Microphysics of Cold Clouds

Microphysics of Cold Clouds Reading Wallace & Hobbs pp 232 – 245

Microphysics of Cold Clouds Objectives Be able to explain why ice crystals grow at the expense of water drops Be able to list the factors that determine the rate that ice crystals grow by deposition Be able to recall the temperature at which ice crystals grow the fastest by deposition in a mixed phase cloud

Microphysics of Cold Clouds Objectives Be able to list the factors that determine ice crystal habit Be able to recall the two basic types of ice crystals Be able to identify ice crystal classification schemes

Microphysics of Cold Clouds Objectives Be able to identify significant ice crystal habits Be able to define riming Be able to describe the positive feedback that riming has on water mass accretion Be able to define graupel

Microphysics of Cold Clouds Objectives Be able to recall the importance of graupel in hail formation Be able to identify the size cut-off between hail and graupel Be able to describe the two modes of hail growth Be able to define aggregation

Microphysics of Cold Clouds Objectives Be able to describe the two factors that determine aggregation Be able to generally comment on the formation of rain from the cold cloud process

Growth of Ice Crystals Growth by Depostion Growth by Riming Growth by Aggregation

Growth by Deposition Ice Crystals Grow by Vapor Diffusion Mixed Phase Cloud

Thermodynamics Review Equilibrium Curve Water Vapor vs. Liquid Water es Equilibrium Pressure Temperature

Thermodynamics Review Supercooled Liquid Water (SLW) Absence of ice Equilibrium with Liquid Water esw SLW Pressure 0.01oC Temperature

Thermodynamics Review What about water vapor vs. ice?

Thermodynamics Review Equilibrium Curve for Ice Temperature Pressure Equilibrium with Liquid Water esw with Ice 0.01oC esi

Thermodynamics Review Mixed Phase Cloud -12oC

Thermodynamics Review Saturated With Respect to Liquid Water Equilibrium with Liquid Water esw Pressure 5 mb -12oC Temperature

Thermodynamics Review Supersaturated With Respect to Ice Equilibrium with Liquid Water esw Pressure 5 mb Equilibrium with Ice 4.7 mb esi -12oC Temperature

Thermodynamics Review Supersaturations of Up to 20% Compare to 1% in Warm Clouds Equilibrium with Liquid Water esw Pressure 5 mb Equilibrium with Ice 4.7 mb esi -12oC Temperature

Growth by Deposition Cloud Droplets Evaporate at the Expense of Ice Crystals

Growth by Deposition Cloud Droplets Evaporate at the Expense of Ice Crystals

Growth by Deposition Cloud Droplets Evaporate at the Expense of Ice Crystals

Growth by Deposition Similar to Growth of Water Drop m = mass of ice t = time r = radius of ice rv,0 = vapor density adjacent to droplet surface rv,oo = vapor density adjacent to droplet surface

Growth by Deposition Flux of Water Vapor is Normal to Surface

Growth by Deposition Ice Crystals Aren’t Always Round

Growth by Deposition Vapor Diffuses to Sharp Points From Many Directions Points Grow More Rapidly

Growth by Deposition Analogy Electric Field Around a Charged Conductor of Irregular Shape Can Be Determined Experimentally in Laboratory

Growth by Deposition Diffusional Capacitance C = Capacitance eo = permittivity of free space = 8.85 x 10-12 C2 N-1 m-2

Growth by Deposition For a Sphere

Growth by Deposition General Form Capacitance (C) Determined Experimentally

Growth by Deposition Simplify Vapor Pressure Away from Crystal Is Not Very Different At Crystal Surface

Growth by Deposition Simplify Ice Crystal Is Not Too Small

Growth by Deposition Simplify

Growth by Deposition Rate of Growth Depends on Shape of Ice Crystal Supersaturation Other Temperature Dependent Factors

Growth by Deposition Maximum in GiSi at -15oC Most Rapid Growth Difference Between esi and es Most Rapid Growth -10 -20 -30 -40 TEMPERATURE (oC) GiSi (kg s-1 m-1) 1 2 3 4

Ice Crystal Habits Variables Temperature Supersaturation Primary Supersaturation Secondary Electric Field Minor

Ice Crystal Habits Basic Habits Plates Hexagonal Plate

Ice Crystal Habits Basic Habits Prisms (or Columns) Column

Ice Crystal Habits Basic Habit Changes Three Times with Decreasing Temperature Prisms Prisms Plates Plates -5 -10 -15 -20 -25 -30 -35 TEMPERATURE (oC)

Ice Crystal Habits Thickness Decreases with Increasing Supersaturation

Ice Crystal Habits Classification Scemes International Commission on Snow and Ice (1951) Nakaya (1954) Magano & Lee (1966)

International Commission on Snow & Ice (1951) Seven Principle Snow Crystal Types Plates Stellar Crystals Caped Columns Columns Needles Spatial Dendrites Irregular Forms Three Additonal Types of Frozen Precipitation Graupel Ice Pellets Hail

International Commission on Snow and Ice (1951) Simple

Nakaya (1954) Seven Major Grouping of Snow Crystals 41 Individual Morphological Types

Magano & Lee (1966) Most Complete Extension of Nakaya 80 Different Morphological Types

Magano & Lee (1966)

Ice Crystal Habits Significant Crystals Plates Prisms

Significant Crystals Plates Dendrite Prettiest Fastest Growing -15oC Stellar Dendrites

Significant Crystals Plates Dendrite Forms Sectored Plate

Significant Crystals Plates Hexagonal Plates High Terminal Velocity Graupel Embryo

Significant Crystals Columns Needles Form in Strong Electric Fields

Significant Crystals Columns Hollow Columns Capped Columns Bullet Combos

Ice Crystal Habits Ice Crystal May Grow Several Different Habits Depends on Supersaturation and Temperature

Growth of Ice Crystals Growth by Depostion Growth by Riming Growth by Aggregation

Growth by Riming Riming “The process by which ice crystals grow through the collision, collection and freezing of supercooled water drops” Fred Remer, 2002

Growth by Riming

Growth by Riming Freezes on Contact

Growth by Riming Freezes on Contact

Growth by Riming Collection efficiency of ice crystal habits vary

Growth by Riming Terminal Velocity of Ice Crystal Increases

Growth by Riming Difficult to Distinguish Original Ice Crystal

Growth by Riming Graupel Heavily rimed ice crystals Often called snow pellets Diameter < 5 mm

Growth by Riming Graupel Shapes Conical Hexagonal Lump or Irregular

Growth by Riming Graupel Serves as Hail Embryo

Growth by Riming Hailstone Diameter > 5 mm Small Hail (Diam. < 6.4 mm) Large Hail (Diam. > 6.4 mm)

Growth by Riming Hailstone Growth Dry Growth Wet Growth

Hailstone Growth Dry Growth Mostly Riming

Hailstone Growth Wet Growth Accretion of Liquid Water and Freezing

Hailstone Growth Both Processes Can Occur at Different Periods of a Hailstone’s Life Time

Growth of Ice Crystals Growth by Depostion Growth by Riming Growth by Aggregation

Glossary of Meteorology Growth by Aggregation Aggregation “The process of clumping together of ice crystals following collision as they fall to form snowflakes” Glossary of Meteorology

Growth by Aggregation

Growth by Aggregation Terminal Fall Speeds Adhesion

Growth by Aggregation Terminal Fall Speeds Columns Plates

Growth by Aggregation Terminal Fall Speeds Columns Increases with Length Needles .5 to .7 ms-1

Growth by Aggregation Terminal Fall Speeds Plates Independent of Diameter Similar Fall Speeds Unlikely to Collide

Growth by Aggregation Riming Greatly Enhances Collisions Various Fall Speeds

Growth by Aggregation Adhesion Type of Ice Crystals Temperature

Growth by Aggregation Adhesion Type of Ice Crystals More Intricate Crystals Become Entwined Upon Collision

Growth by Aggregation Adhesion Temperature Ice Crystals Become Sticky Between –5 to 0oC 0oC

Cold Cloud Process Cloud Droplets Grow at the Expense of Ice Crystals Wegener (1911) Bergeron (1933) Findeisen (1938)

Cold Cloud Process Deposition Can Account for Precipitation Sized Ice Particles 1 mm 30 min.

Cold Cloud Process Deposition Cannot Account for Precipitation Sized Rain Drops 1 mm .3 ms-1 0oC 260 mm

Cold Cloud Process Riming and Aggregation Produces Precipitation Sized Rain Drops 1 mm 1 ms-1 0oC 460 mm

Cold Cloud Process Riming and Aggregation Produces Precipitation Sized Rain Drops 1 cm 1 ms-1 0oC 2 mm