Photon tunneling contributions to extinction for laboratory grown hexagonal columns

A new treatment predicting the extinction and absorption properties of ice particles is evaluated in this study using laboratory measurements of the extinction efficiency, Q(ext). In this treatment, the degree of 'photon tunneling' for ice crystals is unspecified, and laboratory measurements of Q(ext) were used in conjunction with this scheme to quantify the significance of this process by determining a tunneling factor, denoted t(f). The term tunneling here refers to the interaction of a particle with radiation outside its area cross-section. A t(f) of 1.0 corresponds to tunneling exhibited by ice spheres as predicted by Mie theory, while a t(f) of 0 indicates no tunneling. The laboratory work entailed Fourier transform infrared spectroscopy (FTIR) for optical depth measurements in an ice cloud grown in a chamber, over a wavelength range of 2-18 mum From these measurements, the extinction efficiency Q(ext) as a function of wavelength was determined. Ice particle size spectra were measured in the cloud chamber, and were used to predict Q(ext) using the radiation scheme noted above and also using a new implementation of T-matrix, which is based on the exact geometry of a 'pristine' hexagonal ice crystal, without approximating the crystal as a spheroid. Results show that t(f) values determined from the laboratory measurements and the new radiation scheme are qualitatively in agreement with t(f) values based on fundamental theory. Mean Q(ext) errors (relative to measured Q(ext)) over all wavelengths sampled were less than or equal to3.0% when using a constant optimized t(f) in the radiation scheme, and less than or equal to2.3% when using a tf scheme based on complex angular momentum theory. Moreover, Q(ext) as predicted from T-matrix over the wavelength interval 8-12 mum is also in excellent agreement with the measured Q(ext). A single wavelength calculation at 14 mum was performed using the finite difference time domain (FDTD) and T-matrix methods, both of which agreed precisely with the measured Q(ext) value. This validates the integrity of T-matrix, FDTD, the new radiation scheme, and the laboratory measurements for the corresponding range of wavelengths and size parameters. Collectively, these results indicate the tunneling contributions predicted for solid hexagonal columns are realistic. (C) 2001 Elsevier Science Ltd. All rights reserved.