5: Cloud Physics
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By the end of this chapter, you should be able to:
- identify cloud types
- describe the essentials for cloud formation
- on a Koehler curve, explain the behavior of a particle in different supersaturation environments
- explain the lifecycle of cloud formation through precipitation
Clouds and precipitation are integral to weather and can be difficult to forecast accurately. Clouds come in different sizes and shapes that depend on atmospheric motions, their composition, which can be liquid water, ice, or both, and the temperature. While clouds and precipitation are being formed and dissipated over half the globe at any time, their behavior is driven by processes that are occurring on the microscale, where water molecules and small particles collide. We call these microscale processes “cloud microphysics” and microphysics is the focus of this lesson. Three ingredients are required for the formation of clouds: moisture, aerosol, and cooling. If any one of these is missing, a cloud will not form. Over eighty years ago, a simple hypothesis was developed to explain the formation of clouds. This hypothesis has been thoroughly tested and validated and is now called Koehler Theory. We will learn the elements of Koehler Theory and how to use them to determine when clouds will form and when they will not, becoming only haze. Clouds do not automatically precipitate. In fact, most clouds do not. We will learn about the magic required for precipitation to form. Thus, cloud formation through precipitation is a series of microsteps, each of which is necessary, but not sufficient, to achieve precipitation.
- 5.4: How can supersaturation be achieved?
- Three basic mechanisms for cooling the air are RUM: Radiation, Uplift, and Mixing.
- 5.5: Curvature Effect - Kelvin Effect
- Consider the forces that are holding a water drop together for a flat and a curved surface. The forces on the hydrogen bonding in the liquid give a net inward attractive force to the molecules on the boundary between the liquid and the vapor. The net inward force, divided by the distance along the surface, is called surface tension.
- 5.6: Solute Effect - Raoult’s Law
- The atmosphere is not very clean. There are all kinds of dirt and other particles in the atmosphere. Some of these are hydrophilic (i.e., they like water) and water soluble (i.e., they dissolve in water). So let’s see what the effect of soluble CCN might be on the water evaporation rate for a flat water surface. We’ll then put the curvature and the solute effects together.
- 5.7: Vapor Deposition
- The growth of the cloud drop depends initially on vapor deposition, where water vapor diffuses to the cloud drop, sticks, and thus makes it grow.
- 5.8: Did you know most precipitation comes from collision-coalescence?
- There are two types of processes for growth into precipitation drops: warm cloud processes and cold cloud processes. In warm clouds, the processes all involve only liquid drops. In cold clouds, the processes can involve only solid particles, as well as mixed phases (both supercooled liquid and ice). Some of the most important processes involve collisions between drops, whether they be liquid or solid.
- 5.9: An Unusual Way to Make Precipitation in Mixed-Phase Clouds
- Recall that water can exist in liquid form even when T < 273 K. This supercooled liquid needs ice nuclei (IN) in order to become ice, although at a temperature of about –40oC, it can convert to ice homogeneously.