PAR_pt1

 
 PI:             Chuck Trees
 dataset:        0.1 Percent Light Depths
 project/cruise: EQPAC/TT007
 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.

 Parameter     Description                                 Units 

 date        day and month of year 1992 (local time)
 time        time reported in local time
 lat         nominal latitude along 140W                   whole degrees
 PARdepth    depth of 0.1% PAR                             meters