What Is the Shape of a Raindrop? An Answer from Radar Measurements

Abstract

In a previous study, Gorgucci et al. showed the potential advantage of using together polarimetric radar measurements of reflectivity factor, differential reflectivity, and specific differential propagation phase, in order to gather information about the calibration of radar systems. Scarchilli et al. generalized this concept in the self-consistency principle, which stated that, given a drop-shape model to describe the form of raindrops, the corresponding radar measurements are constrained on this three-dimensional surface. In this work the self-consistency principle is collapsed onto a two-dimensional domain defined by the variables: 1) the ratio between specific differential phase and reflectivity factor, and 2) differential reflectivity. In this space the scatter of drop size distribution (DSD) variability is minimized in such a way that drop-shape variability shows up. This methodology is used to observe for the first time the predominant shape of raindrops directly from the radar measurements. The radar polarimetric data were collected in two different climatological regions as central Florida and northern Italy. The significant result shows that the underlying mean axis ratio approaches the model established by Pruppacher and Beard, and the relationship described by Beard and Chuang forms a sort of border for the sphericity of the drop shape.