Retrieving Cirrus Microphysical Properties from Stellar Aureoles

TL;DR

This paper demonstrates how stellar aureoles caused by cirrus clouds can be quantitatively analyzed to retrieve microphysical properties of ice crystals, offering a cost-effective ground-based method for climate research.

Contribution

It introduces a novel analytical approach to interpret stellar diffraction profiles for determining cirrus ice crystal sizes and distributions.

Findings

Stellar aureoles can be followed out to ~0.2 degrees from stars.

Analytic approximations effectively separate diffraction aureoles from background.

Numerical inversion yields particle size distributions between ~50 and 400 microns.

Abstract

The aureoles around stars caused by thin cirrus limit nighttime measurement opportunities for ground-based astronomy but can provide information on high-altitude ice crystals for climate research. In this paper we attempt to demonstrate quantitatively how this works. Aureole profiles can be followed out to ~0.2 degrees from stars and ~0.5 degrees from Jupiter. Interpretation of diffracted starlight is similar to that for sunlight, but emphasizes larger particles. Stellar diffraction profiles are very distinctive, typically being approximately flat out to a critical angle followed by gradually steepening power-law falloff with slope less steep than -3. Using the relationship between the phase function for diffraction and the average Fourier transform of the projected area of complex ice crystals we show that defining particle size in terms of average projected area normal to the propagation direction of the starlight leads to a simple, analytic approximation representing large-particle diffraction that is nearly independent of crystal habit. A similar analytic approximation for the diffraction aureole allows it to be separated from the point spread function and the sky background. Multiple scattering is deconvolved using the Hankel transform leading to the diffraction phase function. Application of constrained numerical inversion to the phase function then yields a solution for the particle size distribution in the range between ~50 microns and ~400 microns. Stellar aureole measurements can provide one of the very few, as well as least expensive, methods for retrieving cirrus microphysical properties from ground-based observations.