Microphysics of Clouds and Precipitation

标题：云和降水的微物理这个项目是对云和降水物理某些方面的理论和观测研究的继续。它包括以下三个主题：1.云滴的凝结生长。云物理中的一个长期难题是观测到的云中液滴大小分布与经典凝聚扩散理论预测的分布之间的差异。观测到的分布比预测的要宽，这对碰撞-合并过程中的雨的发展具有重要的意义，因为宽的分布有利于碰撞和更快地产生大液滴。过去有人提出了许多机制来解释这种差异，尽管没有一种机制被普遍接受。本项目着重于两个效应：较小液滴上较大的平衡蒸汽压造成的分布缓慢展宽，以及液滴附近湿度的细微尺度波动引起的展宽。通过凝聚和合并的同时作用而形成的液滴。碰撞-聚结过程中液滴的生长速率随液滴尺寸的增大而快速、非线性地增大。通常的方法是通过凝聚来计算增长，直到液滴变得足够大，足以发生碰撞，然后从那时起通过碰撞-合并来计算增长，因为它很快就变得比凝结重要得多。然而，即使冷凝导致液滴大小的微小增加也会导致碰撞效率的大幅增加，因此同时作用的两个过程产生的增长比单独处理并将结果相加所获得的增长更快。该项目将开发同时处理凝结和合并的新的计算方法。层状云中雨水的蒸发。雨水通过云底以下的亚饱和空气蒸发并冷却空气。在包括云和降水影响的参数化的大型模式中应考虑到这种影响。这里的方法是计算亚云蒸发与液滴大小分布以及温度、湿度和垂直风速的垂直分布之间的关系。一个目标是发展从云下雷达反射率的垂直剖面估计蒸发量的方法。这些研究有助于对降水发展的基本了解和应用，如雷达遥感降水。

AbstractATM-0314049Srivastava, Ramesh C.University of ChicagoTitle: Microphysics of Clouds and PrecipitationThis project is a continuation of theoretical and observational investigations of certain aspects of cloud and precipitation physics. It comprises the following three topics:1. Growth of cloud drops by condensation. A long-standing puzzle in cloud physics is the discrepancy between observed drop-size distributions in clouds and the distributions predicted from classical condensation-diffusion theory. The observed distributions are broader than those predicted, with important implications for the development of rain by the collision-coalescence process, because broad distributions favor collisions and a more rapid production of large drops. Many mechanisms have been suggested in the past for explaining the discrepancy, though none is generally accepted. This project focuses on two effects: the slow broadening of the distribution caused by the greater equilibrium vapor pressure over the smaller drops, and the broadening caused by fine-scale fluctuations of humidity in the vicinity of drops.2. Growth of drops by the simultaneous action of condensation and coalescence. The rate of growth of drops by the collision-coalescence process increases in a rapid and nonlinear way with increasing drop size. The usual approach is to calculate growth by condensation until the drops become large enough for collisions to occur, and to calculate growth by collision-coalescence from there on, because it quickly becomes much more important than condensation. However, even a small increase in drop size by condensation causes a substantial increase in the collision efficiency, so the two processes acting simultaneously produce more rapid growth than obtained by treating them separately and adding the results. The project will develop new computational methods of treating condensation and coalescence simultaneously.3. Evaporation of rain from stratiform clouds. Rain falling through subsaturated air below cloud base evaporates and cools the air. The effect should be taken into account in large-scale models that include parameterizations for the effects of clouds and precipitation. The approach here is to compute the dependence of sub-cloud evaporation on the drop-size distribution and the vertical profiles of temperature, humidity, and vertical air velocity. An objective is to develop ways of estimating the evaporation from the vertical profile of radar reflectivity below the cloud.These studies contribute to the fundamental understanding of precipitation development and to applications such as the remote measurement of precipitation by radar.