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You are here: GES DISC Home Ozone Keep for Review Old TOMS Nimbus-7 TOMS Version 7^MErythemally Weighted Daily UV Exposures at the Earth's Surface^MNov 1978 - Mar 1993

Nimbus-7 TOMS Version 7^MErythemally Weighted Daily UV Exposures at the Earth's Surface^MNov 1978 - Mar 1993


This CD-ROM contains one file for each day from November 1, 1978 to March 6, 1993 (with a few missing days).Each file contains data representing the relative daily areal exposures of ultraviolet (UV) radiation effective in causing skin irritation, computed at each 1 degree latitude by 1.25 degree longitude pixel, between latitudes 65S and 65N.These data were derived from measurements made by NASA's Total Ozone Mapping Spectrometer (TOMS), which was flown aboard the Nimbus-7 satellite.


The root directory (folder) of this CD-ROM contains a set of subdirectories,y78, y79, ..., y93.Each subdirectory contains a set of files with names of the formyymmdd.erx (e.g. the data for November 1, 1978 are in the file called 781101.erx).


Each data file has a three-line header, followed by 130 12-line records.Each 12-line record contains the data for the 288 pixels in a single latitude band.Each datum is represented by a 3-digit number.The latitude of the midpoint of the band is given as the last number in the twelfth line of each record.In all, each file has 1563 lines and 117,495 bytes.

A sample header is shown here:
Day: 122 May  2, 1979   Production V70 NIMBUS-7/TOMS Erythemal Exposure
Longitudes:  288 bins centered on 179.375 W to 179.375 E  (1.25 degree steps)

Latitudes :  130 bins centered on  64.5   S to  64.5   N  (1.00 degree steps)  
The first line of the header record contains the day number, the date it corresponds to, using common 3-letter abbreviations for month names, and the name of the data product.

The second and third lines of the header are the same for all files in this product.

A sample 12-line record is shown here:
 98101 93 99 90 85 77 77 87 83 88 96 97103104 93 91 93104119122121114114115
109115115110107 99101 95 74 54 44 47 44 53 56 51 65 67 70 72 72 70 82 97119
121118118116114110 95 94 95 93 92 84 37 14 21 29 48 74 91 77 75 84 84  0  0
  0  0  0  0  0  0  0  0  0  0  0  0  0140121112109110108106109114111113113
110119121125121114 88 64 77 87 88 85 84 87 93 71 29 34 63 74 88 99124109122
120119102103123118105 89101120125122125120112100105104107129129117126104101
 98108115123122105118125154158158158157160160168168151148142118105101 95104
126136133106102126128109105100 99112107 95 70 33 20 27 21 19 25 44 78 82105
123130132118 78 83104104107131130130105115122106 99102 92 80 76 73 62 68 87
117117118112 98 95 97109108 89112120119119119118115 94 76 77 41 36 58 41 29
 22 24 29 62109116143147153154154153150148147147130127123126107 44 66 88 97
 97 89 90 90 83 86 85 79 91105 94105108   Lat=  -29.5
This consists of 288 three-digit numbers followed by the latitude at the band center.Southern latitudes are given by negative numbers. The string of zeroes reflects a satellite orbit during which either measurements were not made or the data were lost.

The following FORTRAN-77 code illustrates how the contents of an entire file may be read into the array ERY.
      INTEGER*4 ERY(288,130)
      OPEN(1, FILE='/CDROM/y79/790502.erx', FORM='FORMATTED')
      READ(1,1) ERY
    1 FORMAT(///129(11(1x,25i3,/),1x,13i3/),11(1x,25i3,/),1x,13i3)
When this code is executed, it will fill the array ERY in such a way thatERY(ILON,ILAT)is the diurnal erythemal exposure per unit area within the pixel centered at longitude= -179.375 + (ILON-1)*1.25 and latitude= -64.5 + (ILAT-1), with the convention that negative longitudes are West and negative latitudes are South.


The data in this product have been computed from the NASA/GSFC TOMS version 7.0, level 3 data.These consist of total column ozone and scene reflectivities in the same latitude-longitude pixels.(These data are available as separate products from NASA/GSFC.)These are combined with the results of radiative transfer calculations, terrain height data, and model action spectrum of Caucasian erythemal susceptibility, to produce the data in this product.

Since the action spectrum is defined up to an arbitrary multi- plicative constant, its units are an arbitrary measure of poten- tial erythemal damage per unit exposure; therefore the data in this product, erythemal exposures, are also of arbitrary dimen- sions.We have scaled the data to get the best dynamic range when each datum is expressed as a three-digit decimal number.In order to compute the numbers given in the files, the fluxes were in units of nW/(m^2 nm); the action spectrum was calculated from the expression given below, and scaled by the factor 16/(125 pi), or about 0.004074 .

The definition of the action spectrum for a biological process assumes a linear relation between the action spectrum-weighted exposure and the quantitative biological response.Thus, a twofold increase in the erythemal exposure corresponds to a twofold increase in the potential incidence or severity of erythema.


The Version-7 Level-3 Nimbus-7/TOMS data consist of total column ozone and scene reflectivities derived from measurements of the solar ultraviolet radiation that is backscattered from the atmosphere.Ozone is determined from measurements of radiation with wavelengths less than 318 nm, where ozone is strongly ab- sorbing.The scene reflectivity is determined from the measurement of radiation in a narrow band around 380 nm, where there is neg- ligible absorption by any atmospheric constituents.Low pixel reflectivities are associated with cloudless conditions, while high reflectivities (greater than about 20%) are associated with either cloudy conditions or snow and ice covered ground.Alone, TOMS is unable to distinguish between cloud and snow cover; however, climatological databases of surface reflectivity and snow/ice cover have been used.

The spatial resolution of the Nimbus-7/TOMS does not permit one to determine the nature of the clouds present in the scene; for example, whether it is uniform, overcast, thin clouds, or a field of broken, highly reflective cumulus clouds.For the purpose of this computation, we modelled each pixel in which cloud cover is implied by the TOMS measurement as a uniform overcast of cloud with a putative optical thickness deduced from the 380 nm reflec- tivity, homogeneously distributed in a slab between 700 mbar and 500 mbar.Since each pixel is measured only once for each day for near noon conditions, no account is taken of diurnal variation in cloudiness.

Detailed radiative transfer models were used to estimate the spectral flux at the Earth's surface, given a column ozone amount, TOMS-measured reflectivity, climatological surface reflectivity, terrain height, and solar zenith angle, for a set of wavelengths from 280 nm to 400 nm, which include the UV-B and UV-A regions. The flux decreases sharply with decreasing wavelength shorter than about 310 nm, due to absorption by ozone, while the action spectrum for erythema decreases sharply with increasing wavelength longer than 340 nm.Thus, using the wavelength range 280 nm to 400 nm ensures that all significant contributions to the erythemal exposure have been included in the calculation.

The solar zenith angle as a function of time of day depends on the latitude and solar declination angle.These were used to accomplish the computation of the diurnally-integrated fluxes at each wavelength in the set.The integrated fluxes were multiplied by the model action spectrum values at each wavelength, and summed to give the erythemal exposure.

Symbolically, we can write the erythemal exposure as the following double integral
                 400nm           sunset
                /               /
Erythemal  =  C | dl w(l) S(l)  | dt F(l, Oz, Robs, Rsurf, SZA(t), h)
Exposure        /               /
                280nm           sunrise
C       = Arbitrary scaling constant (16/125/pi)
l       = Wavelength (nm)
w(l)    = Erythemal action spectrum. ([biological effect] J^(-1) m^2)
S(l)    = Solar flux incident on the top of the atmosphere, corrected
          for annual variation in the earth-sun distance. 
          (nW m^(-2) nm^(-1))
t       = Time
F       = Downward flux at the Earth's surface, corrected for
          clouds, for unit incident solar flux at the top of the 
Oz      = TOMS total column ozone
Robs    = TOMS scene reflectivity
Rsurf   = Climatological surface reflectivity
SZA(t)  = Solar zenith angle (also depends on latitude, solar declination)
h       = Terrain height


Wavelengths:All spectral quantities were calculated for all wavelengths 280.0, 280.5, 281.0, ... , 400.0 nm

Solar spectrum:The extraterrestrial solar flux was measured by the SOLSTICE instrument aboard the UARS satellite (data product Version 8).The instrumental resolution was much finer than 0.5 nm, so the spectrum was degraded by splining to the desired set of wavelengths.The Earth-Sun distance varies annually by about 3.4% due to the ellipticity of the Earth's orbit.An expression for the Earth-Sun distance, provided in the Astronomical Almanac, was used.

Action spectrum:The model erythemal action spectrumw(l)was devised by Green, Sawada, and Shettle, and is
                a          c exp2
    w(l) =  ---------- + -----------
             1 + exp1     1 + exp22
    a =    0.04485
    c =    3.9796
    exp1 = exp( (l - l1)/b )
    exp2 = exp( (l - l2)/d )
    b =    3.13
    d =    2.692
    l1 =   311.4
    l2 =   296.5
Total Column Ozone and Scene Reflectivity:The Version 7.0, level-3 value from the Nimbus-7/TOMS instrument was used.

Terrain height:These values are based on a U.S. Department of Defense geographical database, and were degraded to the grid resolution.The height of the ocean surface was set to zero.

Solar Zenith Angle:The solar zenith angle is related to the solar hour angle via the expression
    SZA = cos(Lat - SDA) + [cos(Lat) cos(SDA)] [cos(SHA) - 1]
    Lat     = Latitude
    SZA     = Solar zenith angle
    SHA     = Solar hour angle
    SDA     = Solar declination angle
The hour angle is just the local apparent solar time, expressed as an angle.The declination angle can be computed from the low-precision formulae given in the Astronomical Almanac.

Normalized downward flux:These were calculated as the product of a clear sky flux and a cloud correction factor.The clear-sky fluxes were modelled by a function of the form
    cos( gamma SZA ) exp( -alpha/cos( gamma SZA ) )
                 1 - Rsurf Sb
where alpha, gamma, and Sb values were fit to the results of detailed radiative transfer calculations that used standard, midlatitude ozone profiles and Bass & Paur ozone absorption cross-sections, for each of the wavelengths required, total column ozone amounts of 125 to 575 Dobson Units, and terrain heights from 0 to 6 km.

The cloud correction factor was determined in two steps, using the results of model radiative transfer calculations:First, a putative cloud optical thickness was found as a function of the three quantities, (SDA - Lat), Robs, and Rsurf.Next, the cloud correction factor was determined as a function of wavelength, putative cloud optical thickness, surface reflectivity, terrain height, and solar zenith angle.


Dr. Jay R. Herman
Code 916.0
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771

Dr. Edward A. Celarier
Software Corporation of America
Lanham, Maryland 20706
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Last updated: Jun 15, 2012 10:36 AM ET