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

Water Droplet Growth - Condensation Evolution of droplet size spectra w/time (w/T ∞ dependence for G understood): T (C)G (cm 2 /s) * G (µm 2 /s) x x x x With s env in % (note this is the value after nucleation, << s max ): T=10C, s=0.05% => for small r 0 : r ~ 18 µm after 1 hour (3600 s) r ~ 62 µm after 12 hours * From Twomey, p Diffusional growth can’t explain production of precipitation sizes! Platnick Review:

Platnick Cold Cloud Processes Warm Cloud Processes

review

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

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

Platnick 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”.

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

Ice Molecules Arranged in Lattice Freezing Liquid water Ice

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).

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 Heterogeneous Freezing - conceptual schematic ice nuclei

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

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

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

main types Ice Crystal Habits Variables –Temperature primary –Supersaturation secondary –Electric Field minor Platnick

Growth onT(C)Growth habits prism0 – -4thin plates basal-4 – -10needles prism-10 – -20plates, dendrites basal-20 – -50hollow columns c axis basal face prism face a axis Platnick

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 – 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.

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

Ice cloud microphysics, cont. Platnick

Ice Crystal Habits -dependency on temperature and supersaturation Platnick

MODIS ice crystal library habits/shapes Platnick

Magano & Lee (1966)

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

Ice Multiplication Process Fracture of Ice Crystals Splintering of Freezing Drops During ice particle riming under very selective conditions: 1.Temperature in the range of –3 ° to – 8 °C. 2. A substantial concentration of large cloud droplets (D >25  m). 3. Large droplets coexisting with small cloud droplets.

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

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PHYS Clouds, spring ‘04, lect. 5, Platnick 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 Contact nuclei – An electric dipole effect. Nucleates at ~5-10 C warmer than same nuclei inside droplet. a axis length (A) c axis length organics clays AgI