Numerical simulations of optical properties of Saharan dust aerosols with emphasis on lidar applications

In the framework of the Saharan Mineral Dust Experiment (SAMUM) for the first time the spectral dependence of particle linear depolarization ratios was measured by combining four lidar systems. In this paper these measurements are compared with results from scattering theory based on the T-matrix method. For this purpose, in situ measurements-size distribution, shape distribution and refractive index-were used as input parameters; particle shape was approximated by spheroids. A sensitivity study showed that lidar-related parameters-lidar ratio S-p and linear depolarization ratio delta(p)-are very sensitive to changes of all parameters. The simulated values of the delta(p) are in the range of 20% and 31% and thus in the range of the measurements. The spectral dependence is weak, so that it could not be resolved by the measurements. Calculated lidar ratios based on the measured microphysics and considering equivalent radii up to 7.5 mu m show a range of possible values between 29 and 50 sr at lambda = 532 nm. Larger S-p might be possible if the real part of the refractive index is small and the imaginary part is large. A strict validation was however not possible as too many microphysical parameters influence S-p and delta(p) that could not be measured with the required accuracy.