Photon Tunneling Contributions for Laboratory-Grown Hexagonal Columns

Ice particle-radiation interactions differ from cloud droplet-radiation interactions due to differences in (1) phase functions; (2) the relationships between particle dimension, area, and mass; and (3) photon tunneling effects (e.g., Nussenzveig 1977; Guimaraes and Nussenzveig 1992). Tunneled radiation here can be viewed as non-incident radiation beyond a particle’s physical cross section, which would be absorbed if the particle were a black body. Another type of tunneling is referred to as edge effects that manifest as surface waves. These do not enter the particle’s interior and are not absorbed, but are responsible for large angle diffraction (Mitchell 2000). Although the physical reasons remain unclear, tunneling depends on ice particle morphology, such as aspect ratio, and its contribution to absorption in ice crystals is less than for spheres (Baran et al. 1998). Francis et al. (1999) provided evidence that tunneling at 8.5 μm and 11.1 μm in a cirrus deck sampled microphysically and radiometrically was negligible. The absence of tunneling effects in ice crystals would reduce their absorption efficiency in the thermal infrared (IR) by typically 20%, although in the far IR this reduction (relative to tunneling predicted by Mie theory) can be up to 43%. Hence, IR remote sensing is plagued with large uncertainties until the role of tunneling in ice is resolved.