USING 3D-PRINTED ANALOGUES TO UNDERSTAND THE AERODYNAMICS OF COMPLEX ICE PARTICLES

大气中的许多云都含有冰粒。这些冰粒在气候系统中扮演着重要的角色，因为高空卷云在任何时候都能覆盖全球约30%的面积，并使地球变暖。冰粒对降水的发展也很重要，不仅在寒冷的极地气候中，甚至在中纬度地区（如英国），超过四分之三的降水源自高空的雪花——只是大部分在到达地面之前就融化了。无论是在云中还是在地面的降雪中，冰粒都会沉淀下来。换句话说，它们能够长得足够大，可以从空中掉下来。这有几个含义。其中最明显的是，粒子的下落速度控制着水在大气中的垂直输送，并到达地表。更微妙的是，每个冰粒在空气中的运动直接影响着冰粒生长、蒸发和融化的速度。例如，如果空气足够潮湿，水分子就会扩散到冰晶的表面并沉积在那里，从而导致生长。如果粒子是静止的，那么这种增长就会稳定而缓慢地进行，因为不断增长的冰晶会耗尽它周围的水蒸气，导致分子浓度的浅梯度。如果颗粒在下落，这种生长速度会快得多，因为冰晶不断地落入新鲜潮湿的空气中，导致浓度梯度陡峭。给定大小和形状的冰粒下降的速度，以及冰粒生长速度的提高程度，这些定量细节都是由冰粒周围的气流或其空气动力学决定的。不幸的是，这是云物理的一个领域，我们的理解是非常有限的。简单形状如球体、椭球体和圆盘的空气动力学得到了很好的研究。然而，从对天然冰粒的观察可以清楚地看出，它们的几何结构并不简单。相反，这些粒子的形状往往是复杂和不规则的。关于这些粒子的空气动力学，我们几乎没有高质量的数据。因此，即使是最先进的微物理模型也不得不近似空气动力学对冰过程的影响，就好像这些复杂的不规则粒子是球体或椭球体一样，希望这是一个足够的近似。为了解决这个问题，我们需要在大气中观测到的具有复杂形状的粒子的空气动力学实验数据。障碍在于，对自由落体状态下的天然冰粒进行适当的观察是极具挑战性的。在表面的降雪中，颗粒很小，易碎，容易被风吹走，如果不小心处理，就有可能融化或蒸发。对卷云中下落粒子的直接采样是不可能的。在这两种情况下，都不可能直接确定颗粒周围的气流或气流对微物理过程速率的影响。在这个项目中，我们通过使用类似物来克服这些问题。使用3D打印技术，我们将创建具有与天然冰粒相同复杂几何形状的塑料颗粒。通过将颗粒放入液体罐中，通过实验室的空气和垂直风洞，我们可以确定颗粒的下落速度是如何由它们的大小和几何形状控制的。利用层析粒子成像测速技术的最新发展，我们可以测量下落类似物周围的气流。由此我们可以直接确定气流如何促进颗粒的生长、蒸发和熔化速率。

Many of the clouds in the atmosphere contain ice particles. These ice particles play an important role in the climate system, because high-altitude cirrus clouds cover around 30% of the globe at any one time, and act to warm the planet. Ice particles are also important for the development of precipitation, and not only in cold polar climates: even in mid-latitudes (like the UK), over three quarters of the precipitation that falls originates as snowflakes aloft - it is just that most of it melts before arriving at the surface.Ice particles, both in clouds and in snowfall at the surface, precipitate. In other words, they are able to grow large enough to fall through the air. This has several implications. The most obvious of these is that the rate at which the particles fall out controls the transport of water vertically through the atmosphere and to the surface. More subtly, the movement of each ice particle through the air directly influences the rate at which the particle grows, evaporates and melts. For example, if the air is humid enough, water molecules will diffuse to the ice crystal's surface and deposit there, leading to growth. If the particle is stationary, this growth occurs steadily but slowly, because the growing ice crystal depletes the vapour around it, leading to a shallow gradient in the concentration of molecules. If the particle is falling, this growth can occur much faster, because the ice crystal is constantly falling into fresh, humid air, leading to steep concentration gradients. The quantitative details of exactly how fast an ice particle of a given size and shape falls, and how much the growth rates are enhanced by, is determined by the airflow around the ice particle, or its aerodynamics. Unfortunately, this is an area of cloud physics where our understanding is extremely limited. The aerodynamics of simple shapes like spheres, spheroids and discs is well studied. However it is clear from observation of natural ice particles that they are not simple in their geometry. Instead the particles are often complex and irregular in their shape. We have almost no high-quality data on the aerodynamics of such particles. As a result, even state-of-the-art microphysical models are forced to approximate the aerodynamical effects on ice processes as though these complex irregular particles were spheres or spheroids, hoping that this is an adequate approximation. To solve this problem, experimental data is needed for the aerodynamics of particles with the complex shapes that we observe in the atmosphere. The stumbling block is that making suitable observations of natural ice particles in free-fall is extremely challenging. In snowfall at the surface the particles are small, fragile, easily blown by the wind, and likely to melt or evaporate if not handled with great care. Direct sampling of falling particles in cirrus clouds is impossible. In neither case is it possible to directly determine the airflow around the particle or the influence of that flow on the microphysical process rates.In this project we overcome these problems with the use of analogues. Using 3D printing techniques we will create plastic particles with the same complex geometry as natural ice particles. By dropping the particles in tanks of liquids, and through air in the laboratory and a vertical wind tunnel, we can determine how the fall speed of the particles is controlled by their size and geometry. Exploiting recent developments in tomographic particle imaging velocimetry we can measure the airflow around the falling analogues. From this we can directly determine how the airflow enhances the particle growth, evaporation and melting rates.