Aircraft Measurements of the Microphysical Properties of Stratiform Clouds with Embedded Convection

Abstract

The presence of embedded convection in stratiform clouds strongly affects ice microphysical properties and precipitation formation. In situ aircraft measurements, including upward and downward spirals and horizontal penetrations, were performed within both embedded convective cells and stratiform regions of a mixed-phase stratiform cloud system on 22 May 2017. Supercooled liquid water measurements, particle size distributions, and particle habits in different cloud regions were discussed with the intent of characterizing the riming process and determining how particle size distributions vary from convective to stratiform regions. Significant amounts of supercooled liquid water, with maxima up to 0.6 g m⁻³, were observed between −3°C and −6°C in the embedded convective cells while the peak liquid water content was generally less than 0.1 g m⁻³ in the stratiform regions.

There are two distinct differences in particle size distributions between convective and stratiform regions. One difference is the significant shift toward larger particles from upper −15°C to lower −10°C in the convective region, with the maximum particle dimensions increasing from less than 6000 µm to over 1 cm. The particles larger than 1 cm at −10°C are composed of dendrites and their aggregates. The other difference is the large concentrations of small particles (25–205 µm) at temperatures between −3°C and −5°C in the convective region, where rimed ice particles and needles coexist. Needle regions are observed from three of the five spirals, but only the cloud conditions within the convective region fit into the Hallett-Mossop criteria.

摘要

积层混合云中对流雨核的存在对冰雪晶微物理特性和降水形成有重要影响. 本文分析了 2017 年 5 月 22 日一次混合相层状云降水系统的飞机观测数据. 利用垂直盘旋探测和水平分层探测资料, 分析了不同云区的过冷水含量和冰雪晶形态, 重点对比了层云区和对流雨核区冰雪晶凇附特征和谱型分布. 观测数据表明, 对流雨核区过冷水含量丰富, 主要位于-3至-6°C层, 其极大值可达0.6 g m⁻³, 而层云区过冷水含量一般不超过0.1 g m⁻³.

与层云区相比, 对流雨核区冰雪晶谱分布有两处明显差异. 一个不同之处是, 在对流雨核区从-15到-10°C层, 冰雪晶最大尺寸显著增加, 由不足 6000 μm 增加到 1 cm 以上, 这是由于冰雪晶形态由宽枝状向辐枝状及其聚合体的转变. 另一个不同之处是, 在对流雨核区的-3至-5°C层, 25–205 μm之间的小尺度冰雪晶浓度较大, 并且凇附状与针状粒子共存. 从五次垂直盘旋探测中的三次可观测到明显的针状粒子区, 但是仅有对流雨核区的云系条件符合Hallett-Mossop机制.