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。 在足够冷的云中，包含冰晶和过冷水滴的混合物，晶体通过水蒸气的扩散而生长到足够大的尺寸，以相互凝结形成雪花，或者附着云滴并形成<$球，雪丸或更密集的冰降水形式。 所有这些形式的冰相降水都可能在穿过温暖的空气时融化，并以雨的形式到达地面。 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而生长的--虽然速度比冰晶慢，因为云的环境相对于水来说只有轻微的过饱和，但相对于冰来说可能是强烈的过饱和。 一旦一些液滴达到约20微米的半径，它们开始与较小的液滴碰撞并通过聚结而生长。 云的碰撞合并过程和冰晶过程都可以解释降水的发展，这取决于云的温度。 然而，在碰撞合并过程中的一个不确定的环节是缓慢的液滴增长的冷凝和随之而来的长时间的液滴达到碰撞变得重要的大小。 这个持续研究项目的目标是对暖云中雨的发展和演变有一个物理上一致的理解。 该方法基于（1）对雷达、仪表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中推断出的物理过程。 尚未解决的关键问题是：（1）巨核（大于1微米的吸湿粒子）如何影响不同种类云中的降水发生率？(2)什么是微物理机制，占观察到的雷达特征的不断增长的积云？(3)这些云如何修改后续云开发的环境？(4)风切变等环境参数如何影响降水产生率？(5)巨大的核会影响从不同种类的云落下的总降水量吗？ 这些问题的答案对于理解降水发展至关重要，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.