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Robert S. Schemenauer and Michael J. Curry

Abstract

The capability of a Mee Industries Model 120 ice particle counter (IPC) to differentiate between ice particles and water drops was investigated in laboratory and field studies. The threshold voltage setting as well as the particle size were found to be critical in determining counting efficiency. The results show that ice crystals are counted with an efficiency more than ten times as high as are water drops of the same average size. Increasing the threshold voltage setting of the instrument increases the discrimination factor but also results in a decrease in the absolute number of particles counted. The availability of concurrent information on particle sizes and concentrations from other probes allows the Mee IPC phase determinations to be made with much greater confidence. Methods for utilizing data from the Mee IPC as well as the limitations of the instrument are discussed.

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Michael J. Curry and Robert S. Schemenauer

Abstract

The response characteristics of an optical array precipitation spectrometer probe (PMS-OAP-200Y) have been studied in the laboratory using precision glass beads ejected from a specially constructed air gun. The low-end behavior of the probe is described in terms of a high-pass filter characteristic, which can be used to explain the response of the instrument to a particle population having a wide distribution of sizes.

It is shown that the particle concentrations measured in channel 1 of the OAP-200Y require correction by a factor which is a function of particle size distribution. For typical experimental situations in rain, the correction factor is approximately 1.8. The remaining size channels do not require correction, provided that the probe sample area and channel width have been properly determined.

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Hongli Jiang, William R. Cotton, James O. Pinto, Judy A. Curry, and Michael J. Weissbluth

Abstract

The authors’ previous idealized, two-dimensional cloud resolving model (CRM) simulations of Arctic stratus revealed a surprising sensitivity to the concentrations of ice crystals. In this paper, simulations of an actual case study observed during the Beaufort and Arctic Seas Experiment are performed and the results are compared to the observed data.

It is again found in the CRM simulations that the simulated stratus cloud is very sensitive to the concentration of ice crystals. Using midlatitude estimates of the availability of ice forming nuclei (IFN) in the model, the authors find that the concentrations of ice crystals are large enough to result in the almost complete dissipation of otherwise solid, optically thick stratus layers. A tenuous stratus can be maintained in the simulation when the continuous input of moisture through the imposed large-scale advection is strong enough to balance the ice production. However, in association with the large-scale moisture and warm advection, only by reducing the concentration of IFN to 0.3 of the midlatitude estimate values can a persistent, optically thick stratus layer be maintained. The results obtained from the reduced IFN simulation compare reasonably well with observations.

The longwave radiative fluxes at the surface are significantly different between the solid stratus and liquid-water-depleted higher ice crystal concentration experiments.

This work suggests that transition-season Arctic stratus can be very vulnerable to anthropogenic sources of IFN, which can alter cloud structure sufficiently to affect the rates of melting and freezing of the Arctic Ocean.

The authors find that the Hallett–Mossop riming splintering mechanism is not activated in the simulations because the cloud droplets are very small and cloud temperatures are outside the range supporting efficient rime splintering. Thus, the conclusions drawn from the results presented in this paper may be applicable to only a limited class of Arctic stratus.

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