Field And Modeling Studies Of Warm Cloud Precipitation Physics

两种不同的物理机制解释了降水的形成。在冷到足以包含冰晶和过冷水滴混合物的云层中，晶体通过水蒸气的扩散而生长到足够大的尺寸，以便彼此凝固形成雪花，或积聚云滴形成霰、雪球或密度更大的冰降水。所有这些形式的冰相降水都可能在通过温暖的空气降落并以雨的形式到达地面时融化。在所谓的暖云中，温度不足以形成冰，降水可以通过全水过程产生。云滴最初是通过凝结生长的——尽管速度比冰晶慢，因为云的环境相对于水只是稍微过饱和，但相对于冰可能是强烈过饱和的。一旦一些液滴的半径达到20微米左右，它们就开始与更小的液滴碰撞，并通过合并生长。根据云层温度的不同，无论是碰撞合并过程还是冰晶过程都可以解释降水的发展。然而，碰撞-合并过程中的一个不确定环节是液滴通过凝结生长的缓慢，因此液滴需要很长时间才能达到碰撞变得重要的尺寸。这个正在进行的研究项目的目标是对暖云中雨的发展和演变形成物理上一致的认识。该方法基于(1)对来自雷达、仪器飞机和地面观测数据的分析，以及(2)使用详细的云微物理模型进行模拟，以检查无法仅从观测推断的物理过程。未解决的关键问题是：(1)巨核（大于1微米的吸湿粒子）如何影响不同类型云中降水的发生速率？(2)什么微物理机制解释了观测到的成长积云的雷达特征？(3)这些云如何改变后续云发展的环境？(4)风切变等环境参数如何影响降水生成速率？(5)巨核是否会影响不同类型云的降水总量？这些问题的答案对于理解降水的发展是至关重要的，并且对于与人工云改造、全球降水测量和气候研究相关的问题具有重要意义。

Two different physical mechanisms account for the formation of precipitation. In clouds cold enough to contain a mixture of ice crystals and supercooled water droplets, the crystals grow by the diffusion of water vapor to sizes large enough to coagulate with one another to form snowflakes or to accrete cloud droplets and form graupel, snow pellets, or denser forms of ice precipitation. All of these forms of ice-phase precipitation may melt when falling through warmer air and reach the ground as rain. In so-called warm clouds, which are not cold enough for ice to form, precipitation can be created by an all-water process. Cloud droplets grow initially by condensation - though at a rate slower than ice crystals, because the cloud environment is only slightly supersaturated relative to water though it may be strongly supersaturated relative to ice. Once some of the droplets reach a radius of about 20 micrometer, they begin to collide with smaller droplets and grow by coalescence. Either the collision-coalescence process or the ice-crystal process, depending on cloud temperature, can explain the development of precipitation. However, an uncertain link in the collision-coalescence process is the slowness of drop growth by condensation and the consequent long time for drops to reach a size where collisions become important. The goal of this continuing research project is to develop a physically consistent understanding of the development and evolution of rain in warm clouds. The approach is based on (1) analysis of data from radars, instrumented aircraft, and surface observations, and (2) simulations with a detailed cloud microphysical model to examine physical processes that cannot be inferred from observations alone. Key unresolved questions are: (1) How do giant nuclei (hygroscopic particles larger than 1 micrometer) affect the rate of precipitation onset in different kinds of clouds? (2) What are the microphysical mechanisms that account for the observed radar characteristics of growing cumulus clouds? (3) How do these clouds modify the environment for subsequent cloud development? (4) How do environmental parameters such as wind shear influence the rate of precipitation production? (5) Do giant nuclei affect the total precipitation falling from clouds of different kinds? Answers to these questions are critical for understanding precipitation development and have significance for questions related to artificial cloud modification, global precipitation measurements, and climate studies.