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，足以包含冰晶和过冷水滴的云层中，水蒸气扩散到足够大的尺寸，使其彼此凝结成雪花或附着云滴，形成小球、雪球或更稠密的冰雨，晶体就会生长。所有这些形式的冰相降水在穿过较暖的空气时可能会融化，并以雨的形式到达地面。在所谓的暖云中，它不够冷，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，因为云环境相对于水只是轻微的过饱和，尽管相对于冰可能是强烈的过饱和。一旦一些液滴达到约20微米的半径，它们就开始与较小的液滴碰撞并通过合并生长。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，这取决于云的温度。然而，碰撞-合并过程中一个不确定的环节是液滴凝结增长的缓慢，以及随之而来的液滴达到碰撞变得重要的尺寸所需的很长时间。这一持续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理解。该方法基于(1)分析雷达、仪表式飞机和地面观测的数据，以及(2)使用详细的云微物理模型进行模拟，以检查仅从观测中无法推断的物理过程。尚未解决的关键问题是：(1)巨核(大于1微米的吸湿颗粒)如何影响不同类型云中的降水开始速率？(2)观测到的不断增长的积云的雷达特征的微观物理机制是什么？(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产生速率？(5)巨核是否影响从不同类型云中落下的总降水量？这些问题的答案对于理解降水发展是至关重要的，并对与人工云修改、全球降水测量和气候研究有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.