AT620 Review for Midterm #1 Part 2: Chapters 5-7 Brenda Dolan October 19, 2005 Part 2: Chapters 5-7 Brenda Dolan October 19, 2005

AT620 Review for Midterm #1

AT620 Review for Midterm #1

Part 2: Chapters 5-7Brenda DolanOctober 19, 2005

Part 2: Chapters 5-7Brenda DolanOctober 19, 2005

Chapter 5: Atmospheric Aerosols

Chapter 5: Atmospheric Aerosols

Atmospheric AerosolsAtmospheric Aerosols Aerosol: Small liquid or solid particles

suspended in a medium (the atmosphere). They are very small particles that do not have appreciable fall speeds.

Aerosol: Small liquid or solid particles suspended in a medium (the atmosphere). They are very small particles that do not have appreciable fall speeds.

Cloud Condensation Nuclei (CCN): Aerosols that are activated to serve as cloud nuclei at realistic (low) supersaturations (S that would be found in atmosphere

Cloud Condensation Nuclei (CCN): Aerosols that are activated to serve as cloud nuclei at realistic (low) supersaturations (S that would be found in atmosphere

Condensation Nuclei (CN): All aerosols in atmosphere, including those that are activated at high supersaturations an may not serve as cloud nuclei under normal atmospheric conditions

Condensation Nuclei (CN): All aerosols in atmosphere, including those that are activated at high supersaturations an may not serve as cloud nuclei under normal atmospheric conditions

Atmospheric AerosolsAtmospheric AerosolsClassification Size Sources Sinks Aitken particles r<0.1 µm -Combustion

-Gas to Particle conversion -Coagulation -Capture by cloud droplets

Large Particles 0.1≤r≤1.0 µm -Combustion -Coagulation of Aitken nuclei -Sea salt -Cloud droplet evaporation -plant particulates (pollen, spores, etc)

No efficient sinks -some dry deposition -some precip. scavenging

Giant Particles r> 1.0 µm -Wind blown dust -Sea salt -Industrial emissions (paper mills) -some Aitken nuclei coagulation

-Precipitation scavenging -Dry deposition

Aerosol Production Processes:


1) Gas-to-particle conversion (mostly Aitken production)

-vapors from plant exhaltations and combustion products

-chemical reactions catalyzed by UV radiation

-chemical reactions in small water droplets

(clouds process air, and thus there can be more concentrations of particles that have been processed by a cloud)

1) Gas-to-particle conversion (mostly Aitken production)

-vapors from plant exhaltations and combustion products

-chemical reactions catalyzed by UV radiation

-chemical reactions in small water droplets

(clouds process air, and thus there can be more concentrations of particles that have been processed by a cloud)



2) Mechanical disintegration of the solid and liquid earth surface (mostly large and giant production)

Solid earth

-organic particulates by plants (pollen, seeds, waxes, spores)

-mechanical and chemical disintegration of vegetation free rocks and soils

-volcanic emissions

-particles injected into the atmosphere by industrial processes

(paper mills, steel mills)


Solid earth

-organic particulates by plants (pollen, seeds, waxes, spores)

-mechanical and chemical disintegration of vegetation free rocks and soils

-volcanic emissions

-particles injected into the atmosphere by industrial processes

(paper mills, steel mills)




Ocean

-production of spray droplets at the crest of breaking waves (minor)

-bursting air bubbles that are present at ocean’s surface (few in number but fairly large in size)

-Primarily water-soluble sulfates

3) Extraterrestrial sources (minor source of giant and large)


Ocean

-production of spray droplets at the crest of breaking waves (minor)

-bursting air bubbles that are present at ocean’s surface (few in number but fairly large in size)

-Primarily water-soluble sulfates

3) Extraterrestrial sources (minor source of giant and large)



In terms of mass weighting, natural particles are the greatest sources of aerosols, while anthropogenic particles are minor sources. In terms of numbers, anthropogenic can be quite extensive.

In terms of mass weighting, natural particles are the greatest sources of aerosols, while anthropogenic particles are minor sources. In terms of numbers, anthropogenic can be quite extensive.

Aerosol concentrations Aerosol concentrations

Land 103 to 105 cm-3

Polluted cities 107 cm-3

Rural areas 103 cm-3

Remote regions 10-6 cm-3

Marine 100-600 cm-3

Aerosol Distributions:Aerosol Distributions: In general, aerosol concentrations drop off with height In general, aerosol concentrations drop off with height

Junge layer

-Abrupt increase in aerosol concentrations in the lower stratosphere

-Changes in time and season, but is observed world-wide

-Possibly a result of volcanic eruptions

Junge layer

-Abrupt increase in aerosol concentrations in the lower stratosphere

-Changes in time and season, but is observed world-wide

-Possibly a result of volcanic eruptions In the ocean, aerosol particles are not dominated by

sea-salt particles, but rather oxidation of DMS

Aerosol concentrations can be variable over the oceans, but are significantly less than continental concentrations

In the ocean, aerosol particles are not dominated by sea-salt particles, but rather oxidation of DMS

Aerosol concentrations can be variable over the oceans, but are significantly less than continental concentrations

Measuring aerosols:Measuring aerosols: 1) Electrical aerosol analyzer (EAA)—measure mass and

size of aerosols based on their measured mobility in applied electric field

1) Electrical aerosol analyzer (EAA)—measure mass and size of aerosols based on their measured mobility in applied electric field

2) Optical counters and nephelometers—concentration and size distribution of aerosols is determined by the amount (intensity) of scattered light.

2) Optical counters and nephelometers—concentration and size distribution of aerosols is determined by the amount (intensity) of scattered light.

3) Direct impaction instruments—coated slides are swept through volumes of aerosols. Used for large particles (>0.1 µm)

3) Direct impaction instruments—coated slides are swept through volumes of aerosols. Used for large particles (>0.1 µm)

4) X-ray techniques—evaluate composition of aerosols depending upon the radiation given off

4) X-ray techniques—evaluate composition of aerosols depending upon the radiation given off

5) Aitken nucleus counter —expansion chamber used to create high supersaturations, then they are counted optically.

5) Aitken nucleus counter —expansion chamber used to create high supersaturations, then they are counted optically.

Aerosol removal processes:


Aitken particles

1) Coagulation: brownian motion causes particles to collide and self-collect

2) Capture by cloud droplets: either by condensation of vapor on surface, or direct impact on aerosol by a cloud droplet

Aitken particles

1) Coagulation: brownian motion causes particles to collide and self-collect

2) Capture by cloud droplets: either by condensation of vapor on surface, or direct impact on aerosol by a cloud droplet

Giant particles

1) Sedimentation: Dry deposition due to relatively large fall velocities

2) Precipitation scavenging: collection efficiencies are large

Giant particles

1) Sedimentation: Dry deposition due to relatively large fall velocities

2) Precipitation scavenging: collection efficiencies are large



Large particles: The Greenfeld gap

Large aerosols have the longest life because there is no efficient sink for them. Their fall velocities are not large enough in most cases for dry deposition, and they are in the size range where coaguation is not efficient.

1) Some dry deposition

2) Some precipitation scavenging

Large particles: The Greenfeld gap

Large aerosols have the longest life because there is no efficient sink for them. Their fall velocities are not large enough in most cases for dry deposition, and they are in the size range where coaguation is not efficient.

1) Some dry deposition

2) Some precipitation scavenging



Coagulation

Brownian motion: irregular movement of aerosol particles due to thermal bombardment by air molecules

Smoluchowski’s equation for Coagulation

Coagulation

Brownian motion: irregular movement of aerosol particles due to thermal bombardment by air molecules

Smoluchowski’s equation for Coagulation

δn(x)

δt= K(xc , x ')n(xc )n(x ')dx '− n(x)

0

x /2

∫ K(x, x ')n(x ')dx '0

∞

∫

gain loss

Collection kernel Collection kernel

K(x, x ') =4π(Dx + Dx')(rx + rx' )

Smoluchowski’s equation for Coagulation Describes the change in size spectrum of aerosols

since particles that are moving irregularly have a finite probability of colliding and coagulating with one another and particles with relatively large mobilities collide and coagulate more readily, we need to define some efficiency that is related to mass. This is the diffusivity, D.

Diffusivity is inversely proportional to r and proportional to T

Smoluchowski’s equation for Coagulation Describes the change in size spectrum of aerosols

since particles that are moving irregularly have a finite probability of colliding and coagulating with one another and particles with relatively large mobilities collide and coagulate more readily, we need to define some efficiency that is related to mass. This is the diffusivity, D.

Diffusivity is inversely proportional to r and proportional to T



Dx =kT

6πηrx

Wet Removal Mechanisms

Phoretic effects

Condensation and evaporation of vapor molecules can effect the collection of aerosols, because aerosol particles being bombarded by vapor molecules experience a force directed toward the droplet surface, which enhances the coagulation between cloud droplets and aerosol particles.

Diffusiophoresis: Enhanced diffusion of aerosols to drop, enhancing the collection kernel

Thermophoresis: Diffusion of heat away from growing droplet, which inhibits collection of aerosols


Phoretic effects

Condensation and evaporation of vapor molecules can effect the collection of aerosols, because aerosol particles being bombarded by vapor molecules experience a force directed toward the droplet surface, which enhances the coagulation between cloud droplets and aerosol particles.

Diffusiophoresis: Enhanced diffusion of aerosols to drop, enhancing the collection kernel

Thermophoresis: Diffusion of heat away from growing droplet, which inhibits collection of aerosols




Phoretic effects

Phoretic effects are most important for aerosol particles between 0.1 µm<r<1.0µm, and in this range:

Thermophoresis >> diffusiophoresis

This results in a reduced rate of aerosol particle scavenging by a cloud droplet growing by vapor deposition

This results in an enhanced rate of aerosol particle scavenging by a cloud droplet that is evaporating


Phoretic effects

Phoretic effects are most important for aerosol particles between 0.1 µm<r<1.0µm, and in this range:

Thermophoresis >> diffusiophoresis

This results in a reduced rate of aerosol particle scavenging by a cloud droplet growing by vapor deposition

This results in an enhanced rate of aerosol particle scavenging by a cloud droplet that is evaporating





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Aerosol removal processes: Hydrodynamic capture

A large drop settling through smaller drops will sweep out a volume and collect aerosols with some efficiency, E

Hydrodynamic capture A large drop settling through smaller drops will sweep

out a volume and collect aerosols with some efficiency, E

Ea =πYa

2

π R2

Depends on size of drops and size of aerosols

Most efficient for large and giant aerosols duet to large Vt and cross-sectional area

Depends on size of drops and size of aerosols

Most efficient for large and giant aerosols duet to large Vt and cross-sectional area

Cloud Condensation Nuclei:


~1% of aerosol mass serve as CCN in continental air, while 10-20% serve as CCN in maritime Chemical composition determines the best CCN

hygroscopic

wettable

solubility

~1% of aerosol mass serve as CCN in continental air, while 10-20% serve as CCN in maritime Chemical composition determines the best CCN

hygroscopic

wettable

solubility



CCN measurement techniques

Thermogradient diffusion chamber: Two wetted plates are held at different temperatures,

molecular diffusion not convection leads to:

linear variation of T between plates

linear variation of vapor pressure (e) between plates

Saturation vapor pressure varies exponentially with T

Saturation can be changed by changing the temperature of the two plates

CCN measurement techniques

Thermogradient diffusion chamber: Two wetted plates are held at different temperatures,

molecular diffusion not convection leads to:

linear variation of T between plates

linear variation of vapor pressure (e) between plates

Saturation vapor pressure varies exponentially with T

Saturation can be changed by changing the temperature of the two plates



[DRAW]



World-wide measurements of CCN

continental air masses are richer in CCN than maritime air masses

Concentrations of CCN increase with supersaturation as expected

Remote ocean air contains the fewest CCN

Typically NCCN~100 cm-3 at 1% supersaturation for maritime airmasses

The relationship between CCN and supersaturation is exponential:

World-wide measurements of CCN

continental air masses are richer in CCN than maritime air masses

Concentrations of CCN increase with supersaturation as expected

Remote ocean air contains the fewest CCN

Typically NCCN~100 cm-3 at 1% supersaturation for maritime airmasses

The relationship between CCN and supersaturation is exponential:

NCCN =cSk



Spatial and temporal variation of NCCN

CCN can vary over several orders of magnitude over short periods of time

proximity to CCN sources

wind direction and wind speed (air mass could switch to maritime or continental)

precipitation (cloud formation depletes CCN, precipitation scavenges CCN)

CCN concentrations diminish with height away from ground; but inversions could trap CCN

Spatial and temporal variation of NCCN

CCN can vary over several orders of magnitude over short periods of time

proximity to CCN sources

wind direction and wind speed (air mass could switch to maritime or continental)

precipitation (cloud formation depletes CCN, precipitation scavenges CCN)

CCN concentrations diminish with height away from ground; but inversions could trap CCN



Properties of CCN

Theory: NaCl and large particles serve as CCN

Observations: not NaCl, but sulfides and sulfur compounds; even particles down to 0.02 µm can serve as CCN

Type of could system can influence type of activated CCN (low S, weak verticall motion, etc.)

In reality, atmospheric nuclei are composed of a mixture of particles

Number of CN and CCN are not well correlated

Properties of CCN

Theory: NaCl and large particles serve as CCN

Observations: not NaCl, but sulfides and sulfur compounds; even particles down to 0.02 µm can serve as CCN

Type of could system can influence type of activated CCN (low S, weak verticall motion, etc.)

In reality, atmospheric nuclei are composed of a mixture of particles

Number of CN and CCN are not well correlated

Chapter 6: Observed Microstructure of Warm

Clouds

Chapter 6: Observed Microstructure of Warm

Clouds

Cloud droplet distributionsCloud droplet distributions

CSk combined with radiative cooling through ascent Distribution is no just due to aerosols type or air mass, but also depends on

velocity and liquid water content

when drop concentrations are smaller, drops can grow larger

when there are lots of CCN, they grow smaller (competing for the water)

higher vertical velocities lead to higher concentrations

Activated spectra – bimodal distribution

non-activated spectra – mono-modal distribution

Continental: mean=11.2 µm, mode=12 µm, more narrow droplet size-spectra

Maritime: larger mean and modal diameters, broader droplet-size spectra, but lower concentrations

CSk combined with radiative cooling through ascent Distribution is no just due to aerosols type or air mass, but also depends on

velocity and liquid water content

when drop concentrations are smaller, drops can grow larger

when there are lots of CCN, they grow smaller (competing for the water)

higher vertical velocities lead to higher concentrations

Activated spectra – bimodal distribution

non-activated spectra – mono-modal distribution

Continental: mean=11.2 µm, mode=12 µm, more narrow droplet size-spectra

Maritime: larger mean and modal diameters, broader droplet-size spectra, but lower concentrations

N =C2 /(k+2) 1.62x10−3W3/2

kB(32,k / 2)

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

Raindrop size spectraRaindrop size spectra Marshall-Palmer distribution

Slope depends on rainfall rate

Assume that N0 can be specified

Generalized gamma distribution

The concentration of raindrops is much smaller than the concentration of cloud droplets

Rain drops are obviously much larger than cloud droplets

This implies that only a few cloud droplets make it into raindrops

Marshall-Palmer distribution

Slope depends on rainfall rate

Assume that N0 can be specified

Generalized gamma distribution

The concentration of raindrops is much smaller than the concentration of cloud droplets

Rain drops are obviously much larger than cloud droplets

This implies that only a few cloud droplets make it into raindrops

N(D) =Noe−λD

N(D) =Nt

cs

DDn

⎛

⎝⎜⎞

⎠⎟

cγ−11Dn

exp −(D / Dn)c⎡⎣ ⎤⎦

[LABEL]

Fog size distributionsFog size distributions

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Chapter 7: Theory of Cloud Droplet GrowthChapter 7: Theory of Cloud Droplet Growth

Growth by vapor deposition


Growth by vapor deposition (Diffusional growth)Assumptions

steady state—no accumulation of vapor density

surface of drop is exactly saturated

Growth by vapor deposition (Diffusional growth)Assumptions

steady state—no accumulation of vapor density

surface of drop is exactly saturated

ada

dt=

Dvρl

ρv,∞ −ρv,a⎡⎣ ⎤⎦



Heat budget for a drop growing by vapor deposition: Internal energy:

Heat budget for a drop growing by vapor deposition: Internal energy:

du =dQ =McpdT

No heat storage (du=0): No heat storage (du=0):

dQ =McpdT =0

Three mechanisms for heating1. Condensation

2. Molecular diffusion

3. Radiative heating (cooling)

Three mechanisms for heating1. Condensation

2. Molecular diffusion

3. Radiative heating (cooling)

dQ

dt=

dMdt

L

dQ

dt=−4πaK (Ta −T∞)

dQ

dt=Qr =πa2Qabs[Fnet −σTa

4 ]



Total heat budget (thermodynamic equation): Total heat budget (thermodynamic equation):

Lda

dt=

KρLa

(Ta −T∞ )−QR

4πa2ρL

Combined equation for growth by vapor deposition:

Combined equation for growth by vapor deposition:

ada

dt=G(T,P) S−1−

Aa

+Ba3 −

Lv

RvT2 4πaK

QR

⎡

⎣⎢

⎤

⎦⎥



Combined equation for growth by vapor deposition: Rate of change of radius decreases as drop gets bigger

(doesn’t favor growth of large droplets)

Growth rate increases if saturation ratio increases

Growth rate increases over a solution

Growth rate decreases due to curvature

Radiation can either increase or decrease the growth rate

net effect of this is that drops can cool enough at the top of the could to grow by vapor deposition. Bigger droplets cool more by OLR

Combined equation for growth by vapor deposition: Rate of change of radius decreases as drop gets bigger

(doesn’t favor growth of large droplets)

Growth rate increases if saturation ratio increases

Growth rate increases over a solution

Growth rate decreases due to curvature

Radiation can either increase or decrease the growth rate

net effect of this is that drops can cool enough at the top of the could to grow by vapor deposition. Bigger droplets cool more by OLR



Combined equation for growth by vapor deposition: Assumptions made in deriving this equation:

Transfer of heat and moisture are by steady-state diffusion

The vapor density at the droplet surface is that under which the droplet persists in equilibrium

There is no disturbance of vapor field by neighboring droplets

There is no disturbance of vapor field by motion of the droplet

There is no additional source of heat to or from the droplet other than radiation

Heat storage on the droplet is negligible

Combined equation for growth by vapor deposition: Assumptions made in deriving this equation:

Transfer of heat and moisture are by steady-state diffusion

The vapor density at the droplet surface is that under which the droplet persists in equilibrium

There is no disturbance of vapor field by neighboring droplets

There is no disturbance of vapor field by motion of the droplet

There is no additional source of heat to or from the droplet other than radiation

Heat storage on the droplet is negligible



Combined equation for growth by vapor deposition: Also, this is an assumption that this continuous

diffusion rather than discrete. Thus we can modify the diffusion coefficient with a condensation (accommodation) coefficient, and similarly, the thermal diffusion coefficient.

Large drops also ventilate, which can enhance evaporation and condensation

Large drops can also evaporate

Combined equation for growth by vapor deposition: Also, this is an assumption that this continuous

diffusion rather than discrete. Thus we can modify the diffusion coefficient with a condensation (accommodation) coefficient, and similarly, the thermal diffusion coefficient.

Large drops also ventilate, which can enhance evaporation and condensation

Large drops can also evaporate



Growth example: Growth example:

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Narrows the droplet spectrum in time

shows 1/a dependence

solution effects enhance growth

Narrows the droplet spectrum in time

shows 1/a dependence

solution effects enhance growth



Growth of a population by condensation: Growth of a population by condensation:

As drops grow, they remove S, but as air rises, S increases

As drops grow, they remove S, but as air rises, S increases

dS

dt=P −C



Growth of a population by condensation: Growth of a population by condensation:

In fog, S is lower and only the most chemically active and huge aerosols are activated

Small drops get “starved” of H2O, never reach S large enough to grow

If updraft increases, peak saturation would also increase (cool air faster, takes drops long time to use up H2O) thus smaller drops also activate

In general, it takes days to grow drops to precipitation sizes by vapor deposition alone!!!! TOO SLOW!!

In fog, S is lower and only the most chemically active and huge aerosols are activated

Small drops get “starved” of H2O, never reach S large enough to grow

If updraft increases, peak saturation would also increase (cool air faster, takes drops long time to use up H2O) thus smaller drops also activate

In general, it takes days to grow drops to precipitation sizes by vapor deposition alone!!!! TOO SLOW!!

Collision-Coalescence Growth


E1: Coalescence efficiency

E2: Collision efficiency

E1: Coalescence efficiency

E2: Collision efficiency

K(a1,a2 ) =π(a1 + a2 )2 (v1 −v2 )E1E2

Collection kernel: (units m3/s) Collection kernel: (units m3/s)


Collision-Coalescence Growth E is very small (especially for small drops)

at the beginning because small drops sweep around droplet (following streamlines)

E~1 for a broad range of a1/a2 ratios

Can have efficiencies greater than 1 E drops off as a1/a2 approaches 1

E spikes as a1/a2 is very nearly 1. This is due to wake capture wake capture: as drops are close to

same size, hydrodynamic flow fields interfere and drops slip around each other. But this really doesn’t matter because when a1 and a2 about equal, the difference in their terminal fall speeds is so small it decreases the collection kernel.

E is very small (especially for small drops) at the beginning because small drops sweep around droplet (following streamlines)

E~1 for a broad range of a1/a2 ratios

Can have efficiencies greater than 1 E drops off as a1/a2 approaches 1

E spikes as a1/a2 is very nearly 1. This is due to wake capture wake capture: as drops are close to

same size, hydrodynamic flow fields interfere and drops slip around each other. But this really doesn’t matter because when a1 and a2 about equal, the difference in their terminal fall speeds is so small it decreases the collection kernel.

[DRAW]



Continuous Growth Model or Accretion Model Continuous Growth Model or Accretion Model

dm

dt;

4

3πρw π(a1 + ai )

2 (v1 −vi )E1in(ai )ai3dai

0

ai (max)

∫

if we also assume that a1>>ai and v1>>vi if we also assume that a1>>ai and v1>>vi

dm

dt; πa1

2v1E(ai )wl

Assume coalescence efficiency of unity

Continuous accretion model is applicable when collector droplet is much larger than collected droplets

Fails because it requires an initial broadening of droplet spectra to get drops large enough to be efficient collectors

A given droplet will always grow to the same size when falling through the same droplet population

Assume coalescence efficiency of unity

Continuous accretion model is applicable when collector droplet is much larger than collected droplets

Fails because it requires an initial broadening of droplet spectra to get drops large enough to be efficient collectors

A given droplet will always grow to the same size when falling through the same droplet population



Quasi-stochastic model Quasi-stochastic model

Uses Smolokoskies equation to predict a unique spectrum after some time dt

Use a Monte Carlo distribution

a type of “bin” model that predicts the time rate of change of mass or volume (not radius)

Uses Smolokoskies equation to predict a unique spectrum after some time dt

Use a Monte Carlo distribution

a type of “bin” model that predicts the time rate of change of mass or volume (not radius)

δn(v)

δt= K(vc ,v ')n(vc )n(v ')dv '− n(v) K(v,v ')n(v ')dv '

0

∞

∫0

v /2

∫

gain loss



Quasi-stochastic model Quasi-stochastic model

Similar to aerosols, but K increases as you get to larger droplets (K decreases for smaller aerosols)

Integration limit on Gain term accounts for combinations (don’t want to double count)

Implies that higher droplet concentrations = higher rate of collection but for a given LWC higher concentrations lead to smaller droplets

ie: for same LWC, a cloud with less concentration will grow larger drops than one with more concentration

aerosol # can really affect the cloud/precipitation processes

polluted clouds => much smaller drops

Increasing the LWC can greatly accelerate the collection process

Similar to aerosols, but K increases as you get to larger droplets (K decreases for smaller aerosols)

Integration limit on Gain term accounts for combinations (don’t want to double count)

Implies that higher droplet concentrations = higher rate of collection but for a given LWC higher concentrations lead to smaller droplets

ie: for same LWC, a cloud with less concentration will grow larger drops than one with more concentration

aerosol # can really affect the cloud/precipitation processes

polluted clouds => much smaller drops

Increasing the LWC can greatly accelerate the collection process



Problem of Initial Broadening Problem of Initial Broadening

Problem: How to get droplets to a size where collision-coalescence can kick in

Initial droplet spectra is 4µm to 12µm, so how do we get to sizes 25-30µm to make collision-coalescence productive? Growth by vapor deposition tends to narrow the droplet spectrum

Need broad spectrum of sizes or else the velocities and sizes will be too similar for Collision and Coalescence collection kernel. Thus problem of initial broadening is not just creating drops large enough for Collision-coalescence to begin

Problem: How to get droplets to a size where collision-coalescence can kick in

Initial droplet spectra is 4µm to 12µm, so how do we get to sizes 25-30µm to make collision-coalescence productive? Growth by vapor deposition tends to narrow the droplet spectrum

Need broad spectrum of sizes or else the velocities and sizes will be too similar for Collision and Coalescence collection kernel. Thus problem of initial broadening is not just creating drops large enough for Collision-coalescence to begin




1) Turbulence influences on condensation growth via fluctuation supersaturations

mixing process is inhomogeneous (get pockets of clear air—spaghetti strings)

parts of cloud may be rising and others falling on a small scale leading to evaporation of some drops, leading to relatively larger drops in some areas

Fine scale eddies can centerfuge particles out of regions, increasing the S leading to faster growth of particles that remain

2) Role of GCCN

Can act as “Coalescence Embryos” if soluble, wettable, and large

very small concentrations (similar to raindrop concentrations)

depends on CCN concentration if GCCN is important, because lower CCN clouds drizzle actively without the presence of these GCCN—maritime clouds are prolific collision and coalescence machines and GCCN presence doesn’t matter

1) Turbulence influences on condensation growth via fluctuation supersaturations

mixing process is inhomogeneous (get pockets of clear air—spaghetti strings)

parts of cloud may be rising and others falling on a small scale leading to evaporation of some drops, leading to relatively larger drops in some areas

Fine scale eddies can centerfuge particles out of regions, increasing the S leading to faster growth of particles that remain

2) Role of GCCN

Can act as “Coalescence Embryos” if soluble, wettable, and large

very small concentrations (similar to raindrop concentrations)

depends on CCN concentration if GCCN is important, because lower CCN clouds drizzle actively without the presence of these GCCN—maritime clouds are prolific collision and coalescence machines and GCCN presence doesn’t matter




3) Turbulence influences on droplet collision and coalescence

--> Enhance collision efficiencies (small drops can cross streamlines)

--> Enhance collection kernels (accelerated by air movements)

--> Producing inhomogeneities in droplet concentrations

4) Radiative broadening

assumes droplet stays around top of cloud for a long enough time (strat/fog)

cooling decreases satruation vapor pressure at the surface of the drop, leading to faster growth than drops in the middle of the could

can offset the 1/a dependence since larger drops radiate more

limited to certain classes of clouds, but it is most easy to quantify

Broadening mechanisms are difficult to quantify because of difficulty of studying turbulence.

3) Turbulence influences on droplet collision and coalescence

--> Enhance collision efficiencies (small drops can cross streamlines)

--> Enhance collection kernels (accelerated by air movements)

--> Producing inhomogeneities in droplet concentrations

4) Radiative broadening

assumes droplet stays around top of cloud for a long enough time (strat/fog)

cooling decreases satruation vapor pressure at the surface of the drop, leading to faster growth than drops in the middle of the could

can offset the 1/a dependence since larger drops radiate more

limited to certain classes of clouds, but it is most easy to quantify

Broadening mechanisms are difficult to quantify because of difficulty of studying turbulence.

DropsDrops Drop terminal velocity Drop terminal velocity

Vt is classified for 3 regimes

Vt flattens outs because drops deform

Accumulation zone or “balance zone”—in steady updraft, drops can grow to reach max terminal velocity

Vt is classified for 3 regimes

Vt flattens outs because drops deform

Accumulation zone or “balance zone”—in steady updraft, drops can grow to reach max terminal velocity

Drop breakup Drop breakup When one drop breaks up, can get a spectrum of drop sizes

1) Hydrodynamic instability of large drops due to natural oscillations

2) Collisions with other drops

ring mode (collision with other drops)

bag mode (air trapped inside, blows up like baloon)

3) Langmuir’s chain reaction theory

When one drop breaks up, can get a spectrum of drop sizes 1) Hydrodynamic instability of large drops due to natural oscillations

2) Collisions with other drops

ring mode (collision with other drops)

bag mode (air trapped inside, blows up like baloon)

3) Langmuir’s chain reaction theory

DropsDrops 3) Langmuir’s chain reaction theory 3) Langmuir’s chain reaction theory when a drop grows by collection to the point that it breaks up, it

will produce fragments which are still precipitation sized. Those fragments then collect smaller droplets and be come large enough to break up, ... Requirements:

1. Cloud remains in steady state for extended periods

2. Updraft is strong enough to support lots of hydrometeors

Can be prolific in some warm clouds, leading to bursts of precipitation

Effects raindrop size-distribution

Effects vr=> effects distribution of water

Effects water loading, water in supercooled regions, etc.

Increases number of collectors for a given LWC

when a drop grows by collection to the point that it breaks up, it will produce fragments which are still precipitation sized. Those fragments then collect smaller droplets and be come large enough to break up, ... Requirements:

1. Cloud remains in steady state for extended periods

2. Updraft is strong enough to support lots of hydrometeors

Can be prolific in some warm clouds, leading to bursts of precipitation

Effects raindrop size-distribution

Effects vr=> effects distribution of water

Effects water loading, water in supercooled regions, etc.

Increases number of collectors for a given LWC

Summary of warm-rainSummary of warm-rain Colloidally unstable: warm-based cloud that has a large value of

cloud-base saturation mixing ration and has a potential for condensing a significant amount of LW.

Colloidally stable: larger concentration of CCN and smaller saturation mixing ratio at cloud base leads to a lower potential for liquid water production and must distribute the limited LWC over more droplets, lowering its potential for creating precipitatioin.

Maritime, warm-based clouds: more likely to produce warm rain (fewer, bigger droplets and more likely to have broader droplet spectrum)

Continental, cold-based cloud: activates a larger concentration of CCN and has smaller saturation mixing ratio at cloud base.

Can really have a mixture of all of the above in real clouds

Colloidally unstable: warm-based cloud that has a large value of cloud-base saturation mixing ration and has a potential for condensing a significant amount of LW.

Colloidally stable: larger concentration of CCN and smaller saturation mixing ratio at cloud base leads to a lower potential for liquid water production and must distribute the limited LWC over more droplets, lowering its potential for creating precipitatioin.

Maritime, warm-based clouds: more likely to produce warm rain (fewer, bigger droplets and more likely to have broader droplet spectrum)

Continental, cold-based cloud: activates a larger concentration of CCN and has smaller saturation mixing ratio at cloud base.

Can really have a mixture of all of the above in real clouds

Summary of warm-rainSummary of warm-rainGCCN can move a cloud that is colloidally stable

to colloidally unstable

need to know composition of pollution to understand its influences on the production of warm rain

can effect cloud base temperature

depends on size and hygroscopicity of aerosols

hard to predict if any cloud will rain on any given day...

GCCN can move a cloud that is colloidally stable to colloidally unstable

need to know composition of pollution to understand its influences on the production of warm rain

can effect cloud base temperature

depends on size and hygroscopicity of aerosols

hard to predict if any cloud will rain on any given day...

Documents

AT620 Review for Midterm #1 Part 2: Chapters 5-7 Brenda Dolan October 19, 2005 Part 2: Chapters 5-7 Brenda Dolan October 19, 2005