Small Ice Detector (SID-2H)
START08
PI: Andy Heymsfield (heyms1@ucar.edu)
Data contact: Aaron Bansemer (bansemer@ucar.edu)

The SID-2H measures forward scattering of atmospheric ice and
water particles passing through a laser with a wavelength of 532nm.
Measurements of the scattering intensity are made at 28 angles 
around the particle, allowing the shape of the particle to be 
inferred from its scattering pattern.  Products provided in the
data files include particle size distributions, total particle
concentration, mean diameter, projected area, asphericity, 
particle habit, and probe and housekeeping data.  The SID-2H
was operational on all flights except RF08, when it was not on
board.

Important notes:
1:  Size range
    With final calibrations applied, the size range measured by the 
    SID-2H was from approximately 5 microns to 50 microns.  These
    limits may be influenced by the speed of the aircraft and user
    settings.  The data files contain bins up to 100 microns to 
    account for these changes, but the larger bins (above 50 um) 
    will usually be empty.  

2:  "Liquid" water content
    The variable "LWC" in the data files approximates the integrated 
    volume of condensed water assuming spherical particles with a 
    density of water.  These assumptions were rarely true during 
    START-08, since the particles were typically non-spherical and 
    composed of ice.  Therefore "LWC" will usually be significantly 
    higher than the actual condensed (liquid+ice) water content.  
    
3:  High concentration data
    This instrument did not perform well in clouds with high
    particle concentrations (above 10/cc).  At these concentrations
    many particles could be in the laser beam simultaneously, causing
    artificially high scattering intensity.  The particles reported
    will be over-sized, and habit information may be obscured.  A
    higher than usual number of particles are often rejected due
    to unusual transit times through the laser.   Relative habit, 
    such as percentage of round particles, should still have some 
    qualitative use.

4:  Habit discrimination
    Habits are reported for 4 categories: round, irregular, column,
    and plate.  The habit recognition is done by identifying peaks
    in the scattering patterns.  For example, round particles show no
    peaks, plates show 6 peaks, and columns show 2 or 4 peaks.  A 
    certain number of particles will fall into these categories by 
    pure chance, even if the habit reported in incorrect.  These
    categories are best used to show habit changes over time/distance
    in a relative sense.