Precipitation Processes - Cloud Drops, Growth, and Collision

1. Chapter 7 Precipitation

2. Precipitation Processes • An ordinary cloud droplet is extremely small (~20 micrometers) • 100 times smaller than an average raindrop • if in equilibrium… the size of the droplet does not change • - # of molecules condensing onto the droplet are exactly equal to # evaporating • - saturation vapor pressure = equilibrium vapor pressure

3. Growth of Cloud Drops Atmospheric vertical winds and eddies can keep small and light cloud droplets and condensation nuclei aloft. As the cloud droplets knock and join together, they grow larger and their weight increases, causing them to fall as precipitation.

4. To keep a droplet in equilibrium…. • more water vapor molecules are needed around it to replace those molecules that are constantly evaporating from the surface • smaller droplets have greater curvature and a more rapid rate of evaporation • this process is called the curvature effect • smaller droplet require an even greater vapor pressure to keep them from evaporating away • therefore, when air is saturated with respect to a flat surface, it is unsaturated with respect to a curved droplet of water, which leads to the evaporation of the droplet • to keep tiny cloud droplets in equilibrium with the surrounding air, the air must be supersaturated

5. Vapor Pressure & Saturation Smaller drops have greater curvature and require greater vapor pressure to keep water molecules from evaporating away. As the drop size increases, the required relative humidity (RH) for equilibrium decreases. If the required RH is exceeded, the drop will grow.

6. How do tiny droplets of less than 1 micrometer grow to the size of an average cloud droplet? • hygroscopic particles allow for condensation to occur at relative humidity's less than 100 percent • Example • - when condensation begins on a salt particle, it dissolves forming a solution • -the salt ions bind closely to the water molecules making evaporation difficult • - this condition reduces the equilibrium vapor pressure • - this is known as the solute effect • A droplet containing a CCN (such as salt) can be in equilibrium with its environment at RH < 100%

7. Condensation and precipitation • most clouds cannot produce precipitation (through condensation) • this process is too slow to produce precipitation • it would take several days for this process to produce rain under ideal conditions • observations show that clouds can develop and produce precipitation in under an hour • it takes about 1 million average size cloud droplets (20 micrometers) to produce one average size rain drop • must be other processes at work

8. Collision and coalescence process • Occurs in clouds with tops warmer than -15ºC • some cloud droplets must be larger than others in order for collisions to form a raindrop • larger drops may form on CCN or through random collision of droplets • as cloud droplets fall air slows the drops descent • the amount of air resistance depends on the size of the drop and the rate of the fall • the speed of the falling drop increases until the air resistance equals the pull of gravity • at that point the drop reaches its terminal velocity

9. Collision & Coalescence Process Both a) collisions that join together small cloud droplets b) coalescence that attaches faster and larger droplets with smaller slower droplets work together to assemble nearly 1 million cloud droplets into a raindrop large enough to fall to earth.

10. Collision & Coalescence Process Both a) collisions that join together small cloud droplets b) coalescence that attaches faster and larger droplets with smaller slower droplets work together to assemble nearly 1 million cloud droplets into a raindrop large enough to fall to earth.

11. Warm Clouds • clouds that have above freezing temperatures at all levels • precip forms by collision coalescense • Example: Next Slide • Rain that falls is called warm rain • most important factor in prduction of raindrop is the clouds liquid water content • Other significant factors: • -range of droplets sizes • -cloud thickness • -updrafts of the cloud • -electric charge of the droplets and electric field of cloud

12. Warm Cloud Processes Collision and coalescence operates in warm clouds (> 15° C) to produce rain, and is affected by the clouds liquid water content, droplet sizes, cloud thickness, updrafts, and drop electrical charges. Figure 8.5

13. Ice-crystal Process • called the Bergeron process • extremely important in mid and high latitudes • air below freezing • cloud are called cold clouds • Example: next slide • ice crystals form on ice nuclei • number of ice nuclei available in the atmosphere is small • deposition nuclei - allows water vapor to deposit as ice directly onto their surfaces • freezing nuclei - promote the freezing of supercooled liquid water • contact nuclei - cause freezing after being immersed in liquid

14. Ice Crystal Process Cold clouds may drop below –40° C before small droplets freeze into ice embryos that can serve as condensation nuclei. At very low temperatures, vapor can also condense as ice onto nuclei formed by: a) deposition b) freezing c) contact, primarily in the glaciated region of the cloud.

15. Molecules from Water to Ice Ice crystals have lower saturation vapor pressures than liquid droplets, creating a gradient of high to low water molecules from liquid to ice that encourages ice growth. This growth is critical to the ice-crystal precipitation process.

16. Ice Particle Changes As ice crystals fall and collide with super cooled drops, they get bigger by accretion. Falling icy matter is called graupel, and aggregation describes the joining of two ice crystals into snowflakes.

17. Ice Crystal Growth Ice crystal growth is the dominant cause of precipitation in nimobstratus and cumulonimbus clouds, both of which have lower liquid content than warm-layered clouds such as stratus.

18. Cloud Seeding Artificial seeding, such as Silver Iodide, and natural seeding, such as cirriform ice crystals, are available to increase the number of condensation nuclei and encourage precipitation.

19. Forms of precipitation • Rain • -drizzle (diameters smaller than 0.5mm) • -virga (rain evaporating prior to reaching the surface) • Snow • -flurry (light snow from cumulus clouds) • -squall (intense snow shower) • -blizzard (heavy snow and winds > 30knots) • Sleet (snow that partially melts and freezes again into ice) • Freezing Rain (supercooled liquid water that reaches the ground • Hail

20. Evaporating Rain Rain falling into low humidity air below will cause the drops to decrease in size, possibly evaporating into streaks of dry air as in this virga.

21. Snowflakes & Snowfall Snowflakes are crystalline structures that can have plate, column, dendrite, or needle forms. Air temperature and humidity determine crystal form, and dendrite is the most common habit.

22. Sleet & Freezing Rain Environmental temperatures may reveal a warm zone between two freezing layers. Snow falling into the warm zone will melt and either a) fall as rain and refreeze on contact with the ground, or b) refreeze and fall as sleet.

23. Sleet & Freezing Rain Four vertical temperature profiles are shown to illustrate the phase change that a snowflake may experience in its path toward earth's surface.

24. Rime & Freezing Rain Rime, a granular ice, accumulates when super cooled fog droplets touch a frozen surface. Freezing rain creates incredible strain on branches and other structures, resulting in costly damages.

25. Snow Grains & Snow Pellets Flat and long snow grains fall as frozen drizzle, too small to bounce or shatter. Snow pellets are larger, and have a rounded layer of rimed ice that creates air bubbles and a bounce.

26. Hailstones & Damage Updrafts in a towering cumulus cloud recirculate graupel through an accretion and freezing process that produces large heavy hailstones. Hailstreaks Such storms cause regular property damage, but only 2 U.S. deaths in the 20th Century.

27. Hailstreaks

28. Coffeyville Hailstone Regular and polarized light images of the 14 cm diameter hailstone that fell in Kansas in 1970. Hailstorms cause severe damage to crops and other structures. Farmers name for it?

29. Aurora Hailstone Previous hailstone record holder in the U.S. 17.8 cm in diameter June, 2003

30. Vivian Hailstone Largest hailstone on record in the U.S. 20.32 cm in diameter, 2 lbs in weight July, 2010