Polarization-resolved simulations of multiple-order rainbows using realistic raindrop shapes

- The first five rainbow orders are modeled for realistic, non-spherical raindrop shapes and natural drop size distributions.
- Variations in intensity and position of the multiple-order rainbows due to this distorted drop shape are analyzed.
- Each rainbow order may contain a small amount of circularly polarized light, arising from total internal reflections.
- Anomalous drop size distributions can lead to a twinning of the primary, secondary, tertiary and quinary rainbows at certain solar elevations.

This paper presents selected results of a simulation study of the first five (primary-quinary) rainbow orders based on a realistic, size-dependent shape model for falling raindrops, taking into account that the drops' bottom part is flattened to higher degree than the dome-like top part. Moreover, broad drop size distributions are included in the simulations, as it is one goal of this paper to analyze, whether the predicted amplification and attenuation patterns for higher-order rainbows, as derived from previous simulations with monodisperse drop sizes, will still be pronounced under the conditions of natural rainfall. Secondly, deviations of the multiple rainbow orders' polarization state from the reference case of spherical drops are discussed. It is shown that each rainbow order may contain a small amount of circularly polarized light due to total internal reflections. Thirdly, it is investigated, how the conditions that generate twinned primary rainbows will affect the higher orders. For the simulations, geometric-optic ray tracing of the full Stokes vector as well as an approximate approach using appropriately shifted Debye series data is applied.