The effect of the size of CCN on drizzle and rain formation in convective clouds

Roelof T. Bruintjes

Research Applications Program, National Center for Atmospheric Research, P.O.

Box 3000, Boulder, Colorado, 80307

roelof@ucar.edu

13 June 2004

WMO INTERNATIONAL CLOUD MODELING WORKSHOP

Hygroscopic Seeding: History

Since its inception, the term “hygroscopic seeding” has taken on slightly different meanings depending on the experimental design, type of seeding material used, and the type of cloud that was the subject for experimentation.

In all instances the ultimate goal has been to enhance rainfall by somehow promoting the coalescence process. The direct introduction of “appropriately” sized CCN that can act as artificial rain drop embryos using either water sprays, dilute saline solutions, or grinded salts, are the most common hygroscopic seeding techniques used previously.

The primary objective of introducing artificial rain drop embryos (salt particles larger than 10 m diameter) is to short circuit the action of the CCN population in determining the initial character of the cloud droplet population, and thus, jumpstart the coalescence process.

Some Comments on Hygroscopic Seeding

• Two methods: add giant/ultra-giant, or add large particles. Most experiments have used UGA, but recent experiments use flares that produce both.

• UGA produce embryos that grow to raindrops, but continued influence on the cloud will depend on breakup or some other mechanism to overcome the problem that each embryo otherwise produces one raindrop.

• Large CCN instead rely on acceleration of the production of drizzle which can occur in higher concentrations because the CCN are much smaller.

Measured Size Distributionof Smoke from S.A. Flares:

Two distributions used, one including2D measurements (from anotherflight)

PCASPHistogram: Measured Distribution,PCASP

Smooth Lines: Sum of Fitted Log-Normal Distributions (with two orthree components)

Measurements made by flying behind seeding aircraft. Concentrations diluted(typically by a factor of 100) to allow for dilution as smoke enters cloud.

(a) assumed N=1000(SS/1%)0.5

(b) use transition to Junge distri- bution for intermediate sizes(c) match (limited) observations at largest sizes (Alofs and Liu)(d) assumed ammonium sulfate for natural CCN, KCl for seeded.

Note that assumed size distribution at largesize is critical. Distributions of the formN=C(SS)k have infinite mass unless k>2and give unrealistic concentrations of giant CCN.

Cloud Condensation Nuclei

Comparison of “special” natural and seeded cases:

Significant acceleration of the precipitation process results.

Condensate after 30 min: 0.02 % in raindrops vs 82% or 85% in raindrops

If a size distribution having 1 um geometric-mean diameter (instead of 0.5 um) is used, without the large and giant components of the particle size distribution, the process is still faster and the effect of early broadening of the droplet size distribution is more evident. Drizzle concentrations are enhanced substantially.

0

200

400

600

800

1000

1200

1400

1600

0 0.5 1 1.5

Supersaturation (%)

CC

N c

once

ntra

tion

(cm

-3)

1 730 m (3)1 250 m (6)1 730 m (7)2 650 m (8)Power (1 730 m (7))Power (2 650 m (8))Power (1 250 m (6))Power (1 730 m (3))

(8)

(6)(7)

(3)

0

200

400

600

800

1000

1200

1400

1600

0 0.5 1 1.5

Supersaturation (%)

CC

N c

once

ntra

tion

(cm

-3)

3 550 m asl (5)3 550 m asl (9)1 300 m asl (10)1 730 m asl (11)2 500 m asl (12)Power (2 500 m asl (12))Power (1 730 m asl (11))Power (3 550 m asl (9))Power (3 550 m asl (5))Power (1 300 m asl (10))

(10)(11)(5)(9)

(12)

0

200

400

600

800

1000

1200

1400

1600

0 0.5 1 1.5

Supersaturation (%)

CC

N c

once

ntr

atio

n (

cm-3

)

230 m asl (1)

700 m asl (2)

Power (230 masl (1))Power (700 masl (2))

(1)

(2)

0

200

400

600

800

1000

1200

1400

1600

0 0.5 1 1.5

Supersaturation (%)

CC

N c

once

ntra

tion

(cm

-3)

2 630 m asl (4)

3 250 m asl (13)

Power (3 250 m asl(13))Power (2 630 m asl(4))

(4)(13)

Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) particle analyses

KCl crystals in young smoke from two different flaming fires.

TEM images from more aged smoke (20 to 30 min). KCL has transformed into potassium sulfate and nitrate and form inclusions within organic particles.

Similar chemical transformation have been observed in polluted marine environments (McInnes et al. 1994)

The reaction between sea salt and acific nitrate and sulfate is expected to liberate HCl gas leaving the the particles enriched in nitrate and sulfate.

KNO3(g) + NaCl(p) HCl(g) + NaNO3(p)

H2SO4(g) + 2NaCl(p) 2HCl(g) + Na2SO4(p)

Li et al. 2002: Pristine and partly reacted sea salt particles over the North Atlantic

Sizes and concentrations

1. Biomass burning particles are mostly sub-micron with KCl dominant initially in flaming fires.

2. Initial particles through chemical processes transform from potassium chloride to potassium sulfate and nitrate and are usually smaller.

3. Ultimately in regional haze ammonium sulfate particles dominate by far and are submicron.

4. The larger than 1 µm diameter particles primarily are comprised of NaCl and mineral dust. Closer to the oceans NaCl dominates and further inland mineral dust dominates.

5. Near ocean NaCl also transforms to sodium sulfate and nitrate in polluted environments.

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35

Diameter (microns)

Dro

plet

num

ber c

once

ntra

tion

(cm

-3um

-1) Industrial/Biomass burning

Marine/Biomass burningBiomass burning

•Fresh smoke seem to contain the most effective CCN while the effectiveness diminishes with age especially at supersaturations less than 1% with aerosol particles experiencing chemical transformations.•Aerosol and CCN spectra were found to be fairly homogeneous in regional haze and the CCN number concentration N as a function of supersaturation S can be described by the relation N = 692S0.54.

•Peak cloud droplet concentrations (>800 cm-3) in clouds during SAFARI were typical for clouds

growing in highly polluted environments.

SUMMARY AND CONCLUSIONS

1.E-6

1.E-5

1.E-4

1.E-3

1.E-2

1.E-1

1.E+0

1.E+1

1.E+2

1.E+3

1.E+4

0.1 1 10 100

Diameter (m)

dN/d

LogD

Pristine continentalRural continentalIndustrial continentalLess polluted coastalPolluted coastalPristine cont (10%sol)

Coarse Giant

10% hygroscopic

1% hygroscopic

0

500

1000

1500

2000

2500

0 0.2 0.4 0.6 0.8 1

Supersaturation (%)

CC

N c

once

ntra

tion

(cm

-3)

Aerosol sizedistributions

CCN activationspectra

0

100

200

300

400

500

0 5 10 15Mean droplet radius (m)

Hei

ght a

bove

clo

ud b

ase

(m) Pristine

RuralIndustrial

0

100

200

300

400

500

0 500 1000 1500Droplet concentration (cm-3)

Hei

ght a

bove

clo

ud b

ase

(m) Pristine

RuralIndustrial

0

100

200

300

400

500

0 0.5 1 1.5Supersaturation (%)

Hei

ght a

bove

clo

ud b

ase

(m) Pristine

RuralIndustrial

Effect of accumulation mode aerosol concentration

drizzle raindrops

Pristine continental

Rural continental

Industrial continental

PRODUCTION OFDRIZZLE AND RAINDROPS

WITH INCREASING ACCUMULATION MODE

AEROSOL CONCENTRATION

Pristinecontinental

Industrialcontinental

Pollutedmaritime

No large aerosols With large aerosols

drizzle raindrops

EFFECT OF LARGEAEROSOLS ON DRIZZLE AND

PRECIPITATIONPRODUCTION

POLLUTEDCOASTAL

Aerosols<1m diameter

Aerosols<10m diameter

Aerosols<50m diameter

INDUSTRIALCONTINENTAL

Aerosols<1m diameter

Aerosols<10m diameter

Aerosols<50m diameter

PRISTINECONTINENTAL

Aerosols<1m diameter

Aerosols<10m diameter

Aerosols<50m diameter

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30

Time (minutes)

Dro

plet

con

cent

ratio

n (c

m-3

)

C1: Less polluted coastal (aerosols < 1 m)

C2: Less polluted coastal (aerosols < 10 m)

C3: Less polluted coastal (aerosols < 50 m)

PC1: Polluted coastal (aerosols < 1 m)

PC2: Polluted coastal (aerosols < 10 m)

PC3: Polluted coastal (aerosols < 50 m)

C1

C3 C2

PC1

PC3 PC2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30

Time (minutes)

Dro

plet

con

cent

ratio

n (c

m-3

)

P1: Pristine continental (aerosols < 1 m)

P2: Pristine continental (aerosols < 10 m)

P3: Pristine continental (aerosols < 50 m)

R1: Rural continental (aerosols < 1 m)

R2: Rural continental (aerosols < 10 m)

R3: Rural continental (aerosols < 50 m)

P1

P2

P3

R1

R2

R3

CONCENTRATIONOF DROPLETS

WITH DIAMETER >40 m

Continentalconditions

Coastalconditions

•The cloud droplet size distribution is dependent on the chemistry, size and concentrations of the aerosol population and aerosols.•Course mode aerosols (CCN) between 0.8 and 5 µm diameter governs the drizzle production in convective updrafts.• Transfer of water via coalescence process to rain water proceeds more rapidly with higher drizzle concentrations.•Drizzle also mixes through larger parts of the cloud resulting in larger parts of the cloud developing an effective coalescence process.

•These studies support the rainfall enhancement studies using hygroscopic flares.

SUMMARY AND CONCLUSIONS

THE END