Halo polarization profiles and sampled ice crystals: observations and interpretation

Simultaneous two-wavelength polarization and radiance distributions have been obtained for 22 degrees parhelia in four Antarctic ice-crystal swarms that extended to ground level. Samples of crystals that produced these parhelia were collected and replicated. The wavelength dependence of the width of the halo polarization peak agrees with Fraunhofer diffraction theory, indicating that the broadening of the halos is caused primarily by diffraction. However, the observed broadening is much more than predicted from the size distribution of the replicated crystals. From one halo display to the other, the ratio of observed/predicted broadening is erratic, suggesting size-dependent collection efficiency in the sampling. This would imply that, for South Pole conditions, halo polarimetry (or even photometry) is a more reliable method for crystal size determination than actual sampling. It also implies that shapes of the sampled crystals need not necessarily be representative for the shapes of the halo-making crystals in the swarm. Our previous hypothesis [Appl. Opt. 33,4569 (1994)], that a spread of interfacial angles is the dominating cause of halo broadening, has proved untenable.