CLoud Analysis - Part 8
The way in which the cloud microphysics processes control the properties of cloud particles in each volume of air can be understood in terms of the rates at which water mass enters and leaves the particular air volume and the rate at which water mass is changed from one form to another within the volume of air (see diagram). The two processes by which water mass enters and leaves a particular volume are by atmospheric motions carrying vapor at a rate τ*-1 and by falling of cloud particles relative to the air (called sedimentation); upward air motions can reduce the sedimentation rate or even reverse its direction. The sedimentation rate (or fall rate, τ-1fall) increases as particle size increases because larger particles fall faster.
The air motions transport water vapor into the volume but also serve another fundamental purpose for water clouds on Earth: upward motions cause the air to cool (because pressure decreases with altitude - likewise, downward motions cause the temperature to increase) and change the relation between the actual vapor pressure of water vapor and the value that is in equilibrium with the condensed phases of water (liquid or ice), which depends on temperature. When the water vapor is supersaturated (actual vapor pressure exceeds equilibrium value), indicated by a relative humidity above 100%, the excess vapor can condense onto cloud particles (condensation). In the polar regions and at very high altitudes, an air parcel may also cool by radiative heat loss. In Earth's atmosphere, cloud particles begin their growth as water deposits on a background of very small aerosol particles that are produced by natural processes and human pollution and carried into the cloud by air motions. This fact means that the number of cloud particles whose growth is initiated depends on the number and characteristics of these aerosols as well as the rate at which the air parcel cools.
After the initiation stage, no more new particles are created and the excess vapor (the amount above the saturation value) is consumed by the growth of the existing cloud particles, which depends on the rate of air parcel cooling, temperature, the phase of the particle (liquid or ice), the size of the particles, and the number of growing particle per unit volume. The condensation rate decreases as the particles grow larger. When the cooling rate of air is relatively slow, the size of the cloud particles are determined in one of two ways. In some cases, as in fair weather cumulus, the growth of the particles is slower than the rate at which the larger particles fall out of the particular volume of air, so that the size of the particles is determined simply by how long the upward motion lasts; in other words, the particle size is determined by the length time available for condensation growth. When the upward motion ceases or reverses to downward motion, the relative humidity falls below 100% and the cloud droplets evaporate rapidly (illustrated by τ-1supply and τ-1remove in the diagram). In other cases, as in cirrus clouds, the growth of the ice crystal size decreases the condensation rate and increases the sedimentation rate until an approximate balance between the two is produced (supply of mass by vapor condensation and removal of mass by particle sedimentation illustrated by τ-1supply and τ-1remove in the diagram). In this case, particles fall through the cloud base, where the relative humidity is < 100%, and evaporate. Since the relative humidity just below cloud base is usually much less than 100%, whereas in the cloud the relative humidity is typically just barely above 100%, the evaporation rate is somewhat faster than the condensation rate in the cloud, so the particles do not fall very far.
However, many ice clouds form by a two-step process, initial condensation onto liquid droplets followed by freezing. Because the saturation vapor pressure over ice is lower than that over liquid at temperatures below freezing, the cloud base of the freezing-droplet cloud is initially at a higher altitude and the typical supersaturation governing the condensation is 10 or more times larger than in liquid water clouds. Moreover, ice crystals falling from their initial base level continue to grow because the relative humidity with respect to ice is still > 100%. Consequently, cirrus cloud layers can grow downward becoming quite thick and producing much larger cloud particles.
When the upward motions of air are more vigorous, the cloud particles can grow much more rapidly. When they exceed a certain size (about 10-15 microns), their falling motions relative to the air and to each other (since different sized particles fall at different rates) causes collisions to occur. At smaller sizes, the particles cannot push through the air between them so collisions are prevented by this air "cushion". The collision rate (τ-1growth in the diagram) of the larger particles depends on how many particles there are and how fast they fall, which depends on how large the particles are. If the colliding particles stick together to form an even larger particle, the rate of such collisional growth increases very rapidly, despite the fact that the number of particles available decreases. Very quickly, the balance in the cloud is between growth by collisions (called coalescence) and removal of particles by sedimentation. Coalescence rapidly produces cloud particles that are so large that their sedimentation rate now exceeds the rate at which the particles can evaporate when they leave the cloud; this situation produces precipitation, which is called rainfall or snowfall if it reaches the surface.
The efficiency with which liquid droplets stick together is nearly perfect, although collisions of very large precipitation-sized droplets can also cause breakup into many smaller droplets. The sticking efficiency for ice crystals depends on temperature: at very cold temperatures, the ice crystals act as solid particles (like dust) and do not stick very much, whereas, as temperatures approach the freezing point, some liquid water is usually present that helps colliding ice crystals stick together (the water causes sticking then freezes). Once precipitation begins to form, events happen so fast that growth by vapor condensation is no longer very important, the cloud water is depleted and the cloud dissipates. However, very violent weather produces such a large supply of water and suspends the precipitation-sized particles for so long, that long-lived cloud systems with very heavy precipitation rates can be sustained for hours or even more than a day.