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 μm. 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 3.0% when using a constant optimized t_f in the radiation scheme, and 2.3% when using a t_f scheme based on complex angular momentum theory. Moreover, Q_ext as predicted from T-matrix over the wavelength interval 8–12 μm is also in excellent agreement with the measured Q_ext. A single wavelength calculation at 14 μm 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.