The Roles of Aerosols and Entrainment and Mixing in the Warm Rain Process

热带和中纬度地区的大量降雨来自云层，这些云层不够冷，不足以容纳冰晶。这场所谓的“暖雨”是由云滴碰撞聚合而成，云滴是凝结而成的。降水形成所需的时间取决于云滴的浓度及其聚集的趋势。非沉淀云中的典型液滴直径约为10毫米。这么小的水滴相互碰撞的频率不够高，不足以解释降雨的形成。在观察到的时间内(云出现后约20分钟)产生毛毛雨(100毫米)需要存在一些直径约20毫米的水滴。相对于较小的液滴，这些液滴的下落速度足够快，足以与它们相撞，形成更大的液滴。一般而言，促进碰撞和合并并从而引发降水的云滴群体是那些包含液滴大小扩散的云滴：扩散越大，碰撞速率就越大，云转化为毛毛雨和雨的速度就越快。暖雨理论中的一个经典问题是，凝聚作用于被认为是现实中的凝聚核群体，不会产生有利于碰撞和合并的宽广液滴光谱。这个问题是通过数值模拟和观测分析相结合的方法来解决的。指导性的假设是，水滴光谱的展宽可以通过不同历史的云团混合来解释。该研究计划包括计算一组水滴的轨迹，这些水滴从一个正在发展的云中的不同位置开始，但同时到达相同的位置。由于动荡和不同的增长历史，来源区域可能很大。该分析将湍流扰动增长轨迹的思想与大涡模拟产生的气流和参数化降水相结合。这项工作的一个新方面是，计算是在时间上倒着进行的：给定大小的水滴的历史可以从规定的起点追溯到云底的原始位置。并不是所有的规模都会被证明是起源于云基础。迭代程序被用来确定哪些大小可以归因于在云底形成的液滴。那么，这些就是与模型一致的尺寸。通过将预测的水滴大小与1995年在佛罗里达州进行的小积云微物理研究中飞机测量的水滴大小进行比较，对该模型进行了评估。

A substantial amount of the rain in the tropics and midlatitudes falls from clouds that are not cold enough to contain ice crystals. This so-called "warm rain" is produced by the collision and coalescence of cloud droplets, which are formed by condensation. The time required for precipitation to develop is determined by the concentration of cloud droplets and their tendency to come together. A typical droplet in a non-precipitating cloud has a diameter of about 10 mm. Drops this small do not collide with each other frequently enough to account for the formation of rain. The creation of drizzle drops (100 mm) in the times observed (about 20 min after the cloud appears) requires the presence of some drops of about 20 mm diameter. These fall fast enough relative to the smaller droplets to collide with them and form still larger drops. In general, the cloud droplet populations that promote collisions and coalescence and can thereby initiate precipitation are those containing a spread of droplet sizes: the greater the spread the greater the collision rate and the faster the conversion of cloud to drizzle and rain. A classical problem in the theory of warm rain is that condensation acting on what are thought to be realistic populations of condensation nuclei does not produce the broad droplet spectra that are conducive to collisions and coalescence. This problem is approached by a combination of numerical modeling and the analysis of observations. The guiding hypothesis is that the broadening of droplet spectra may be explained by the mixing of cloudy air parcels with different histories. The research plan includes computing an ensemble of trajectories of droplets that start from different positions within a developing cloud but reach the same place at the same time. Because of turbulence and different growth histories, the source region can be large. The analysis combines the idea of turbulently-perturbed growth trajectories with the airflow and parameterized precipitation generated by large eddy simulation. A novel aspect of the work is that the calculations are performed backwards in time: the history of a drop of a given size is traced back from a prescribed starting point to an original position at cloud base. Not all sizes will turn out as originating at cloud base. An iterative procedure is used to determine which sizes can be ascribed to droplets that form at cloud base. These, then, are the sizes that are consistent with the model. The model is evaluated by comparing the predicted drop sizes with those measured by aircraft in the Small Cumulus Microphysics Study conducted in Florida in 1995.