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