Whitecaps, sea-salt aerosols, and climate

Magdalena D. Anguelova

Oceans and Ice Branch Seminar

College of Marine Studies

University of Delaware

18 October, 2001

Outline

Sea-salt aerosols and climate Sea spray Whitecap coverage estimation

Climate studies

© Ocean Drilling Program

Cloud feedbacks - 20 W m-2

Modeling cloud feedback

Cess et al., 1990

“I used to think of clouds as the

Gordian knot of the problem,” says

cloud specialist V. Ramanathan of

Scripps. “Now I think it’s the aerosols.

We are arguing about everything.”

R. A. Kerr, Science , 1997, 276

Aerosol radiative forcing

Anthropogenic aerosol loading; Aerosol radiative forcing:

• Negative• 0.5 to 2 W m-2

Aerosol effects

Direct;

Radiative forcing on climate in 2 distinct ways:

Aerosol effects

Indirect;

Radiative forcing on climate in 2 distinct ways:

• Cloud properties;

• Cloud lifetime.

Direct;

Include aerosol direct and indirect

forcingin climate models.

Recognized Need

Types of aerosols Natural Anthropogenic

Baseline

Clean atmosphere is affected only by

natural background aerosols.

Sea-salt aerosols are the dominant

aerosol species in background atmosphere.

Direct effect of sea-salt aerosols

Cooling 0.6 to 2 W m-2 (Winter and Chýlek, 1997)

Potential of –4 W m-2

(Quinn et al., 1996)

Andreae (1995)

Indirect effect of sea-salt aerosols Dominate the activation of CCN; Compete with SO4

2- aerosols.Activation of CCN begins

on the largest and

most soluble particles.

• Larger;• More hygroscopic.

• SS concentrations;

• SO42- concentrations

• Cloud updraft:• Total CCN • Total CCN

Halogen chemistry

Multiphase reactions

Site for chemical reactions

Halogen chemistry

Multiphase reactions

Site for chemical reactions

• Reactive Cl and Br;

Cl, Br

CH4

DMS

OH

Halogen chemistry

Multiphase reactions

Site for chemical reactions

• Reactive Cl and Br;• Tropospheric O3:

– Greenhouse gas;– Pollutant; Cl, Br

Clean air Polluted air

NOx

Halogen chemistry

Multiphase reactions

Site for chemical reactions

• Reactive Cl and Br;• Tropospheric O3:

– Greenhouse gas;– Pollutant;

• Sink of S.

H2SO4

O3

DMS SO2 SO42-

Industrial

SO42-

Sea-salt aerosol effects must be accounted for.

Outline

Sea-salt aerosols and climate Sea spray Whitecap coverage estimation

As waves break,

air blobs break up,

and forms clouds of bubbles.

Wave breaking

The fate of the bubbles

…dissolve and disappear… …stabilize and join… …rise and burst…

The fate of the bubbles

…dissolve and disappear… …stabilize and join… …rise and burst…

Upon bursting, bubble caps shatter

Film drops

Resch and Afeti (1991)

As the bubble cavity collapses...

Jet drops

MacIntyre (1974)

Under very high winds drops are torn from the wave crests and blown directly into the air.

Spume drops

Sea spray

In the air:• Moisture equilibrium;• Change of size and

phase state;

sea-salt aerosols.

r, m5001001010.1

Andreas (1998)

Sea spray sizes

Residence time (Andreas, 1992)

Heat exchangeAerosol forcing

• > 20 m• < 20 m

film jet spume

Modeling sea-salt aerosols Many processes:

• Generation;• Transport;• Diffusion and convection;• Chemical and physical

transformations:– in clear air;– in clouds;– below clouds;

• Wet and dry deposition.

• Generation;

Sea spray generation function, F

Monahan et al. (1986)

r, m5001001010.1

Via bubbles

Tearing

= dF0 /dr + dF1 /dr

Rate of production of sea spray per unit area per increment of droplet radius, r (s-1 m-2 m-

1).

dF /dr

Best generation function

Among 14 proposed functions (Andreas, 2001)

dr

dfrUW

dr

dF 024.010 )(76

dr

df Explicit forms for 4 size regions covering 1 to 500 m range.

41.310

610 108.3)( UUW (Monahan and

O’Muircheartaigh, 1980)

Improved generation function?

)()(),(

1010 rfUf

dr

UrdF

W (U10 , T, Ts , S, f , d , C )

W (U10)Best available

(Monahan and O’Muircheartaigh, 1986)

Improved generation function?

)()(),(

1010 rfUf

dr

UrdF

W (U10 , T, Ts , S, f , d , C )

W (U10)Best available

(Monahan and O’Muircheartaigh, 1986)

Improved generation function?

)()(),(

1010 rfUf

dr

UrdF

W (U10 , T, Ts , S, f , d , C )

W (U10)Best available

(Monahan and O’Muircheartaigh, 1986)

Improved generation function?

)()(),(

1010 rfUf

dr

UrdF

W (U10 , T, Ts , S, f , d , C )

W (U10)Best available

(Monahan and O’Muircheartaigh, 1986)

Improved generation function?

)()(),(

1010 rfUf

dr

UrdF

W (U10 , T, Ts , S, f , d , C )

W (U10)Best available

(Monahan and O’Muircheartaigh, 1986)

Improved generation function?

)()(),(

1010 rfUf

dr

UrdF

W (U10 , T, Ts , S, f , d , C )

W (U10)Best available

(Monahan and O’Muircheartaigh, 1986)

Improved generation function?

)()(),(

1010 rfUf

dr

UrdF

W (U10)Best available

W (U10 , T, Ts , S, f , d , C )

(Monahan and O’Muircheartaigh, 1986)

Need of database

W (U10 , T, Ts , S, f , d , C )

477 points

Existing database• 16 cruises (1969 –

1984);• Photographs.

Need of database

W (U10 , T, Ts , S, f , d , C )

477 points

Existing database• 16 cruises (1969 –

1984);• Photographs.

Need of database

W (U10 , T, Ts , S, f , d , C )

477 points307 points

Existing database• 16 cruises (1969 –

1984);• Photographs.

Need of database

W (U10 , T, Ts , S, f , d , C )

477 points307 points

Existing database• 16 cruises (1969 –

1984);• Photographs.

New meth

od

Outline

Sea-salt aerosols and climate Sea spray Whitecap coverage estimation

Whitecaps signature

High Reflectivit

y

High Emissivity

Reflectivity

Emissivity

Vis IR mWUV

e = (es + er)(1-W ) + W ef

The concept

Ocean composite emissivity

e – es – er ef – es – er W =

e , es , er , ef

e as W The task

t

Top of theatmosphere

eTs

TCB

TBU TBD

Ocean Ts

TB

Radiometer

TB = teTs + t2(1 - e) TCB+ TBU + t(1 - e) TBD

Calculate composite emissivity e

eTs = TB

e = TB - TBU - t TBD - t2 TCB

t Ts - t TBD - t2 TCB

TB -- SSM/I

Ts -- AVHRR

TCB = 2.7 K

TBU

TBD

tWentz (1997) V, L (SSM/I)

Calculate composite emissivity e

es = 1 - rs

1 + i = +

s -

0

- i

Fresnel formula: rs = f ( , )

Calculate specular emissivity es

Debye equation:

Klein and Swift (1977)

= 2f , f = 19 GHz

, 0 , ,

s , = f (Ts , S )

Ts -- AVHRR

S -- NOAA Atlas

3 - Q + Q f =

2 Q - 2Q + 3

Rosenkranz and Staelin (1972)

ef = 1 - rf

Fresnel formula: rf (f , )

Calculate foam emissivity ef

Q = Volume of water

Volume of mixture

Q = 2 %

Calculate rough sea emissivity er

er = (A + B 2) U10

Ts

Pandey and Kakar (1982)

, -- knownA , B -- given coefficientsU10 -- SSM/ITs -- AVHRR

What else?

Analytical expressions; Data (TB, U10, V, L, Ts, S ); Preparation; Error analysis; Calculate W;

Results

Emissivities

All emissivities vs. Ts

All emissivities vs. Ve =TB - TBU - t TBD - t2 TCB

t Ts - t TBD - t2 TCB

All emissivities vs. Ve =TB - TBU - t TBD - t2 TCB

t Ts - t TBD - t2 TCB

All emissivities vs. Ve =TB - TBU - t TBD - t2 TCB

t Ts - t TBD - t2 TCB

All emissivities vs. Ve =TB - TBU - t TBD - t2 TCB

t Ts - t TBD - t2 TCB

Retrieved emissivities 27 March (86), 1998

Restrictions for W estimation

rsf

rs

eee

eeeW

2 – 10 %

W < 0

e < es + er

Results

Whitecap coverage

W = 0 to 24%W = 3.16%3.4% (Blanchard,

1963)

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

Longitude

Lat

itu

de

0.00 0.02 0.04 0.06 0.08 0.10

Whitecap coverage, W

Whitecap coverage 27 March (86), 1998

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

Longitude

Lat

itu

de

0.00 0.02 0.04 0.06 0.08 0.10

Whitecap coverage, W

Validation: 41.310

610 108.3)( UUW

W = 0 to 17%W = 1.43%

Validation New method – Wind Formula

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

Longitude

Lat

itu

de

-0.15 -0.10 -0.05 -0.00 0.05 0.10 0.15 0.20 0.25

New method - Wind formula

The effect of Ts

W as Ts ; as Ts ;

The effect of Ts

Suppress at high latitudes; Boosts at mid latitudes;

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

Longitude

Lat

itu

de

0 5 10 15 20 25 30 35 40

Wind speed, U10(m s-1)

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

Longitude

Lat

itu

de

-2.0 3.8 9.7 15.5 21.3 27.2 33.0

Sea surface temperature, Ts (oC)

Validation: in situ data

Validation: in situ data

Another hour to talk about

Database organization; Regressions; Modified formula; Global distribution of sea-salt

aerosols; Estimations for climate processes.

Questions