Article,
Droplet spectra measurements with the FSSP-100. Part II: Coincidence effects
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY, 22 (11): 1756-1761 (2005)
DOI: 10.1175/JTECH1818.1
Abstract
At high droplet number concentration, measurements of the droplet number
concentration, of the droplet spectrum, and of the liquid water content
with the FSSP-100 are significantly affected by the coincidences of
particles in the probe's detection beam and by count losses during the
dead time of the probe's electronics. Accurate statistical procedures
have been proposed in the literature for the correction of the
coincidence and dead-time losses in the estimation of the droplet number
concentration. There are, however, no techniques available for
correction of the spectral distortion, hence of the derived liquid water
content. The model of probe functioning described in Part I of this
series is now used on a stochastic mode in order to simulate all
possible events of coincidences and dead-time losses. Observed features,
such as variations of the depth of field and beam edge acceptance ratios
with the droplet concentration, are correctly reproduced by the
simulator. The three most currently used techniques for the correction
of the droplet concentration are then evaluated with the simulator. The
distortion of the spectral shape is also examined. The simulator shows
that the measured droplet mean volume diameter increases with the
droplet number concentration, by up to 40% at a concentration of 1500
cm(-3). An empirical procedure is finally proposed for the correction of
the spectral distortion and of the derived liquid water content.
