met215: advanced physical meteorology ice clouds: nucleation and growth

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MET215: Advanced Physical Meteorology Ice Clouds: Nucleation and Growth. Menglin S. Jin. Sources: Steve Platnick. Diffusional growth can’t explain production of precipitation sizes!. Review:. Water Droplet Growth - Condensation. - PowerPoint PPT Presentation

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  • MET215: Advanced Physical MeteorologyIce Clouds: Nucleation and GrowthSources: Steve PlatnickMenglin S. Jin

  • Water Droplet Growth - CondensationEvolution of droplet size spectra w/time (w/T dependence for G understood):With senv in % (note this is the value after nucleation, for small r0:r ~ 18 m after 1 hour (3600 s)r ~ 62 m after 12 hours* From Twomey, p. 103.PlatnickReview:

    T (C)G (cm2/s)* G (m2/s)-103.5 x 10-90.3506.0 x 10-90.60109.0 x 10-90.902012.3 x 10-912.3

  • PlatnickCold Cloud ProcessesWarm Cloud Processes

  • review

  • Ice Clouds: Nucleation and GrowthNucleation

    Homogeneous, heterogeneous, ice nucleiHabits (shapes)

    Sources: Steve Platnick

  • Ice Clouds: Nucleation and GrowthNucleation

    Homogeneous, heterogeneous, ice nucleiHabits (shapes)

    Ice crystal growth

    Growth from vapor (diffusion)Bergeron process (growth at expense of water droplets)Ice multiplication processCollision/coalescence (riming, aggregation)

    Size distributions

    Microphysical measurements, temperature dependenciesSources: Steve Platnick

  • PlatnickIce Clouds NucleationSome 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 fluxin equilibrium, molecular clusters in Boltzmann distribution Chance aggregation number/concentrations increases with decreasing temperature.ice embryo

  • Ice Molecules Arranged in LatticeFreezingLiquidwaterIce

  • Ice Clouds Heterogeneous NucleationOverview

    Vapor deposition directly to aerosol particle (insoluble or perhaps dry soluble particles). Contact freezing: particle collides with water dropletCondensation 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 nucleiminerals (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 nucleiIce nuclei provide a surface for liquid water to form ice structureIce embryo starts at a larger sizeFreezing occurs at warmer temperatures than for homogeneous freezing

    Heterogeneous Freezing - conceptual schematicice nuclei

  • ContactWater droplet freezes instantaneously upon contact with ice nuclei

    Condensation followed by instantaneous freezingNuclei acts as CCN, then insoluble component freezes dropletHeterogeneous Freezing - conceptual schematic

  • ImmersionIce nuclei causes freezing sometime after becoming embedded within droplet

    DepositionIce forms directly from vapor phaseHeterogeneous 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 crystalssplintering of freezing drops

    Other

    In situ measurement problems

  • Ice Crystal HabitsVariablesTemperatureprimarySupersaturationsecondaryElectric FieldminorPlatnick

  • Platnick

    Growth on T(C)Growth habitsprism0 -4thin platesbasal-4 -10needlesprism-10 -20plates, dendritesbasal-20 -50hollow columns

  • PlatnickIce 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.

  • Ice Cloud Microphysics CRYSTAL-FACE, A. HeymsfieldPlatnick

  • Ice cloud microphysics, cont.Platnick

  • Ice Crystal Habits-dependency on temperature and supersaturationPlatnick

  • MODIS ice crystal library habits/shapesPlatnick

  • Magano & Lee (1966)

  • 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:Diffusion growth (C is capacitance of particle in units of length, current flow to a conductor analogy for molecular diffusion ):Ice Particle Growth - Condensation PlatnickSolution: same as water droplet with C vs. r, Ls vs. L, es,i vs. es,w

  • Ice Multiplication ProcessFracture of Ice CrystalsSplintering of Freezing DropsDuring 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.

  • Ice Precipitation ParticlesAt 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 crystalsPlatnick

  • extras

  • PHYS 622 - Clouds, spring 04, lect. 5, PlatnickIce 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.a axis length (A)c axislengthorganicsclaysAgI.52010515

    **From Bill Olsen & sep.; Gs from Twomey text. Note: for comparison with R&Y, p. 104. With s in absolute, at 10C, G=90 m2/sec (reasonably close to Fig. 7.1).*Wang and Olsen.

    *Wang and Olsen slide.****Wang and Olsen.*Wang and Olsen.

    *From Wang and Olsen.*H. Houghton: Mason & van end Heuvel (1959), si~12% at -12 C for AgI.*Wang and Olsen.

    *Wang and Olsen.

    *Wang and Olsen.

    Wang and Olsen.

    **Wang and Olsen slide.*Wang and Olsen slide.*P&K + Krider notes.*For ice crystals, you have another unknown - particle habit.

    C-F Citation:VIPS - video particle imager (down to 5-10 m), well-defined volume, capable of determining size distributionsCPI - down to ~ 20 mAndy says in strong updrafts, high concentration of small spherical-like ice particles from homogeneous nucleation

    => Some assumption regarding habit must be made to enable the retrieval method previously described.*sep slide.*Wang and Olsen slide. Heymsfield et al. ???*Aggregates include rough surfaces a al Cox and Munk, otherwise smoothD is maximum dimension (e.g, size dist. x-axis)Heymsfield replicator images?*Wang and Olsen slide.*Rogers and Yau.

    *Wang and Olsen slide.*Krider notes.**Krider notes.

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