10/25/99

PRECIPITATION


Precipitation is any form of water (liquid or solid) that falls from a cloud and reaches the ground. There are, obviously, lots of different types of precipitation: rain, snow, hail, sleet, etc. Each type of precipitation has undergone its own unique process of formation prior to reaching the surface. It may surprise you to learn that most of the precipitation that falls from our summer thunderstorms began in frozen form. Since precipitation formation involves water changing from one phase to another (e.g. vapor to liquid or liquid to solid), take a look at Figure 2.9 in Danielson to remind yourself of the phase transitions and the names of each one.

When water vapor condenses or freezes, it prefers to condense or freeze onto something - it needs a surface (preferable some liquid water or ice already present). Away from the surface of the earth (in the "free atmosphere"), the only available surfaces are tiny solid particles suspended in the air (dust, smoke particles, sea salt, etc.) These are called condensation nuclei. They are generally quite small; typical sizes are 0.01 to 1 micrometers in diameter.

The formation of precipitation begins with the process of nucleation, which is the deposition, freezing, or condensation of water vapor in the free air onto condensation nuclei. Let's say you manage to get some water vapor to condense onto a condensation nuclei. What will determine whether this initial water droplet will continue to grow or not? The answer is that the relative humidity of the air around the droplet will determine its growth. If we assume the nuclei is "normal" (i.e. neither attracts nor repels water molecules), then


We see that under "normal" circumstances, drop growth by condensation or depostion will only occur when the environment is supersaturated - when RH > 100 %. This is fairly rare. How then will a cloud ever form?

The answer is that some condensation nuclei are hygroscpic, they attract water. Because they attract water molecules, hygroscopic nuclei allow drop growth to occur when RH is equal to or less than 100 %. Thus these type of nuclei are very important for cloud formation.

It is common to observe tiny droplets of liquid water in clouds even when the temperature is below freezing. When liquid water is present at below freezing temperatures, it is called supercooled water. Remember that water molecules want to have surfaces to condense (or in this case, to freeze) onto - call them ice nuclei in this case. Since there are very few ice nuclei in a typical cloud, you end up with supercooled water droplets hanging around looking for something to condense on. If it gets cold enough (below -39 C), these supercooled water droplets will freeze even in the absence of ice nuclei. This is called spontaneous nucleation.

Drop growth by condensation or deposition of water vapor onto appropriate nuclei is very slow. Figure 7.1 in Danielson shows that it takes more than 2 hours to grow a 30-micrometer diameter cloud droplet from a condensation nucleus by condensation alone. We know from routine observations that a thunderstorm can form and produce rain in less time than this. How is this possible, given the fact that drop growth by condensation is so slow?



10/27/99

Growth by collision and coalescence (Warm rain process)

The handout distributed today shows that typical condensation nuclei are 0.1 to 1 micrometer in diameter. Typical cloud droplets are observed to be 1-30 micrometers in diameter. The smallest precipitation particles are about 0.2 millimeters in diameter (or about 200 micrometers), and they can be up to 5 millimeters (5000 micrometers) in size. Condensation and depostion alone can grow typical cloud droplets in a period of 20-120 minutes, but you would have to wait almost forever for these processes to produce a precipitation-sized particle that is heavy enough to fall. Luckily, there are other ways to produce precipitation-sized particles that work much faster.

The growth of precipitation particles beyond the cloud droplet stage can be accomplished by the combined effects of collision and coalescence, and also by the Bergeron process (aka the three-phase process). Collision and coalescence invlolve interaction of liquid water droplets with other liquid water droplets - it is most important in clouds with temperatures above freezing - hence the name "warm rain process" (Danielson p. 190). The Bergeron process is most important in clouds with temperatures below freezing. It involves interactions between ice particles, supercooled water, and water vapor - hence the name "three-phase process").

Collision is self-explanatory. Liquid cloud droplets carried by air motions within a cloud can collide. Obviously, the most effective type of collisions will involve one big droplet moving through a group of smaller droplets. If the droplets are all moving at different speeds, that will also increase the likelihood of collisions.

Coalescence refers to the fact that water is "sticky". If two water droplets come into contact (say by collision) then they will stick together and make one larger droplet. Not all droplets that collide will stick together - they could bounce off each other. Therefore, collision and coalescence are not an entirely efficient process. Furthermore, these colliding droplets will tend to limit the size that a precipitation particle can reach - no larger than about 5 millimeters in diameter.


10/29/99

Growth by the Bergeron Process (three-phase process)

As stated above, much of the precipitation (frozen or liquid) reaching the earth's surface began high up in cumulus clouds where the temperature is below freezing. In this environment, the Bergeron process is much more important for the production of precipitation particles. The physical principle that drives the Bergeron process is the fact that the saturation mixing ratio over ice is less than the saturation mixing ratio over supercooled water.

If you have an ice crystal suspended in a cold cloud which is surrounded by supercooled water droplets, the difference in saturation mixing ratio between ice and liquid means that air next to the ice crystal will be at or above saturation, while air next to the liquid droplets will be below saturation. Water molecules will evaporate from the supercooled droplets and deposit onto the ice crystal under these conditions - the ice crystal grows at the expense of the supercooled droplets (see Figure 7.5 in Danielson for a detailed explanation of why this is so). This process is so effective within a cold (i.e. below freezing) environment due to the fact that there are many more supercooled droplets in a typical cloud than there are ice crystals. This means there are lots of supercooled droplets to "feed" each crystal.

Under the Bergeron process, precipitation particles can grow very large very fast. Once they grow large enough to fall into regions where the temperature is above freezing, collision and coalescence can also contribute to further growth. For precipitation to actually reach the surface, however, there has to be sufficient moisture in the cloud for droplet growth through condensation & deposition, and through collision and coalescence. There also has to be the right number of ice crystals versus supercooled water droplets for the Bergeron process to take place. And finally, the air between the cloud base and the surface has to be moist enough so that the precipitation particles do not evaporate before reaching the surface. Precipitation particles that fall from a cloud and evaporate before reaching the surface are called virga.