README FILE

                 New UV Irradiance and Exposure Data Product

NASA Goddard Space Flight Center
Code 916 
Greenbelt, MD 20770



Data Product

  The solar irradiances used in the calculation have units of
mW m^{-2} nm^{-1} (milliwatts per square meter per nanometer; the
"square meter" refers to the area on a horizontal surface, the 
"nanometer" refers to the wavelength of the light).  Thus, after
integrating over wavelength and time, one is left with units of
J m^{-2} (Joules per square meter).  For the 4 individual wavelengths 
the irradiance is calculated by integrating over a triangular slit 
function that is 0.55 nm wide FWHM (full width at half maximum) and 
normalizing by the area under the slit function.  The true vacuum 
central wavelengths are 305.088716, 310.089969, 324.093502, and 
380.107892 nm.  The apparently small shifts of ~0.1 nm from integer 
wavelengths can cause errors of a few percent when comparing with 
measured values.  This occurs because of the rapidly changing solar 
spectrum and changes in ozone absorption with wavelength.

In the future, correction tables will be provided for different slit 
widths and band centers.


Format of the Data Files

  Each file consists of 2700 data records.  The data records are
organized in 180 groups of 15 records.  The first 14 records in
each group contain 25 three-digit value codes; the 15th record
contains 10 value codes, followed by the numerical value of the
center latitude of the band.  Each group corresponds to a
single 1 degree wide latitude band, and each value code corresponds
to a cell of dimensions 1 degree latitude by 1 degree longitude.

  Each value code consists of three digits: a 1 digit exponent (E), 
and a 2 digit mantissa (M).  A decimal point is implied between the 
two digits of the mantissa.  Together,  E  and  M  encode a value 
of  M*10^E .  For example, the value code "342" represents the value 
4.2*10^3 .  Note that a value code of "999" is a fill-value, used
where TOMS data were unavailable.

  The following format specifications are recommended for reading 
the data records (all at once) from a UV data file.

The value codes may be read as integers into a single integer array
of dimensions (360,180) (using the FORTRAN or IDL subscripting 
convention).  They must then be decoded into the values they represent.
The recommended format specification for reading the data is

   (179(14(1X,25I3,/),1X,10I3,/),14(1X,25I3,/),1X,10I3)

Alternatively, the values may be read into an integer array of 
dimensions (2,360,180), where the element (1,ilon,ilat) (FORTRAN)
or (0,ilon,ilat) (IDL) is the exponent  E  for the value at 
the grid cell (ilon,ilat), and the element (2,ilon,ilat) (FORTRAN)
or (1,ilon,ilat) (IDL) is the mantissa  M .  The  recommended format 
specification in this case is

   (179(14(1X,25(I1,I2),/),1X,10(I1,I2),/),14(1X,25(I1,I2),/),1X,10(I1,I2))



Code Samples

  Code samples are included in the following files:

      read_new_uv.fc
      read_new_uv.fp


Contacts

Dr. Jay. R. Herman
Code 916.0
NASA Goddard Space Flight Center
Greenbelt, MD 20770
herman@tparty.gsfc.nasa.gov