Abstract
Abstract. 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 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 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 μm, 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 μm (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 production.
Highlights
The investigating of climate, radiative transfer or numerical forecast modelling require a good knowledge of the microphysical properties of clouds
Our results clearly show that the second mode in the range 20–35 μm of the Forward Scattering Spectrometer Probes (FSSP)-100 size distribution is associated with the presence of ice particles
In this paper we have analyzed cloud in situ measurements performed in boundary layer clouds during ASTAR2007 and POLARCAT experiments
Summary
The investigating of climate, radiative transfer or numerical forecast modelling require a good knowledge of the microphysical properties of clouds. Korolev et al (2011) have shown that the response of the FSSP-100 in ice clouds can be almost entirely due to ice crystal shattering on the inlet tube, and this can be almost greatly eliminated by removing the sample tube, and using deflecting probe tips In this context, this paper is a contribution to the interpretation of the effects of ice particles on FSSP measurements using a data set for Arctic mixed phase clouds. The interpretation of the measurements from independent techniques leads to a clear identification of the effects of ice crystals on FSSP particle size distributions This feature is confirmed with additional data obtained during POLARCAT, studied in order to experience a wide range of ice crystal conditions.
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