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

在热带和中纬度地区，大量的雨水来自于温度不足以包含冰晶的云层，福尔斯。 这种所谓的“暖雨”是由凝结形成的云滴碰撞和合并而产生的。 降水形成所需的时间取决于云滴的浓度及其聚集的趋势。 非降水云中的典型水滴直径约为10毫米。如此小的水滴相互碰撞的频率不足以解释降雨的形成。 在观测时间内（云出现后约20分钟）产生毛毛雨滴（100 mm）需要存在一些直径约20 mm的滴。 它们相对于较小的液滴下落得足够快，从而与它们碰撞并形成更大的液滴。 一般来说，促进碰撞和合并并因此能够引发降水的云滴群是那些包含水滴大小分布的云滴群：分布越大，碰撞率越大，云转化为毛毛雨和雨的速度越快。 暖雨理论中的一个经典问题是，凝结作用于被认为是现实的凝结核种群上，不会产生有利于碰撞和合并的宽液滴光谱。 这个问题是接近的数值模拟和观测分析相结合。 指导性假设是，液滴光谱的加宽可以通过具有不同历史的多云空气团的混合来解释。 该研究计划包括计算液滴轨迹的集合，这些轨迹从发展中的云内的不同位置开始，但同时到达同一地点。 由于湍流和不同的生长历史，源区可能很大。 分析结合的想法，连续扰动的增长轨迹的气流和参数化的降水产生的大涡模拟。 这项工作的一个新颖之处在于，计算是在时间上向后进行的：给定大小的液滴的历史可以从规定的起点追溯到云底的原始位置。 并不是所有的大小都是从云基地开始的。 一个迭代的过程是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。 通过将预测的水滴尺寸与1995年在佛罗里达进行的小积云微物理研究中飞机测量的水滴尺寸进行比较，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.