflectivity of the lower boundary
        * T(i) is the amount of direct plus diffuse radiation reaching
          the surface, then diffusely transmitted upward to the
          satellite
        * S(i) is the fraction of radiation reflected by the surface
          that is scattered back to the surface by the atmosphere. The
          term 1/( 1- R*S) effectively accounts for multiple
          reflections between the ground and the atmosphere.

     In the above, I(i) and Ig(i) depend upon total ozone amount, the
     effective scene pressure level and reflectivity, the solar zenith
     angle and the satellite viewing angle. The purely atmospheric
     contribution Ia(i) as well as T(i) depend upon all of the above
     except the reflectivity R. The values of I(i), Ia(i) and to a
     lesser extent Ig(i) are also somewhat dependent upon the shape
     (i.e., vertical distribution) of the ozone profile.

     Once the measured backscattered radiances have been corrected for
     the effects of wavelength drift and changes in the instrument
     optics and sensitivity (see McPeters et al., 1996); McPeters and
     Komhyr, 1991) a quantity called the "N-value", or N(i), is
     computed. it is defined as:

             N(i) = -100 log[ I(i) / F(i) ]

     where the ratio I/F denotes the backscattered radiance I(i)
     normalized by the direct solar radiation, F(i), incident at the
     level of the sensor.

     Given the optical properties of the atmosphere at each TOMS
     wavelength, a set of tables is created relating total ozone to I/F
     (and thus N) for several independent variables. These include

        * Climatological ozone profiles (23 profiles, 3 low latitude,
          10 mid latitude, 10 high latitude)
        * Pressure of the reflecting surface, (2 pressures, 400 mb and
          1000 mb)
        * Solar zenith angle (10 angles, 0 to 88 degrees)
        * Satellite zenith angle at the IFOV (6 angles, 0 to 63
          degrees)

     The scene reflectivity R is not included as a table variable since
     the tabulated quantities of interest, S(i), T(i) and Ia(i), do not
     depend upon it. The theoretical values of I/F at each of the 6
     TOMS wavelengths are calculated using the radiative transfer
     methodology of Dave (1964).

     The computation of total column ozone is accomplished by computing
     radiance ratios called Pair values, which are ratios of I/F at a
     longer wavelength, which is relatively insensitive to ozone, to
     that of a shorter, ozone-sensitive wavelength. The pair values are
     defined as:

             A-Pair = N(313 nm) - N(331 nm)
             B'-Pair = N(318 nm) - N(340 nm)
             C-Pair = N(331 nm) - N(340 nm)

     Pairs are chosen about 20 nm apart or less, so that scattering
     effects are about the same, and the relative attenuation of the
     pair is sensitive mostly to ozone absorption. In addition, the
     ratios of the radiances help to minimize calibration errors and
     wavelength independent effects. Different pairs of wavelengths are
     used for different conditions, i.e., for large ozone amounts at
     low sun angles the A-pair becomes less sensitive to changes in
     total ozone since 313 nm senses higher in the atmosphere (Klenk et
     al., 1982). It also becomes more sensitive to ozone profile shape;
     thus more weight will be placed upon the derived B'- and C-Pair
     ozone values in this case.

     Table interpolation is used to extract four total ozone estimates
     corresponding to each of three measured Pair values, i.e., at 1000
     mb and 400 mb, and for two standard latitude bands on either side
     of the actual latitude measured for each pressure. Prior to this,
     surface reflectivities for the two pressures are computed from the
     two longest TOMS wavelengths, 360 nm and 380 nm, which are not
     sensitive to ozone, as follows:

             R = (I - Ia)/(T - S*(I-Ia) )

     where Ia, T, and S are obtained from the tables for the given sun
     and satellite angles and for each of the two pressures, and I is
     the satellite-measured radiance at 360 nm or 380 nm.

     An estimated scene pressure (Ps) is then computed using a
     climatological cloud top pressure (Pc), and actual terrain
     pressure (Pt) as follows:

             Ps = (1-w)*Pc + w*Pt

     where w is a weighting function based upon the measured surface
     reflectivity R at 1000 mb and on the presence or absence of
     snow/ice.

     The Pair ozone values for the latitude and derived scene pressure
     (Ps) of the measurements are obtained by linear interpolation in
     latitude between the values for the two surrounding latitudes,
     followed by linear interpolation in pressure between the 400 mb
     and 1000 mb values.

     Finally, a "best ozone" value is obtained as a weighted average of
     the total ozone values derived from the A, B', and C-pairs. The
     weighting takes into account the sensitivities of the individual
     Pair values to the profile shape, the solar zenith angle and to
     changes in the total ozone itself.

     The above description is taken from the Version 6 User's Guide.
     The Version 7 data are produced using a revised instrument
     calibration based on analysis of the entire 14.5 year data record,
     as well as an improved algorithm. Improvements include:

        * corrected 0.2 nm wavelength calibration error; this error
          caused a 3% absolute offset relative to Dobson
        * use of wavelength "triplets" that correct for errors linear
          in wavelength
        * improved ISCCP cloud height climatology, higher resolution
          terrain height maps
        * use of improved profile shape selection to improve total
          ozone at very large solar zenith angles
        * use of a more accurate model for partially-clouded scenes
        * improved radiative transfer calculations for table generation

     The new Version 7 User's Guide describes the algorithm in detail.
     It can be obtained from the Ozone Processing Team's Nimbus-7/TOMS
     page.

     In addition to deriving ozone values for every individual
     field-of-view over a day, the Ozone Processing Team has created a
     daily gridded product. Out of the approximately 200,000
     measurements per day, only those values not contaminated by
     stratospheric aerosols, volcanic sulfur dioxide, inconsistent pair
     values of ozone, and/or implausible reflectivity or ozone values
     are included in this product. These individual measurements have
     been averaged into grid cells 1.25 degree in longitude by 1
     degrees in latitude. These have been written as ASCII data files
     and images on a set of CD-ROMs, as well as in HDF format, and
     constitute the data source from which this 1 degree by 1 degree
     monthly average data set was constructed. The following steps were
     performed at the Goddard DAAC:

     These data were further processed by the Goddard DAAC. Processing
     included, regridding, and reformatting the output data product.
     The regridding of these data from 1.25 degree x 1 degree to 1
     degree x 1 degree was implemented as follows:

       1. For each monthly 1.25 x 1 data file, every data value in each
          latitude band was replicated by the target number of grid
          cells in a latitude band within the final output data file,
          360, and assigned to a temporary array. Each original
          latitude band had 288 data values which when replicated 360
          times produced a temporary array of 103680 data values for
          that latitude band.

       2. The first 288 (temporary array) data values were compared
          against the fill value for these data. Any values that were
          not fill values were then summed, and a count of data value
          and fill value occurrence was kept.

       3. A test for fill value occurrence was performed. If fill value
          constituted more than 50% of contributing values then the
          fill value was assigned to that grid cell. Otherwise, the
          average was computed for the target grid cell from only those
          points constituting data values. When assigning fill values,
          a new fill value was used to provide greater uniformity with
          other existing data sets held at the Goddard DAAC.

       4. Steps 2 and 3 were repeated for the next 288 values within
          the temporary array until all values were summed, tested for
          fill value occurrence, and assigned to a target grid cell.

       5. Steps 2, 3, and 4 were repeated for each of the next 179
          latitude bands.

       6. To validate that the regridded data do not introduce any
          spurious artifacts into the original data, a visual
          comparison of the two data sets was performed and the ozone
          values in randomly selected, localized regions were examined
          to ensure spatial coherence in the regridded data set and a
          high degree of similarity with the original data set.

     Scientific Potential of Data
     Total ozone data as derived from the TOMS instrument are useful
     for understanding a variety of phenomena involving both short-term
     stratospheric fluctuations and long-term climate change.
     Stratospheric ozone modulates the incoming (and biologically
     harmful) solar ultraviolet radiation stream through absorption in
     much the same way as tropospheric carbon dioxide traps outgoing
     infrared radiation emitted by Earth's surface and atmosphere. Just
     as an increase or decrease of carbon dioxide in the lower
     atmosphere may have a climatic impact over the long term, so may
     changes in the ozone content of the upper atmosphere. Beside the
     study of long-term climate change, other specific examples of
     scientific applications of this data set include the following.

        * input to global solar radiation models for use in determining
          the proportion of Ultraviolet-B (UV-B), Ultraviolet-A (UV-A),
          and Photosynthetically Active Radiation (PAR) penetrating the
          biosphere (Eck et al., 1995)

        * determination of the long-term trends in total ozone on both
          regional (e.g., Antarctica) and global scales (Bowman 1988)

        * study of spatial and temporal patterns, seasonal cycles, and
          interannual variability of ozone (Bowman, 1986)

        * use as a tracer of stratospheric dynamics in the 30-40 mb
          region where the bulk of the ozone resides, including
          correlations with wind and temperature patterns, especially
          for transient phenomena such as Sudden Stratospheric Warmings
          (Miller et al. 1976) and periodic phenomena such as the
          Quasi-Biennial Oscillation (QBO) (Lait et al. 1989)

        * input to radiative transfer models for use in providing
          atmospheric corrections to satellite-observed radiances
          (e.g., AVHRR and CZCS) for the determination of ocean color
          and vegetation indices (Gordon and Clark 1981)

        * intercomparison and validation with results derived from
          other total ozone instrumentation such as the TIROS
          Operational Vertical Sounder (TOVS) (Lienesch and Pardey,
          1985)

     Validation of Data
     Numerous comparisons were made with various ground based Dobson
     networks and with measurements taken from the summit of Mauna Loa.
     The later comparison is made due to the close coincidence of
     measurements by the two instruments, differing by 100 km in space
     and 1 hour in time. Differences range from 2% to 4% for Dobson
     network comparison, and 3 1/2% to 5% for the World Standard.

Contacts

     Points of Contact
     Science questions concerning the production and validation of this
     data set should be directed to:

          Dr. Richard D. McPeters
          Code 916
          NASA Goddard Space Flight Center
          Greenbelt, MD 20771
          e-mail: [e-mail]
          301-614-5224 (voice)
          301-614-5268 (fax)

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

          EOS Distributed Active Archive Center (DAAC)
          Code 902.2
          NASA Goddard Space Flight Center
          Greenbelt, Maryland 20771
          e-mail: [e-mail]
          301-614-5224 (voice)
          301-614-5268 (fax)

References

     Bhartia, P.K., J. Herman, R.D. McPeters, and O. Torres, 1993. The
     effect of Mt. Pinatubo aerosols on total ozone measurements from
     backscatter ultra violet (BUV) Experiments. J. Geophys. Res., 98,
     18547-18554.

     Bowman, K.P., 1986, Interannual variability of total ozone during
     the breakdown of the Antarctic circumpolar vortex, Geophys. Res.
     Lett.,, 13, 1193-1196.

     Bowman, K.P., 1988, Global trends in total ozone, Science, 239,
     48-50.

     Dave, J.V. 1964. Meaning of successive iteration of the auxiliary
     equation of radiative transfer. Astrophys. J., 140, 1292-1303.

     Dave, J.V. 1978, Effect of aerosols on the estimate of total ozone
     in an atmospheric column from the measurements of its ultraviolet
     radiance, J. Atmos. Sci., 35, 899-911.

     Eck, T.F., P.K. Bhartia, and J.B. Kerr, 1995, Satellite estimation
     of spectral UVB irradiance using TOMS derived total ozone and UV
     reflectivity, Geophys. Res. Lett., 22(5), 611-614.

     Gordon, H.R., and D.K. Clark, 1981, Clear water radiances for
     atmospheric correction of Coastal Zone Color Scanner imagery,
     Appl. Optics, 20, 4175-4180.

     Klenk, K.F., P.K. Bhartia, A.J. Fleig, V.G. Kaveeshwar, R.D.
     McPeters, and P.M. Smith, 1982, Total ozone determination from the
     Backscattered Ultraviolet (BUV) Experiment, J. Appl. Meteor., 21,
     1672-1684.

     Lait, L.R., M.R. Schoeberl, and P.A. Newman, 1989, Quasi-biennial
     modulation of the Antarctic ozone depletion, J. Geophy. Res., 94,
     559-571.

     Lienesch, J.H., and P.K.K. Pardey, 1985, "The use of TOMS data in
     evaluating and improving the total ozone from TOVS measurements",
     Rep. NOAA-TR-NESDIS-23, Issue 22, 3814-3828.

     Liou, K.-N., 1980, An Introduction to Atmospheric Radiation,
     Academic Press, New York.

     McPeters, R., and W.D Komhyr. 1991. Long-term changes in the Total
     Ozone Mapping Spectrometer relative to world standard Dobson
     Spectrometer 83. J. of Geophys. Res., 96, 2987-2993.

     McPeters, R.D., P.K. Bhartia, A.J. Krueger, J. R. Herman, B.
     Schlesinger, C.G. Wellemeyer, C. J. Seftor, G. Jaross, S.L.
     Taylor, T. Swissler, O. Torres, G. Labow, W. Byerly, and R.P.
     Cebula, 1996. Nimbus-7 Total Ozone Mapping Spectrometer (TOMS)
     Data Products User's Guide. NASA Reference Publication 1384.

     Miller, A.J., R.M. Nagatani, K.B. Labitzke, E. Klinker, K. Rose,
     and D.F. Heath, 1976, Stratospheric ozone transport during the
     mid-winter warming of December 1970-January 1971, paper presented
     at Joint Symposium on Atmospheric Ozone, Dresden, Germany, August
     9-16, 1976.

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

                 [NASA] [GSFC]  [Goddard DAAC] [cidc site]

                  NASA  Goddard      GDAAC        CIDC

Last update:Tue Aug 19 11:27:57 EDT 1997
Page Author: Frank Corprew -- [e-mail]
Web Curator: Daniel Ziskin -- [e-mail]
NASA official: Paul Chan, DAAC Manager -- [e-mail]