研究团队通过比较高分辨率的积云密集云观测与最先进的大涡模拟，以及基于拉格朗日粒子的微物理方案，为湍流对云滴大小分布和降雨形成的演变的显著影响提供了大量证据。湍流聚结必须包含在模型中，以准确地表示观测到的雨滴大小分布，特别是云中较低高度的细雨雨滴大小。与只考虑重力聚结的模拟相比，湍流导致了更早的降雨形成和更大的降雨积累。
研究观测到云底上方的雨大小分布遵循幂律标度，该标度偏离了考虑纯重力碰撞核或忽略液滴惯性效应的湍流核的理论标度，为云中的湍流合并提供了额外的证据。相比之下，作为云凝结核(“巨型CCN”)的大气溶胶对雨的形成没有显著影响，因为相对于上升的积云热气流的寿命，它们达到平衡湿尺寸的时间较长。总体而言，湍流液滴合并对暖积云的降雨形成起主导作用，而巨型CCN的影响有限。
据悉，雨的形成是控制云的生命周期和辐射作用力的关键因素，因此它是天气和气候的关键因素。云微物理—湍流相互作用发生在大尺度范围内，在分辨率有限的大气模式中很难表现出来。基于过去的实验和理想化的数值模拟，已经假设云湍流通过增强雨滴碰撞合并加速降雨的形成。
附：英文原文
Title: Are turbulence effects on droplet collision–coalescence a key to understanding observed rain formation in clouds
Author: Chandrakar, Kamal Kant, Morrison, Hugh, Grabowski, Wojciech W., Lawson, R. Paul
Issue&Volume: 2024-6-25
Abstract: Rain formation is a critical factor governing the lifecycle and radiative forcing of clouds and therefore it is a key element of weather and climate. Cloud microphysics–turbulence interactions occur across a wide range of scales and are challenging to represent in atmospheric models with limited resolution. Based on past experiments and idealized numerical simulations, it has been postulated that cloud turbulence accelerates rain formation by enhancing drop collision–coalescence. We provide substantial evidence for significant impacts of turbulence on the evolution of cloud droplet size distributions and rain formation by comparing high-resolution observations of cumulus congestus clouds with state-of-the-art large-eddy simulations coupled with a Lagrangian particle-based microphysics scheme. Turbulent coalescence must be included in the model to accurately represent the observed drop size distributions, especially for drizzle drop sizes at lower heights in the cloud. Turbulence causes earlier rain formation and greater rain accumulation compared to simulations with gravitational coalescence only. The observed rain size distribution tail just above cloud base follows a power law scaling that deviates from theoretical scalings considering either a purely gravitation collision kernel or a turbulent kernel neglecting droplet inertial effects, providing additional evidence for turbulent coalescence in clouds. In contrast, large aerosols acting as cloud condensation nuclei (“giant CCN”) do not significantly impact rain formation owing to their long timescale to reach equilibrium wet size relative to the lifetime of rising cumulus thermals. Overall, turbulent drop coalescence exerts a dominant influence on rain initiation in warm cumulus clouds, with limited impacts of giant CCN.
DOI: 10.1073/pnas.2319664121
Source: https://www.pnas.org/doi/abs/10.1073/pnas.2319664121