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Journal article

Some effects of ice crystals on the FSSP measurements in mixed phase clouds

Febvre G, Gayet J, Shcherbakov V, Gourbeyre C, Jourdan O...(+5 more)

Atmospheric Chemistry and Physics, vol. 12, issue 19 (2012) pp. 8963-8977

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In this paper, we show that in mixed phase clouds, the presence of ice
crystals may induce wrong FSSP 100 measurements interpretation
especially in terms of particle size and subsequent bulk parameters. The
presence of ice crystals is generally revealed by a bimodal feature of
the particle size distribution (PSD). The combined measurements of the
FSSP-100 and the Polar Nephelometer give a coherent description of the
effect of the ice crystals on the FSSP-100 response. The FSSP-100
particle size distributions are characterized by a bimodal shape with a
second mode peaked between 25 and 35 mu m related to ice crystals. This
feature is observed with the FSSP-100 at airspeed up to 200 m s(-1) and
with the FSSP-300 series. In order to assess the size calibration for
clouds of ice crystals the response of the FSSP-100 probe has been
numerically simulated using a light scattering model of randomly
oriented hexagonal ice particles and assuming both smooth and rough
crystal surfaces. The results suggest that the second mode, measured
between 25 mu m and 35 mu m, does not necessarily represent true size
responses but corresponds to bigger aspherical ice particles. According
to simulation results, the sizing understatement would be neglected in
the rough case but would be significant with the smooth case.
Qualitatively, the Polar Nephelometer phase function suggests that the
rough case is the more suitable to describe real crystals.
Quantitatively, however, it is difficult to conclude. A review is made
to explore different hypotheses explaining the occurrence of the second
mode. However, previous cloud in situ measurements suggest that the
FSSP-100 secondary mode, peaked in the range 25-35 mu m, is likely to be
due to the shattering of large ice crystals on the probe inlet. This
finding is supported by the rather good relationship between the
concentration of particles larger than 20 mu m (hypothesized to be ice
shattered-fragments measured by the FSSP) and the concentration of
(natural) ice particles (CPI data). In mixed cloud, a simple estimation
of the number of ice crystals impacting the FSSP inlet shows that the
ice crystal shattering effect is the main factor in observed ice

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