A Process Study of the Dependence of Ice Crystal Absorption on Particle Geometry: Application to Aircraft Radiometric Measurements of Cirrus Cloud in the Terrestrial Window Region

Abstract

The processes that contribute to the absorption of infrared radiation by atmospheric ice crystals are studied. The processes are separated into the geometric optics (i.e., refraction, internal, and external reflection) and above-edge (i.e., the capture of photons beyond the physical cross section of the particle via tunneling through an inertial barrier) contributions. The geometric optics and above-edge contributions to ice crystal absorption are compared and contrasted assuming ice spheres, randomly oriented hexagonal ice columns, and randomly oriented ice aggregates (i.e., systematically increasing particle complexity). The geometric optics and above-edge absorption coefficients have been calculated using the complex angular momentum approximation applied to the ice sphere, and a composite method has been used to calculate the two sets of absorption coefficients for the hexagonal ice column and ice aggregate based on the finite difference time domain and improved geometric optics methods. The impact of the geometric optics and above-edge contributions to retrieval of ice crystal effective size (re) is studied for each particle geometry using aircraft-based downwelling radiometric measurements of cirrus at the wavelengths of 8.55 and 11.0 ?m. The retrieved re is compared with in situ measurements of crystal effective size. The profile averaged value of re is estimated to be in the range 32?49 ?m. The retrieved re assuming the ice sphere with the geometric optics contribution only is found to be 30.4 ± 14.2 ?m, while with the above-edge contribution included, it is 15.5 ± 7.2 ?m. The impact of the ice sphere above-edge contribution acts to reduce the retrieved re by about half, and this results in the retrieved re being about a factor of 2?3 less than the in situ measurements of re. Interestingly, as the particle complexity increases from the ice sphere to the ice aggregate, the impact of the above-edge contribution on the retrieval of re is found to systematically diminish. For the ice aggregate, the retrieved re with the geometric optics contribution only is found to be 27.4 ± 4.3 ?m. However, with the above-edge contribution included, it is found to be 28.6 ± 3.9 ?m. Clearly, as particle complexity increases and particle symmetry decreases, the impact of the above-edge contribution on the retrieval of re at the wavelengths of 8.55 and 11.0 ?m is considerably diminished. However, in general the above-edge contribution should not be ignored and a full electromagnetic solution is still preferred in the resonance region. The findings also indicate that ice aggregates are a better representation of cirrus cloud midinfrared radiative properties than pristine solid hexagonal ice columns.