In this study, characteristic optical sizes of ice crystals in synoptic cirrus are determined

using airborne measurements of ice crystal size distributions, optical extinction and

water content. The measurements are compared with coincident visual observations

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5 of ice cloud optical phenomena, in particular the 22 and 46 halos. In general, the

scattering profiles derived from the in-situ cloud probe measurements are consistent

with the observed halo characteristics. It is argued that this implies that the measured

ice crystals were small, probably with characteristic optical radii between 10 and 20 µm.

There is a current contention that in-situ measurements of high concentrations of small 10 ice crystals reflect artifacts from the shattering of large ice crystals on instrument inlets.

Significant shattering cannot be entirely excluded using this approximate technique,

but it is not indicated. On the basis of the in-situ measurements, a parameterization

is provided that relates the optical effective radius of ice crystals to the temperature in

mid-latitude synoptic cirrus. 15 1 Introduction

In cold clouds, the density in air of ice mass (W ) and optical extinction (βext ) is most

usually related through an optical length scale termed the effective radius:

3W

re = (1)

2ρi βext

where, ρi is the bulk density of ice (Foot, 1988). Defined this way, re represents the 20 characteristic radius of an ensemble of hydrometeors, as represented by the ratio of

their physical volume to their radiative extinction cross-section at visible wavelengths.

This paper examines the values of re used for describing ensembles of ice crystals

in numerical models of cirrus. The importance of correctly assessing the value of this

parameter is that, as noted by Stephens et al. (1990), until it is well constrained, it is

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