Dataset: PAR_pt1

   PI:              Chuck Trees
   of:              San Diego State University
   dataset:         0.1 Percent Light Depths
   dates:           February 4, 1992 to March 8, 1992
   location:        N: 12  S: -12  W: -140.0  E: -140.0
   project/cruise:  EQPAC/TT007 - Spring Survey
   ship:            Thomas Thompson
 
   PI Notes:
 
   Enclosed are the depths of the 0.1% light levels as calculated from
   scalar PAR (broadband detector, 400-700 nm, Biospherical
   Instruments).  As some of you remember from the Spring Survey Cruise,
   I was trying to get the PI's to use percent light level depths based
   on downwelling vector irradiance at 488 nm, instead of the PAR
   values.  All near-surface downwelling optical measure- ments are noisy
   and it is difficult to extrapolate to the surface.  One method which
   helps in fitting this data near the surface is to plot it in log space
   and for many wavelengths the irradiance is linear.  Downwelling
   irradiance at 488nm has this near-surface attribute.  Unfortunately, a
   PAR profile is curved near the surface (enhanced attenuation in the
   red).  making extrapolation difficult and introducing errors in
   estimating light depths.  Quoting Gordon and McCluney (1975, Appl.
   Opt. 14,2) "Furthermore, for broadband sensing systems, z90 can be
   determined only from spectral irradiance measurements..".  z90 is the
   depth at which 90% of the diffusely reflected irradiance originates
   that a satellite can observe, which is also the 37% light level.  With
   this disclaimer I hope you have fun with the data.