Atmos. Chem. Phys. Discuss., 12, 7909-7947, 2012
© Author(s) 2012. This work is distributed
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Effects of ice crystals on the FSSP measurements in mixed phase clouds
G. Febvre1, J.-F. Gayet1, V. Shcherbakov1,2, C. Gourbeyre1, and O. Jourdan1
1Laboratoire de Météorologie Physique, UMR6016 CNRS/Université Blaise Pascal, Clermont-Ferrand, France
2LaMP, Institut Universitaire de Technologie d'Allier, Montluçon, France

Abstract. In this paper, we show that in mixed phase clouds FSSP-100 measurements may be contaminated by ice crystals, inducing wrong interpretation of particle size and subsequent bulk parameters. This contamination is generally revealed by a bimodal feature of the particle size distribution; in other words, in mixed phase clouds bimodal features could be an indication of the presence of ice particles. 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 μm 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 μm and 35 μm, does not necessarily represent true size responses but likely corresponds to bigger aspherical ice particles. According to simulation results, the sizing understatement would be neglected in the rough case but would be major 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. Previous cloud in situ measurements suggest that the FSSP-100 secondary mode, peaked in the range 25–35 μm, is likely to be due to the shattering of large ice crystals on the probe tips. This finding is supported by the rather good relationship between the concentration of particles larger than 20 μm (hypothesized to be ice shattered-fragments measured by the FSSP) and the concentration of (natural) ice particles larger than 100 μm (CPI data). The shattering efficiency is defined as the ratio of the measured ice shattered-fragments to the number of natural ice particles (with d>100 μm) impacting the probe leading edge. In the present study the shattering efficiency is evaluated to ~7%. It is found that about 400 ice fragments may result from the shattering of one equivalent irregular shaped ice crystal with a mean volume diameter of 310 μm. Obviously, these values could be strongly dependent on the inlet design, the airspeed and the robustness of ice crystals via the impact kinetic energy to surface energy ratio providing the particle breakup.

Citation: Febvre, G., Gayet, J.-F., Shcherbakov, V., Gourbeyre, C., and Jourdan, O.: Effects of ice crystals on the FSSP measurements in mixed phase clouds, Atmos. Chem. Phys. Discuss., 12, 7909-7947, doi:10.5194/acpd-12-7909-2012, 2012.
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