Tunneling optical resonances in light-droplet interactions: simulations of spaceborne cloud droplet observations.

Abstract

A fruitful approach for light-droplet interaction in atmospheric optics, the quantum optical analogy combined with partial wave analysis, is used to characterize (location, width) very sharp tunneling optical resonances (TORs). For this purpose, a fast and flexible technique of computation, the transfer matrix method, has been developed; it is described herein. This paper proposes explicit calculations of TORs, considering isolated droplets and a droplet population, and so goes further than earlier studies. Applied in the context of POLDER observations, from the visible to the near infrared, it is shown that TORs enhance cross sections with respect to Mie's theory as currently computed in atmospheric optics. Precisely, for a typical warm cloud droplet population, this enhancement can reach almost 30% in the case of absorption. Scattering and extinction cross sections are also higher than those of Mie's theory, but to a lesser extent. As a conclusion, it is suggested that tunneling should be taken into account more explicitly when addressing light scattering by droplets. No scattering inversion technique is investigated in this paper, but for the time being, the results presented can be used as look-up tables (at least orders of magnitude) for practical computations in atmospheric remote sensing. Finally, the approach presented herein is proposed as a perspective to be used in different situations involving other spherical (or almost spherical) scatterers that otherwise should be addressed approximately.