Whitecaps, sea-salt aerosols, and climate

Magdalena D. Anguelova

Physical Oceanography

Dissertation Symposium

College of Marine Studies, University of Delaware

June 17-21, 2002Breckenridge, Colorado

What?

Outline

How?

Why?

Work

Background

Results

Aerosol effects

IPCC, 2001

Assessment: Effect of anthropogenic aerosols = Effect of all aerosols – Effect of natural aerosols

Aerosol radiative forcing Defined as…

natural

Background atmosphere Natural aerosols; Baseline of an unperturbed

atmosphere.

Background baseline

Sea-salt aerosols are the dominant aerosol species in background

atmosphere.

Sea-salt aerosols

Natural aerosols; Baseline of an unperturbed

atmosphere.

Formation of sea-salt aerosols

Sea spray;

Droplet sizes:• 0.5-500 m;• < 20 m;

Sea-salt aerosols:• Phase state;• Sizes: 0.025 to 20 m.

Sea-salt aerosol effects must be accounted for in climate models.

Motivation

Modeling sea-salt aerosols

Generation; Transport; Diffusion and convection; Chemical and physical

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

Wet and dry deposition.

Generation

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

1).

Sea spray generation function

dr

dF

dr

dfrf )( Explicit forms for 4 size regions

covering 1.6 to 500 m range. Andreas (2001)

41.310

61010 108.3)()( UUWUf (Monahan and

O’Muircheartaigh, 1980)

Best)()( 10 rfUf

Improved generation function?

)()( 10 rfUfdr

dF

r0 1.6 mMeasurements

0.1

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

(Monahan and O’Muircheartaigh, 1986)

) , C ), T, Ts , S , f , d , C ), Ts , S , f , d

Whitecap coverage

Need for a database

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

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

1

2

3

45 6

7

9

1011

1213

14

1516

Need of a database

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

1

2

3

45 6

7

9

1011

1213

14

1516

307 points477 points

New meth

od

Outline

Work

Background

Results

Ocean emissivity is compositee

Method concept

e as W Emissivity of foam-free ocean is low.

Emissivity of foam-covered ocean is high.

e – es – er ef – es – er

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

The task: calculate emissivities

Composite emissivity e:

Specular emissivity es:

Foam emissivity ef :

Roughness correction er:

Radiative transfer equation

Fresnel formula, Debye equation

Fresnel formula, empirical relation

Empirical relation

TB, V, L

Ts, S

Ts, S

U10, Ts

SSM/I

AVHRRNOAA

SSM/IAVHRR

Valid estimation of W

rsf

rs

eee

eeeW

W < 0

e < es + er

Brightness Temperature, TB (K)

100 120 140 160 180

Whi

teca

p co

vera

ge, W

(%

)

0

10

20

30

40

50

L = 0 mmS = 35 psuTs = 20 oCQ = 2 %U10 = 9 m s-1

V mm

8

20

35

50

Brightness Temperature, TB (K)

100 120 140 160 180

Whi

teca

p co

vera

ge, W

(%

)

0

10

20

30

40

50

L = 0 mmS = 35 psuTs = 20 oCQ = 2 %U10 = 9 m s-1

V mm

8

20

35

50

Brightness Temperature, TB (K)

100 120 140 160 180

Whi

teca

p co

vera

ge, W

(%

)

0

10

20

30

40

50

L = 0 mmS = 35 psuTs = 20 oCQ = 2 %U10 = 9 m s-1

V mm

8

20

35

50

Brightness Temperature, TB (K)

100 120 140 160 180

Whi

teca

p co

vera

ge, W

(%

)

0

10

20

30

40

50

L = 0 mmS = 35 psuTs = 20 oCQ = 2 %U10 = 9 m s-1

V mm

8

20

35

50

2 – 10 %

Error of W

Method accuracy

0 10 20 30 40 50 60 70 80 90 1000

40

80

120

160

200

240

280

320

Relative error of W

Cou

nt

Relative error, W/W (%)

Cou

nt

-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 coverage27 March

1998

Validation with in situ data

Validation with in situ data

Magnitude; Trend:

• Suppression at high winds;

• Enhancement at moderate winds.

Variability!

Outline

Work

Background

Results

Database

Use:• Investigate spatial and temporal

characteristics of global whitecap coverage;

• Evaluate whitecap contribution to climate processes.

• Parameterize effects of additional factors on whitecaps;

Content:• Daily and monthly estimates of W and W

for the entire 1998;• Collocated measurements of U10, Ts, S;

Spatial distribution Same magnitude; Different spatial

features:• More uniform;• 3% instead of 1%.

-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

March 1998 W U10

3

-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

-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

Effects of additional factors

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

-3 0 3 6 9 12 15 18 21 24 27 30 33

Wind speed, U10 (m s-1)

Sea surface temperature, Ts (oC)

Wind fetch and duration; Surface-active material.

March 1998

Ocean surface albedo Natural

climate agent; Average:

0.11 W m-2;

Anthropogenic agents:

•Stratospheric ozone (0.18 W m-2)

•Biomass burning (0.21 W m-2)

•Land use (0.22 W m-2)

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

Longitude

Lat

itud

e

0.00 0.04 0.08 0.12 0.16 0.20

Radiative flux changes, F (W m‑2)

CO2 transfer velocity

5 – 150 cm h-1; Average:

56.8 cm h-1; Tropics are

source of CO2; Southern

Ocean is sink of CO2.

CO2 transfer velocity, kCO2 (cm h‑2)

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

Longitude

Lat

itud

e

0 30 60 90 120 150

Flux = kCO2 C

Improved generation function?

)()(),(

1010 rfUf

dr

UrdF

r0 1.6 mMeasurements

0.1

W(U10)

(Monahan and O’Muircheartaigh, 1986)

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

Modified generation function

1010

0

),,,,,,(

dr

dfCdfSTTUW

dr

dFs 206.1 0 r

2010

0

),,,,,,(

dr

dfCdfSTTUW

dr

dFs 6.14.0 0 r

3010

0

),,,,,,(

dr

dfCdfSTTUW

dr

dFs 4.01.0 0 r

Assimilating new method estimates

Andreas, 2001

Monahan et al., 1986

Future work

m

m

m

Sea-salt aerosol loading

Magnitude; Weak wind dependence;

5.0 10.0 15.0 20.0 25.0 30.0

0

50

100

150

200

250

300

Cou

nt

Ts in places with

U10 = 10 m s-1

Sea surface temperature, Ts oC 5.0 10.0 15.0 20.0 25.0 30.0

0

50

100

150

Cou

nt

Ts in places with

U10 = 15 m s-1

Haywood et al., 1999Model - Experiment

Spatial distribution of sea-salt

-180 -120 -60 0 60 120 180

90

60

30

0

-30

-60

-90

Longitude

Lat

itud

e

1.0e+005 4.0e+005 7.0e+005 1.0e+0061105 4105 7105 1106

Number flux, dF/dr0 (# m-1 m-2 s-1)

Direct effect: 15 W m-2

Conclusions

Whitecap coverage estimation

Whitecap coverage database

Generation of sea-salt aerosols