Articles | Volume 10, issue 6
Research article
21 Nov 2016
Research article |  | 21 Nov 2016

Relating optical and microwave grain metrics of snow: the relevance of grain shape

Quirine Krol and Henning Löwe

Abstract. Grain shape is commonly understood as a morphological characteristic of snow that is independent of the optical diameter (or specific surface area) influencing its physical properties. In this study we use tomography images to investigate two objectively defined metrics of grain shape that naturally extend the characterization of snow in terms of the optical diameter. One is the curvature length λ2, related to the third-order term in the expansion of the two-point correlation function, and the other is the second moment μ2 of the chord length distributions. We show that the exponential correlation length, widely used for microwave modeling, can be related to the optical diameter and λ2. Likewise, we show that the absorption enhancement parameter B and the asymmetry factor gG, required for optical modeling, can be related to the optical diameter and μ2. We establish various statistical relations between all size metrics obtained from the two-point correlation function and the chord length distribution. Overall our results suggest that the characterization of grain shape via λ2 or μ2 is virtually equivalent since both capture similar aspects of size dispersity. Our results provide a common ground for the different grain metrics required for optical and microwave modeling of snow.

Short summary
Optical and microwave modelling of snow involve different metrics of "grain size" and existing, empirical relations between them are subject to considerable scatter. We introduce two objectively defined metrics of grain shape, derived from micro-computed tomography images, that lead to improved statistical models between the different grain metrics. Our results allow to assess the relevance of grain shape in both fields on common grounds.