ATS 351 Lab 7 Precipitation

1. March 7, 2006 ATS 351 Lab 7Precipitation

2. Droplet Growth by Collision and Coalescence • Growth by condensation alone takes too long • Occurs in clouds with tops warmer than 5°F (-15°C)‏ • Greater the speed of the falling droplet, the more air molecules the drop encounters • Important factors for droplet growth • High liquid water content within the cloud • Strong and consistent updrafts • Large range of cloud droplet sizes • Thick cloud

3. Collision and Coalescence

5. Droplet Growth by the Bergeron process • Cold clouds • Homogeneous nucleation of ice • Vapor deposition • Accretion • Aggregation

6. Homogeneous nucleation of ice • Freezing of pure water • Enough molecules in the droplet must join together in a rigid pattern to form an ice embryo • Smaller the amount of pure water, the lower the temperature at which water freezes • Supercooled droplets • Water droplets existing at temperatures below freezing • Homogeneous nucleation (freezing) occurs at temperatures of –40°C • Vapor deposition • From vapor to solid • Not likely in our atmosphere

7. Ice nuclei • Ice crystals form in subfreezing air on particles called ice nuclei • Ice nuclei are rare; only one out of 10 million aerosols is an effective ice nuclei • Fewer sources than CCN • Desert and arid regions: silicate particle (dominant)‏ • Clay particles: for temperatures between –10 and –20°C • Volcanic emissions • Combustion products • bacteria • IN may be de-activated when exposed to atmospheres with high concentrations of Aitken nuclei produced by industrial processes • Oceans are NOT good sources of IN

8. IN requirements • Insolubility • If soluble, cannot maintain molecular structure requirement for ice • Size • Must be comparable, or larger than, that of a critical ice embryo (typically 0.1 microns)‏ • Chemical bond • Must have similar hydrogen bonds to that of ice available at its surface • Crystallographic • Similar lattice structure to that of ice (hexagonal)‏ • Active Site • Pits and steps in their surfaces

9. Heterogeneous nucleation • Vapor deposition • Direct transfer of water vapor to nucleus • Condensation-freezing Condensation of vapor onto surface, followed by freezing • Immersion • Ice nucleus immersed within a drop • Contact • Collision with supercooled droplets, freezing upon impact

10. Growth mechanisms • Vapor deposition • Saturation vapor pressure over water greater than over ice • Supercooled liquid droplets more readily evaporate and contribute to the vapor pressure than sublimation from ice • When ice and liquid coexist in cloud, water vapor evaporates from drop and flows toward ice to maintain equilibrium • Ice crystals continuously grow at the water droplet’s expense • The process of precipitation formation in cold clouds by ice crystal diffusional growth at the expense of liquid water droplets is known as Bergeron process

12. Growth mechanisms • Diffusional growth alone not sufficient for precipitation formation • Accretion • Ice crystals collide with supercooled droplets, which freeze upon impact • Forms graupel • May fracture or split as falls, producing more ice crystals

13. Growth mechanisms • Aggregation • Collision of ice crystals with each other and sticking together • Clumping of ice crystals referred to as a snowflake

14. Precipitation Types- Ice Habits Environmental Temperature (°C)‏ Crystal Habit 0 to -4 thin plates -4 to -6 needles -6 to -10 columns -10 to -12 plates -12 to -16 dendrites, plates -16 to -22 plates -22 to -40 hollow needles

15. Snow • Snowflakes can generally fall 300m (1000ft) below the freezing level before completely melting • Dry vs. wet • Moist air slightly above freezing, snowflakes slightly melt forming thin film of water along edges; snowflakes stick together • Extremely cold air with a low moisture content, small, powdery flakes fall

16. 43oF and Snow? • Snow occurs when air temperature above freezing if very dry air • Evaporative cooling can allow a rainy day to change to snowfall • Need a wet-bulb temperature at freezing or below

17. Graupel • Ice crystals falls through cloud, accumulating supercooled water droplets that freeze upon impact • Creates many tiny air spaces • These air bubbles act to keep the density low and scatter light, making the particle opaque • When ice particle accumulates heavy coating of rime, it’s called graupel

18. Hail • Hailstones form when either graupel particles or large frozen drops grow by collecting copious amounts of supercooled water • Graupel and hail stones carried upward in cloud by strong updrafts and fall back downward on outer edge of cloud where updraft is weaker • Hail continues to grow and carried into updraft until so large that it eventually falls out bottom of cloud

19. Hail growth • As hailstone collects supercooled drops which freeze on surface, latent heat released, warming surface of stone • At low growth rates, this heat dissipates into surrounding air, keeping surface of stone well below freezing and all accreted water is frozen • Referred to as dry growth of hailstone

20. Hail growth • If hailstone collects supercooled drops beyond a critical rate or if the cloud water content is greater than a certain value, latent heat release will warm surface to 0°C • Prevents all accreted water from freezing • Surface of hailstone covered by layer of liquid water • Referred to as wet growth of hailstone

21. Hail layers • Alternating dark and light layers • Wet growth • solubility of air increases with decreasing temperature so little air dissolved in ice during wet growth • Ice appears clear • Dry growth • Hailstone temperature close to environmental temperature so at cold temperatures, large amount of air dissolved • Ice appears opaque

26. Lake effect snow

27. Lake effect snow • Heating • Water warmer than land in fall and early winter • Unstable environment

28. Lake effect snow • Air rises, quickly reaching saturation due to addition of moisture from lake (evaporation)‏

29. Lake effect snow

30. Lake effect snow

31. Lake effect snow • Wind fetch • Length of trajectory of wind across lake • Greater the distance the wind blows over warm water, the greater the convection • Frictional difference • When wind moves from over water to land, friction slows it down, resulting in surface convergence and lifting • Large-scale forcing • Enhancement of lake-effect snow

32. Case study (Dec 1998)‏

33. Case study (Dec 1998)‏

34. Case study (Dec 1998)‏

35. Case study (Feb 2007)‏

36. Global Distribution of Precipitation • Annual precipitation on earth is equal to the annual evaporation. • The general circulation of the atmosphere gives clues as to where maxima and minima in precipitation can be found. • Precipiation minima are found in regions of widespread subsidence • Precipitation maxima are found in regions of widespread upward vertical motion

37. Rain Shadow • A rain shadow is an arid region on the lee side of a mountain range • Caused by the adiabatic cooling and warming of air parcels as they travel over the topography • Why the western slopes in CO receive more snowfall than the eastern slopes.