Microphysics of Cold Clouds

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

Microphysics of Cold Clouds Objectives Be able to define cold clouds, mixed phase cloud and glaciated cloud Be able to define supercooled liquid water Be able to define homogeneous and heterogeneous nucleation

Microphysics of Cold Clouds Objectives Be able to generally describe the process of homogeneous nucleation Be able to recall the relationship between homogeneous nucleation, temperature and drop size Be able to generally describe why heterogeneous nucleation occurs at warmer temperatures

Microphysics of Cold Clouds Objectives Be able to describe the four different modes of heterogeneous nucleation Be able to recall that the temperature of heterogeneous nucleation depends on the mode of nucleation Be able to define ice nuclei

Microphysics of Cold Clouds Objectives Be able to identify the biggest source of ice nuclei Be able to recall the Fletcher Relationship of ice nuclei size distribution Be able to explain why concentrations of ice crystals in cloud exceed ice nuclei concentrations

Cold Cloud Some Part of Cloud Extends Above 0oC 0oC

Mixed Phase Cloud Contains Both Liquid Water Drops and Ice Particles 0oC

Glaciated Cloud Consists Entirely of Ice Particles 0oC

Supercooled Liquid Water Liquid Water That Exists at Temperatures Colder Than 0oC 0oC

Supercooled Liquid Water Liquid Water Molecule Hydrogen Bonds

Supercooled Liquid Water Metastable State Want to Freeze 0oC

Fusion Ice Water Molecules Arranged in Lattice Fusion Liquid Water Ice

Fusion Homogeneous Nucleation Heterogeneous Nucleation

Homogeneous Nucleation Water Molecules Arrange Themselves into a Lattice 0oC Ice Embryo

Homogeneous Nucleation Embryo Grows by Chance Aggregation 0oC Ice Embryo

Homogeneous Nucleation Ice Nucleus Is in Constant Flux Molecule Come & Go 0oC Ice Embryo

Homogeneous Nucleation Chance Aggregation Increases with Decreasing Temperature 0oC Ice Embryo

Homogeneous Nucleation Chance Aggregation Also Depends on Drop Size (or Surface Tension) 0oC Ice Embryo

Homogeneous Nucleation Ice Embryo Must Reach Critical Size Before Fusion Occurs Spontaneously 0oC Ice Embryo

Homogeneous Nucleation Spontaneous Nucleation Occurs At 5 mm -39oC

Homogeneous Nucleation Spontaneous Nucleation Occurs At 5 mm 40 mm -36oC -39oC

Homogeneous Nucleation Spontaneous Nucleation Rarely Occurs in the Atmosphere 0oC

Fusion Homogeneous Nucleation Heterogeneous Nucleation

Heterogeneous Nucleation Fusion Is Aided by Foreign Substances Called Ice Nuclei 0oC Ice Nuclei

Heterogeneous Nucleation Ice Nuclei Provide a Surface for Liquid Water to Attach 0oC Ice Nuclei

Heterogeneous Nucleation Ice Embryo Starts At a Larger Size 0oC Ice Nuclei

Heterogeneous Nucleation Fusion Occurs at Much Warmer Temperature 0oC Ice Nuclei -39oC

Heterogeneous Nucleation Modes Condensation Followed by Freezing Contact Immersion Deposition

Heterogeneous Nucleation Modes Condensation Followed by Freezing Ice nuclei acts as CCN, then freezes droplet

Heterogeneous Nucleation Modes Contact Water droplet freezes instantaneously upon contact with ice nuclei

Heterogeneous Nucleation Modes Immersion Causes freezing after becoming embedded within droplet

Heterogeneous Nucleation Modes Deposition Ice forms directly from vapor

Heterogeneous Nucleation Modes One particle may nucleate in different ways

Heterogeneous Nucleation Modes Temperature at which nucleation occurs also depends on mode 0oC

Glossary of Meteorology Ice Nuclei “Any particle that serves as a nucleus leading to the formation of ice crystals without regard to the particular physical processes involved in the nucleation.” Glossary of Meteorology

Ice Nuclei Crystal Structure Similar to Ice Nucleate at warmer temperatures

Ice Nuclei Pure Substances Minerals Organic Material

Ice Nuclei

Ice Nuclei Terrestrial Source Biggest Contributor Ice Nuclei Concentrations Decrease with Altitude More Ice Nuclei in Continental Air

Ice Nuclei Measurement Techniques Cloud Chambers Filter Systems

Ice Nuclei Measurement Problems Do not know mode of activation Contact, Immersion or Condensation Followed by Freezing? History of Aerosol ‘Trained’ or Preactivated Ice Nuclei May Not Activate Until Later

Ice Nuclei High Degree of Variablity Order of Magnitude

Ice Nuclei Observed Concentrations (Active) 100 10 1 .1 .01 .001 -10 -20 -15 -25 Temperature (oC) Concentration (#/L)

Ice Nuclei Fletcher Relationship 1/L @ -20oC Decreases by order of magnitude every +4oC 100 10 Concentration (#/L) 1 .1 .01 .001 -10 -15 -20 -25 Temperature (oC)

Ice Nuclei Fletcher Relationship a = .6 N = concentration per liter 100 10 1 Concentration (#/L) .1 .01 .001 -10 -15 -20 -25 a = .6 N = concentration per liter DT = change in temperature Temperature (oC)

Ice Nuclei Typical aerosol concentration about 104 cm-3 One in 10 billion (108) aerosols acts as an ice nuclei (at -20oC) 100 10 1 Concentration (#/L) .1 .01 .001 -10 -15 -20 -25 Temperature (oC)

Ice in Cloud Observed Concentrations Much Higher Than Ice Nuclei Concentrations 100 10 1 .1 .01 .001 -10 -20 -15 -25 Temperature (oC) Concentration (#/L) -5

Ice in Cloud Depends Upon Temperature Age of Cloud Type of Cloud

Ice in Cloud Observed Concentrations Increases with Decreasing Temperature 100 10 1 .1 .01 .001 -10 -20 -15 -25 Temperature (oC) Concentration (#/L) -5

Ice in Cloud Probability of ice crystals 100 10 1 .1 .01 .001 -10 -20 -15 -25 Temperature (oC) Concentration (#/L) -5

Ice in Cloud Probability of ice crystals increases with age of cloud Old clouds Glaciated Higher concentrations Young Clouds Water droplets

Ice in Cloud Higher probability of ice crystals in stratus than in cumulus of same cloud top temperature

Ice in Cloud Higher ice crystal concentrations in clouds with broad spectra

Ice in Cloud Discrepancy Between Ice Nuclei Concentration Observed Ice Crystal Concentration 100 10 1 .1 .01 .001 -10 -20 -15 -25 Temperature (oC) Concentration (#/L) -5

Ice in Cloud Primary Method Heterogeneous Nucleation

Ice in Cloud Seconday Method Ice Multiplication Process Fracture of Ice Crystals Splintering of Freezing Drops

Ice Multiplication Process Fracture of Ice Crystals

Ice Multiplication Process Splintering of Freezing Drops