Millimeter wave scattering from ice crystal aggregates: comparisons with radar measurements at X- and Ka-Band using cloud model simulations

Description

Arctic clouds are often mixed-phase, such that the radiative properties of the clouds are a strong function of the relative amounts of cloud liquid and ice. The poorly understood ice phase processes are the regulators of the liquid water fraction; in particular, ice precipitation, which in the Arctic is dominated by pristine ice and aggregates. This study evaluates results of different ice microphysics representations in a cloud-resolving model (CRM) using cloud radar measurements. An algorithm is presented to generate realistic aggregates of ice crystals from which radar backscatter cross-sections may be calculated using a generalized solution for a cluster of spheres. The aggregate is composed of a collection of pristine ice crystals, each of which is constructed from a cluster of tiny ice spheres. Each aggregate satisfies the constraints set by the component crystal type and the mass-dimensional relationship used in the cloud-resolving model, but is free to adjust its aspect ratio. This model for calculating radar backscatters is compared to two spherical and one spheroidal (bulk model) representation for ice hydrometeors. It was found that the refined model for representing the ice hydrometeors, both pristine crystals and their aggregates, is required in order to obtain good comparisons between the CRM calculations and the radar measurements. The addition of the radar-CRM comparisons to CRM-in situ measurements comparisons allowed conclusions about the appropriateness of different CRM ice microphysics representations.