Observation, analysis, and reconstruction of a twinned rainbow

A photograph of a twinned rainbow, obtained on 11 May, 2012, in Dresden, Germany, is precisely calibrated with respect to lens projection and camera orientation. Since the twinning was only located in a part of the picture, it was possible to read out the red-green-blue intensity data from both a twinned and nontwinned position of the rainbow. These data were fitted with modeled spectra for polydisperse drop distributions, which were calculated with a Debye series algorithm and shifted in the scattering angle to account for the nonspherical shape of natural raindrops. The shift data were acquired from raytracing through realistic raindrop shapes modeled by two conjoined half-spheroids of different oblateness. Effective drop size distributions along the line of sight are derived from the fit for the two sampling positions and used to generate a true-color simulation of the original photograph. By this, the optical determination of physical rainfall properties is demonstrated.