MET215: Advanced Physical Meteorology Ice Clouds: Nucleation and Growth

1. MET215: Advanced Physical Meteorology Ice Clouds: Nucleation and Growth Menglin S. Jin Sources: Steve Platnick

3. Diffusional growth can’t explain production of precipitation sizes! Review: Water Droplet Growth - Condensation Evolution of droplet size spectra w/time (w/T∞ dependence for G understood): With senv in % (note this is the value after nucleation, << smax): *From Twomey, p. 103. T=10C, s=0.05% => for small r0: r ~ 18 µm after 1 hour (3600 s) r ~ 62 µm after 12 hours Platnick

5. Cold Cloud Processes Warm Cloud Processes Platnick

6. review

11. Ice Clouds: Nucleation and Growth • Nucleation • – Homogeneous, heterogeneous, ice nuclei • – Habits (shapes) Sources: Steve Platnick

12. Ice Clouds: Nucleation and Growth • Nucleation • – Homogeneous, heterogeneous, ice nuclei • – Habits (shapes) • Ice crystal growth • – Growth from vapor (diffusion) • – Bergeron process (growth at expense of water droplets) • – Ice multiplication process • – Collision/coalescence (riming, aggregation) • Size distributions – Microphysical measurements, temperature dependencies Sources: Steve Platnick

13. Ice Clouds – Nucleation • Some nucleation pathways • Homogeneous freezing of solution droplets (w/out assistance of aerosol particles) • requires very cold temperatures (~ -40 C and below) • Heterogeneous freezing (via aerosol particles that may or may not contain/be imbedded in water). Ice nuclei not well understood. • Contact freezing (ice nuclei contact with solution droplet) • Deposition on ice nuclei • reference: P. Demott, p. 102, “Cirrus”, Oxford Univ Press, 2002; Rogers and Yau, “A short course in cloud physics”. Platnick

14. Homogeneous Freezing - conceptual schematic • Water molecules arrange themselves into a lattice. • Embryo grows by chance aggregation . • Ice nucleus cluster number/concentrations are in constant flux • in equilibrium, molecular clusters in Boltzmann distribution • Chance aggregation number/concentrations increases with decreasing temperature. ice embryo

15. Ice Molecules Arranged in Lattice Liquid water Freezing Ice

17. Ice Clouds – Heterogeneous Nucleation • Overview • Vapor deposition directly to aerosol particle (insoluble or perhaps dry soluble particles). • Contact freezing: particle collides with water droplet • Condensation freezing: from mixed aerosol particle (soluble component of particle initiates condensation, insoluble component causes freezing instantly) • Immersion freezing: same as above but insoluble particle causes freezing at a later time, e.g., at a colder temperature (but at temperatures greater than for homogeneous freezing) • Theoretical basis less certain than for homogeneous freezing. • Ice nuclei • minerals (clay), organic material (bacteria), soot, pure substances (AgI) • Deposition requires high supersaturation w.r.t. ice (e.g., 20% for AgI at -60 C, Detwiler & Vonnegut, 1981).

18. Heterogeneous Freezing - conceptual schematic • Freezing is aided by foreign substances, ice nuclei • Ice nuclei provide a surface for liquid water to form ice structure • Ice embryo starts at a larger size • Freezing occurs at warmer temperatures than for homogeneous freezing ice nuclei

19. Heterogeneous Freezing - conceptual schematic • Contact • Water droplet freezes instantaneously upon contact with ice nuclei • Condensation followed by instantaneous freezing • Nuclei acts as CCN, then insoluble component freezes droplet

20. Heterogeneous Freezing - conceptual schematic • Immersion • Ice nuclei causes freezing sometime after becoming embedded within droplet • Deposition • Ice forms directly from vapor phase

26. Ice Clouds – Ice Nuclei (IN) • Measured ice particle number concentration: < 1/liter to ~10/liter • Large discrepancies between measured IN and ice number concentration • IN vary with temperature, humidity, supersaturation. • Secondary production (limited understanding): • shattering of existing crystals • splintering of freezing drops • Other • In situ measurement problems

27. main types Ice Crystal Habits • Variables • Temperature • primary • Supersaturation • secondary • Electric Field • minor Platnick

28. c axis a axis basal face prism face Platnick

29. Ice Nuclei (IN), cont. • Internal nuclei • Water ice lattice held together by hydrogen bonds. Aerosol with hydrogen bonds at surface with similar bond strengths, as well as rotational symmetry which exposes H-bonding groups allowing interaction with water molecules, will be good IN. Example: organics. • Geometrical arrangement of aerosol surface molecules also important. Surface matching ice lattice structure will serve as good IN (e.g., AgI). Best IN will have similar bond length. Bond length differences give rise to stresses which creates an energy barrier to nucleation. Therefore expect easier nucleation at colder temperatures. See Pruppacher & Klett, Fig. 9-12. • Lab experiments indicate that ice nucleation is a local phenomenon proceeding at different active sites on the surface. • Contact nuclei • An electric dipole effect? Nucleates at ~5-10 C warmer than same nuclei inside droplet. Platnick

30. 25 July 2002 (VIPS) 25 July 2002 (VIPS) CPI: 7 July 2002 Ice Cloud MicrophysicsCRYSTAL-FACE, A. Heymsfield Platnick

31. Ice cloud microphysics, cont. Platnick

32. Ice Crystal Habits -dependency on temperature and supersaturation Platnick

33. MODIS ice crystal library habits/shapes Platnick

34. Magano & Lee (1966)

35. Ice Particle Growth - Condensation Diffusion growth (C is “capacitance” of particle in units of length, current flow to a conductor analogy for molecular diffusion ): C is a useful analogy, but difficult to analytically quantify except for simple shapes). Note that C = r for spheres, 2p/r for hexagons, … With ice supersaturation defined as: Solution: same as water droplet with C vs. r, Ls vs. L, es,i vs. es,w Platnick

44. Ice Multiplication Process • Fracture of Ice Crystals • Splintering of Freezing Drops • During ice particle riming under very selective conditions: • Temperature in the range of –3 ° to –8 °C. • A substantial concentration of large cloud droplets (D >25 m). • Large droplets coexisting with small cloud droplets.

45. Ice Precipitation Particles • At surface: • Hail: alternating layers of clear ice (wet growth) & opaque ice • Graupel: “soft” hail < 1 cm diameter, white opaque pellets, consists of central crystal covered in rimed drops • Sleet: transparent ice, size of rain drops • Snow: coagulation of dendritic crystals Platnick

46. extras

47. Ice Nuclei (IN), cont. • Internal nuclei • Nuclei have similar bond length. Bond length differences give rise to stresses which creates an energy barrier to nucleation. Therefore expect easier nucleation at colder temperatures. See Pruppacher & Klett, Fig. 9-12. • Contact nuclei • An electric dipole effect. Nucleates at ~5-10 C warmer than same nuclei inside droplet. c axis length 15 organics 10 . clays 5 AgI a axis length (A) 5 20 PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick