Atmo 171 SEction 2 - Lecture Notes Week 10



3/20/00

CLOUD FORMATION



Clouds form as air rises and cools to its dew point temperature. At this point, the air is saturated and water vapor begins to condense into the liquid water droplets that make up a cloud. This occurs at the base of the cloud, which can also be referred to as the condensation level.

How do we get rising air in the first place? Normally, the atmosphere is in a state of balance (called hydrostatic balance) where the downward force of gravity and the upward pressure gradient force of air molecules cancels out - meaning that there is normally no net force causing air to rise or sink. However, certain processes can overcome this balance and lead to rising air, and thus cloud formation. These processes of cloud formation (Figure 5.8 in Ahrens) are:

  1. Convection (also called "free convection" or "buoyant lifting"): This occurs when heated air becomes less dense than the surrounding air and begins to rise. If the overlying atmosphere is stable, then a rising air parcel will soon become cooler than its surroundings and cease to rise. The resulting cloud that forms will not have a large vertical extent, but instead will be somewhat flat (e.g., stratocumulus, cumulus humilis). If, on the other hand, the atmosphere is unstable or conditionally unstable, convection may result in a large, towering cloud forming (i.e. cumulus congestus or cumulonimbus). Daytime heating of the earth's surface drives the convective activity we see in Tucson.

  2. Topographic (also called "orographic lifting" or "forced lifting"): This refers to rising air motions that result from air flow over a large obstacle such as a mountain range. Horizontal winds are forced to rise up and over the obstacle (see Figure 5.12 in Ahrens). This rising motion can be enough to cause cloud formation and possibly precipitation. On the back (or "leeward") side of the obstacle, air sinks. Recall that sinking air becomes compressed and heats up, which prevents cloud formation. This leads to the formation of a rain shadow on the leeward side of a mountain range.

  3. Convergence: Within a low pressure system (also called a cyclone), air flows counterclockwise (in the Northern Hemisphere) and in toward the center. This convergence of air at the surface results in rising air motion. For this reason, mid-latitude low pressure systems are commonly associated with cloudy and rainy conditions. Within a high pressure system (or anticyclone), air flow clockwise and outward from the center. The diverging surface air at the center of an anticyclone is balanced by downward motion, which prevents cloud formation. Thus high pressure systems are associated with clear conditions.

  4. Frontal lifting: Recall that fronts are boundaries between two different air masses. A cold front marks the boundary between warm air and advancing cold air. The colder air, being more dense or "heavier" than the warm air, acts as a wedge that pushes the warm air up and out of the way. This leads to rising motion and cloud formation ahead of a cold front. In the case of a warm front, advancing warmer air encounters a cold air mass and, being "lighter" or less dense, runs up along the boundary of the warm front. This leads to rising motion and cloud formation that occurs well ahead of the location of the surface frontal boundary (see Figure 5.8d).

It should be emphasized that the stability of the atmosphere plays an important role in determining the type of cloud that forms as a result of any of these processes. Rising motion in an unstable or conditionally unstable environment will favor vertical development, and increase the liklihood of precipitation.

3/22/00

PRECIPITAION


We define precipitation as any liquid or frozen water that falls from a cloud and reaches the ground we it can be measured. Water vapor can be converted into precipitation through the processes of condensation, freezing, and deposition (i.e., going directly from vapor to solid form).

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 or ice nuclei, depending on the process. They are generally quite small; typical sizes are 2 micrometers (or 0.002 millimeters) 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,i.e., 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. Efforts to enhance precipitation through "cloud seeding" add more particles to the atmosphere to increase the likelihood of nucleation.

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 freeze on. If it gets cold enough (at or below -40 C), these supercooled water droplets will freeze even in the absence of ice nuclei. This is called spontaneous nucleation.

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 ice crystal process (aka the Bergeron process).

Precipitation Formation by Collision and Coalescence


Collision and coalescence invlolve interaction of liquid water droplets with other liquid water droplets - it is most important in clouds with temperatures above freezing - and so together they are referred to as a "warm rain process". 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 may 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.


3/24/00

PRECIPITATION (cont'd)



The Ice Crystal Process of Precipitation Formation


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 vapor pressure above supercooled liquid water is greater than the saturation vapor pressure over ice. If you have an ice crystal suspended in a cold cloud which is surrounded by supercooled water droplets, the difference in saturation vapor pressure between ice and liquid means that there are more vapor molecules next to the supercooled droplet and less vapor molecules next to a neighboring ice crystal. This difference drives a process known as diffusion that moves vapor molecules away from the droplet and onto the ice crystal, where they deposit themselves. To replace these diffusing molecules and maintain saturation, water evaporates from the supercooled droplet.

Under these conditions the ice crystal grows at the expense of the supercooled droplets (see Figure 5.19 in Ahrens). This process is very 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.

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