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

by G. Febvre, J. F. Gayet, V. Shcherbakov, C. Gourbeyre, O. Jourdan
Atmospheric Chemistry and Physics ()
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In this paper, we show that in mixed phase clouds, the presence of ice\ncrystals may induce wrong FSSP 100 measurements interpretation\nespecially in terms of particle size and subsequent bulk parameters. The\npresence of ice crystals is generally revealed by a bimodal feature of\nthe particle size distribution (PSD). The combined measurements of the\nFSSP-100 and the Polar Nephelometer give a coherent description of the\neffect of the ice crystals on the FSSP-100 response. The FSSP-100\nparticle size distributions are characterized by a bimodal shape with a\nsecond mode peaked between 25 and 35 mu m related to ice crystals. This\nfeature is observed with the FSSP-100 at airspeed up to 200 m s(-1) and\nwith the FSSP-300 series. In order to assess the size calibration for\nclouds of ice crystals the response of the FSSP-100 probe has been\nnumerically simulated using a light scattering model of randomly\noriented hexagonal ice particles and assuming both smooth and rough\ncrystal surfaces. The results suggest that the second mode, measured\nbetween 25 mu m and 35 mu m, does not necessarily represent true size\nresponses but corresponds to bigger aspherical ice particles. According\nto simulation results, the sizing understatement would be neglected in\nthe rough case but would be significant with the smooth case.\nQualitatively, the Polar Nephelometer phase function suggests that the\nrough case is the more suitable to describe real crystals.\nQuantitatively, however, it is difficult to conclude. A review is made\nto explore different hypotheses explaining the occurrence of the second\nmode. However, previous cloud in situ measurements suggest that the\nFSSP-100 secondary mode, peaked in the range 25-35 mu m, is likely to be\ndue to the shattering of large ice crystals on the probe inlet. This\nfinding is supported by the rather good relationship between the\nconcentration of particles larger than 20 mu m (hypothesized to be ice\nshattered-fragments measured by the FSSP) and the concentration of\n(natural) ice particles (CPI data). In mixed cloud, a simple estimation\nof the number of ice crystals impacting the FSSP inlet shows that the\nice crystal shattering effect is the main factor in observed ice\nproduction.

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