The importance of ice particle shape on UV measurements of polar mesospheric clouds: SBUV/2 observations

Abstract

We present calculations of scattering of solar UV radiation from polar mesospheric clouds (PMC) for the wavelengths and scattering angles appropriate to the SBUV/2 series of satellite experiments. Our model includes for the first time the effects of non-sphericity of ice particles whose evidence has been recently gained from lidar depolarization experiments. Our results apply to the scattering cross section averaged over a particle size ensemble which spans the mean size and size ranges expected for PMC. We demonstrate that for the size range 20–100 nm (effective spherical particle radius), the wavelength range 252–292 nm, and the scattering angle range 100– 140 ∘ , non-spherical particles cause only moderate ( < 35 % ) deviations from conventional Mie theory, which assumes spherical shapes. The total UV scattering efficiency of PMC particles per unit mass of water vapor is not significantly affected by shape effects, in the range of parameters studied. On the other hand, our study suggests that shape effects need to be considered in the analysis of UV spectra of PMC, such as those of Carbary et al. [2004. Evidence for bimodal particle distribution from the spectra of polar mesospheric clouds. Geophysical Research Letters 31 (13), 3108] and DeLand et al. [2005. Spectral measurements of PMCs from SBUV/2 instruments. Journal of Atmospheric and Solar-Terrestrial Physics, to appear, doi: 10.1016/j.astp.2005.08.006]. In particular, attention must also be paid to the scattering angle dependence of the measurements, and the range of scattering angles involved in compiling average spectra of PMC.