Large increase in cloud drop concentrations and albedo over sea surfaces

colder than 7°C

Daniel Rosenfeld, Tom Goren, David Giguzin

The Hebrew University of Jerusalem, Israel

Wm-2-50 -30 0 30 50

Absorbed Shortwave Radiation Mean Error – CMIP5

SOCRATES hypotheses for explaining the error

1. Too little low water clouds

2. Wrong cloud microphysics

3. Wrong aerosol transport

4. Persistent supercooled water clouds without freezing

5. Remote sensing errors

Yes for 1-4, But why?

Zabori et al., ACP 2012SST

Sea

spr

ay N

The research questions1. How do SST, wind speed (WS) and

marine productivity (P) affect cloud drop number concentrations (Nd)?

2. How does Nd affect cloud solar radiative effect (CRE)?

Data sources:Nd – retrieved from MODISWS – retrieved from scaterometersSST – NOAA/ESRL daily SST productP – Oregon State U. Ocean Productivity page

SST

SST, Wind Speed, Productivity

4622 scenes of 100x100 km

0

50

100

150

200

250

300

0 5 10 15 20 25

SST<5 C10>SST>5C15>SST>10C

20>SST>15CSST>20 C

Nd [

cm-3

]

Wind speed [m/s]

E

Nd does not depend on WS

Nd

cm-3

0

50

100

150

200

250

300

0 5 10 15 20 25 30

WS<5 m/s10>WS>5m/s15>WS>10m/s

20>WS>15m/sWS>20 m/s

Nd [

cm-3

]

Sea Surface Temperture [C]

C

Nd increases with colder SST, with no effect of WS

0

50

100

150

200

250

300

0 200 400 600 800 1000 1200 1400

SST>20C20>SST>15C15>SST>10C10>SST>5CSST<5C

Nd [c

m-3

]

Productivity

A

Nd decreases with productivity ?!The increase of Nd with colder SST is dominant.

0

50

100

150

200

250

Nd

cm

-3

SST>7 SST<7

<10WS: >1710-17 <10 >1710-17 ms-1

N=863 1192 624 566 608 769

SST>7°C SST<7°CWS

Nd

cm-3

Nd increases with colder SST, with no effect of WS

Normalized number size distributions for winter experiments resulting from bubble bursting of sea water at different temperatures (reproduced from Figure 11 of Zabori et al., ACP 2013). Strong winds particles and are capable of rising larger particles to cloud base, which lower supersaturation and suppress the activation of the added smaller particles into cloud drops, regardless of the concentrations of the small particles.

50

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80

90

100

-130

-120

-110

-100

-90

-80

-70

Nd T<7CNd T>7C

CF% T<7CCF% T>7C

CRE T<7CCRE T>7C

Nd [

cm-3

];

Clo

ud

fra

ctio

n [

%]

Clo

ud

Rad

iative Effect [w

m-2]

Wind Speed [ms-1]

<10 10-17 >17

50

60

70

80

90

100

-130

-120

-110

-100

-90

-80

-70

Nd T<7CNd T>7C

CF% T<7CCF% T>7C

CRE T<7CCRE T>7C

Nd [

cm-3

];

Clo

ud

fra

ctio

n [

%]

Clo

ud

Rad

iative Effect [w

m-2]

Wind Speed [ms-1]

<10 10-17 >17

50

60

70

80

90

100

-130

-120

-110

-100

-90

-80

-70

Nd T<7CNd T>7C

CF% T<7CCF% T>7C

CRE T<7CCRE T>7C

Nd [

cm-3

];

Clo

ud

fra

ctio

n [

%]

Clo

ud

Rad

iative Effect [w

m-2]

Wind Speed [ms-1]

<10 10-17 >17

-250

-200

-150

-100

-50

0

14 30 54 98 169

CR

E [

W m

-2]

N=332

8243541800

1312

Nd [cm-3]

A

0

0.2

0.4

0.6

0.8

1

14 30 54 98 169

Clo

ud

Fra

cti

on

N=332824

354

1800

1312

Nd [cm-3]

B

Negative Solar Cloud Radiative Effect increases with Nd

Cloud Fraction increases with Nd

-140

-120

-100

-80

-60

-40

-20

0.3

0.4

0.5

0.6

0.7

0.8

0.9

10 100

CRE = 66.4 - 88.4 log(Nd) R= 0.998CF= 0.0454 + 0.34 log(Nd) R= 0.996

CR

E [

W m

-2] C

lou

d F

ractio

n

Nd [cm-3]

A

-140

-120

-100

-80

-60

-40

-20

0.3

0.4

0.5

0.6

0.7

0.8

0.9

10 100

CRE = 66.4 - 88.4 log(Nd) R= 0.998CF= 0.0454 + 0.34 log(Nd) R= 0.996

CR

E [

W m

-2] C

lou

d F

ractio

n

Nd [cm-3]

A

Variable

SST<7°C SST>7°C

Means diffN Mean SE N Mean SE

Nd 1943 93.9 1.0 2679 57.6 0.7 36.3

CRE 1943 -110.5 1.0 2679 -79.4 1.1 -31.1

CF 1943 0.74 0.007 2679 0.60 0.007 0.14

Variable

SST<7°C SST>7°C

Means diffN Mean SE N Mean SE

Nd 1943 93.9 1.0 2679 57.6 0.7 36.3

CRE 1943 -110.5 1.0 2679 -79.4 1.1 -31.1

CF 1943 0.74 0.007 2679 0.60 0.007 0.14

CRE=-23 Wm-2-23/-31≈3/4

75% of the Nd effect on CRE is mediated by increasing CF, i.e., cloud lifetime effect.

Conclusions1. The dependence of Nd on SST explains enhanced solar

reflectance of ~30 Wm-2 in the high latitudes of the Southern Oceans.

2. Effects of wind speed and productivity are not obvious.

3. The enhanced Nd occurs probably due to the increased sub-micron sea spray particles at the colder SST. The physical reason for that has yet to be discovered.

4. Most (3/4) of the effect of Nd on CRE is mediated through the cloud lifetime effect, by suppressing coalescence and glaciation. Only 1/4 remains for the cloud brightening effect by the added Nd.

5. This might explain also the abundance of supercooled cloud water in the Southern Oceans.

6. The found effect matches the missing solar reflectance in the GCMs. The SST effect might be the missing process in the models.

0

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<30

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>60

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<30

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>60

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<30

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>60

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<30

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300

-600

>60

0

Clo

ud

dro

p n

um

ber

co

nce

ntr

atio

ns

[cm

-3]

T>7 T>7 T<7T<7WS<10 WS>10

Prod =

SST=

340 840 619 1090 265390 178 432 113 29N= 312 14

10.9 11.9 13.9 3.6 4.715.1 16.0 2.1 4.2 4.918.9 5.4