Overview of Clouds 1. Nucleation of Water Vapor 2. Warm Clouds 3. Water Content and Entrainment

1. PHYS-575/CSI-655Introduction to Atmospheric Physics and ChemistryLecture Notes #6Cloud Microphysics – Part 1 Overview of Clouds 1. Nucleation of Water Vapor 2. Warm Clouds 3. Water Content and Entrainment 4. Droplet Growth (Warm Clouds) 5. Microphysics of Cold Clouds 6. Artificial Modification of Clouds 7. Thunderstorm Electrification 8. Cloud and Precipitation Chemistry

2. Announcements: April 2, 2007 • Office Hours: Monday-3:00-5:00 pm, by appointment (other times possible) • Homework #4 • 5.12, 5.14, 5.18 (make table, show work), 5.19, 5.24 • Due: April 2 • Homework #5 (Assigned early due to second exam) Part 1: 6.8 – a, b, d, f, I, k, n, r, s, t, x, bb Part 2: Clouds Homework: Clouds in a Glass of Beer (Bohren article) and Physics Today article. Due: April 16 • Exam #2: April 16 Instructor Travel • April 3-6 • April 19-20 • April 24-26

3. Overview of Clouds When the temperature in the Earth’s atmosphere drops below the condensation temperature, water vapor condenses or freezes out; the numerous water droplets and/or ice crystals make up clouds. • Influences of Clouds: • Reflect and absorb solar radiation • Reflect and absorb terrestrial radiation • Latent heat release  atmospheric heating

4. Cloud Types Cloud types are usually classified grouped into "low", "middle", and "high" clouds, referring to the altitudes they occur at. "Low" clouds are generally below about 6,500 ft. "Middle" clouds range from about 6,500 ft to 20,000 ft, and high clouds range between 20,000 and 40,000+ feet in altitude. As seen in the photos above, low clouds include cumulus, stratus, and stratocumulus; middle clouds include altocumulus and altostratus; and high clouds include cirrus, cirrocumulus, and cirrostratus. If low stratus clouds are raining, they are usually called nimbostratus. Cumulonimbus clouds (thunderstorms) often span all three cloud heights, with bases from 1,000 to 5,000 feet and tops sometimes reaching 60,000 feet. http://www.weatherquestions.com/cloud_types.jpg

5. Earth’s Highest Clouds: Noctilucent Clouds From “Observing Noctilucent Clouds” by M. Gadsden and P. Parviainen IAGA, 1995

6. Noctilucent Clouds

7. AIM: Aeronomy of Ice in the MesosphereAre Noctilucent Clouds the “miner’s canary” of Global Change? Launch Date: 3/29/07 http://aim.hamptonu.edu/

8. Clouds on Other Planets Methane (CH4) & Hydrocarbons Sulfuric Acid (H2SO4) Water, Carbon Dioxide CH4 + various Hydrocarbons, Carbon vapor CH4, Ammonia (NH3), Ammonium Sulfate (NH4SH), He

9. Formation of Water Vapor Clouds on Earth http://www.tonya.me.uk/Marine/graphics/clouds/clouds1.gif

10. 1. Nucleation and Water Vapor Condensation

11. Vertical Motion and Condensation Upward motion leads to cooling, via the FLT. Cooling increases the relative humidity. When the relative humidity exceeds 100%, then condensation will occur.

12. Homogeneous Nucleation Theory Start with a moist parcel of clean air and let it cool (say, by moving it vertically). Drops do not form immediately upon super-saturation, but drop embryos are formed by chance collisions of water molecules. Sticking tends to stabilize the embryos, but thermal motion tends to disrupt them. To form an embryo that is stable, the drop must be a critical size which is more energetically stable than the same amount of water in the vapor phase, otherwise the embryo will re-evaporate and disappear due to thermal agitation. Gibbs Free Energy (G) is a measure of both energy and entropy. Minimizing G will simultaneously minimize energy and maximize entropy – just what is required for a stable system. Enthalpy (U + PV) is a thermodynamic variable. G = H – TS = U + PV – TS  dG = dU – TdS – SdT + PdV + VdP FLT: TdS = dU + PdV dG = - SdT + VdP In equilibrium dG = 0 The key is to write dG in terms of the properties of the drop embryo.

13. Homogeneous Nucleation - continued dG = - SdT + VdP Consider a constant temperature system (liquid + vapor) where the partial pressure of the liquid varies from e to e + de. IGL  eVv = RvT Vapor: dGv = Vvde Liquid: dGl = Vlde V is the specific volume; Vv >> Vl, and the IDL for the vapor Vv = RvT/e, so d(Gv - Gl) = (Vv – Vl) de = Vvde = RvT (de/e) = RvT d(ln e) Integration (fixed T) gives: Gv(T,e) – Gl(T,e) = RvT (ln e) + constant At equilibrium, e = es(T), so the constant = - RvT (ln es) Gv(T,e) – Gl(T,e) = RvT ln (e/es)

14. Saturation Vapor Pressure:Clausius-Clapeyron Equation of State es(T) = CL e-Ls/RT es(T) = Saturation vapor pressure at temperature T CL = constant (depends upon condensable) Ls = Latent Heat R = Gas constant

15. Homogeneous Nucleation - Continued Initially, before a drop forms, the total G0 = Gv(T,e)M0 where M0 = total mass At some time later we have a droplet of radius R and mass Ml = 4/3 πR3ρl The total G of the system, including energy in the form of surface tension σ (surface energy per unit area), is: G = Gv(T,e)Mv + Gl(T,e)Ml + σ 4πR2 By conservation of mass, Mv = M0 – Ml, so the change in Gibbs Free Energy G – G0 = (Gl – Gv) Ml + σ 4πa2 Finally we get G – G0 = - 4/3 πR3 ρl RvT ln [e/es(T)] + σ 4πR2 Or as in the text:ΔE = ΔG = σ 4πR2 – 4/3 πR3 nkT ln(e/es)

16. Homogeneous Nucleation - Continued Thus the change in Gibbs Free Energy in the formation of a droplet of radius R, is given by: ΔE = σ 4πR2 -4/3 πR3 nkT ln(e/es) For unsaturated conditions, droplets aren’t stable and thus evaporate. For saturated and supersaturated conditions, droplets above a critical radius r, are stable and subsequently grow.

17. Homogeneous Nucleation – Critical Size for Growth ΔE = σ 4πR2 -4/3 πR3 nkT ln(e/es) To find the critical size for which growth is more stable than evaporation: Set d(ΔE)/dR = 0 and solve for the value of R* The Critical Radius: Kelvin’s Equation For saturated and supersaturated conditions, droplets above the critical radius R*, are stable and subsequently grow. Given a radius r, we can calculate the vapor pressure of water vapor for which growth is possible.

18. Size of Stable Drop Embryos Typical super-saturation in the atmosphere is only about 101% relative humidity. Thus droplets must be about 0.1 microns in size to be stable. This requires about 106 water molecules. However, the critical radius can be arbitrarily small for a pre-existing, hydrophilic, atmospheric particle. Thus heterogeneous nucleation is the dominant source of water droplets in the atmosphere.

19. Supersaturation Near Droplets

20. Cloud Condensation Nuclei

21. Cloud Condensation Nuclei (CCN) CCN are pre-existing atmospheric particles that come from a large variety of sources: Dust Volcanoes Factory smoke Fires and soot Sea Salt Di-methyl Sulfate (Phytoplankton) Abundance ranges from 103-105 per cubic centimeter, larger over continents and urban areas. Two Types: Hydrophilic/Hydroscopic: water sticks readily Hydrophobic: repels water http://apollo.lsc.vsc.edu/classes/met130/notes/chapter6/ccn.html

22. Aerosol Particle Sizes – Bi-modal Distributions http://www.defra.gov.uk/environment/airquality/aqs/air_measure/images/02.gif

23. 2. Microstructures of Warm Clouds

25. Cloud Optical Thickness

26. Ship Tracks

27. 3. Cloud Liquid Water Content and Entrainment

28. Cumulus Cloud Entrainment

29. Model Simulation of Entrainment

30. Questions for Discussion • Is precipitation possible without CCN? • Why does the relative humidity rarely exceed 101%? • If precipitation did not occur, how would vapor be lost from the atmosphere? • Is growth of an ice particle the same as growth of a liquid water droplet?