INFLUENCE ON OPTICAL DEPTH AND ALBEDO OVER THE NORTH ATLANTIC SHOWN BY SATELLITE MEASUREMENTS AND CHEMICAL TRANSPORT MODELING

Stephen E. Schwartz and Carmen M. Benkovitz

Harshvardhan

Photooxidants, , and Haze across the Arctic and North Atlantic: Transport Observations and Models Lamont-Doherty Earth Observatory, Palisades, New York June 12-15, 2001 AEROSOL INFLUENCES ON RADIATION BUDGET AND CLIMATE

Direct Effect (Clear sky) Light scattering -- Cooling influence Light absorption -- Warming influence, depending on surface

Indirect Effects ( influence cloud properties) More droplets -- Brighter (Twomey) More droplets -- Enhanced cloud lifetime (Albrecht)

Semi-Direct Effect Absorbing aerosol heats air and evaporates clouds DEPENDENCE OF CLOUD ALBEDO ON CLOUD DEPTH Influence of Cloud Drop Radius and Concentration

LWC = 0.3 g m-3 g = 0.858

r = 4 8 16 µm

CLOUD DEPTH Twomey, Atmospheric Aerosols, 1977 SENSITIVITY OF ALBEDO AND FORCING TO CLOUD DROP CONCENTRATION ∆ (Global MeanShortwaveForcing,Wm

0.14 ∆ 0.025 0.08 (Top-of-Atmosphere Albedo) 8 ∆ 0.12 0.020 (Global MeanAlbedo) 0.10 0.06 6 0.015 0.08 0.04 4 0.06 0.010

(Cloud-Top Albedo) (Cloud-Top 0.04 0.02 2 ∆ 0.005 0.02 0.00 0.00 0.000 0 1.3 2 3 4 5 6 -2

1 ) 1 Relative Number Density of Cloud Drops

Schwartz and Slingo (1996) CLOUD DROPLET NUMBER CONCENTRATION Dependence on Non-Seasalt Sulfate 1000

100 CDNC (cm-3) CDNC (cm-3) * Leaitch, 1992 A Quinn, 1993 ˆ Hegg, 1993 B Berresheim, 1993 E Van Dingenen, 1995

10 2 10-2 10-1 100 101 102

Boucher and Lohmann, 1995 NRC REPORT TO PRESIDENT HIGHLIGHTS IMPORTANCE OF AEROSOL INDIRECT FORCING The greatest uncertainty about the aerosol climate forcing—indeed, the largest of all the uncertainties about global climate forcings—is probably the indirect effect of aerosols on clouds. Aerosols serve as condensation nuclei for cloud droplets. Thus, anthropogenic aerosols are believed to have two major effects on cloud properties, the increased number of nuclei results in a larger number of smaller cloud droplets, thus increasing the cloud brightness (the Twomey effect); and the smaller droplets tends to inhibit rainfall, thus increasing cloud lifetime and the average cloud cover on Earth. Both effects reduce the amount of sunlight absorbed by the Earth and thus tend to cause global cooling.

Committee on the Science of Climate Change National Research Council June 6, 2001 NRC REPORT TO PRESIDENT HIGHLIGHTS IMPORTANCE OF AEROSOL INDIRECT FORCING (cont’d) The existence of these effects has been verified in field studies, but it is extremely difficult to determine their global significance. Climate models that incorporate the aerosol-cloud physics suggest that these effects may produce a negative global forcing on the order of 1 W/m2 or larger. The great uncertainty about this indirect aerosol climate forcing presents a severe handicap both for the interpretation of past climate change and for future assessments of climate changes.

Committee on the Science of Climate Change National Research Council June 6, 2001 INDIRECT FORCING OF SULFATE AEROSOL Annual-mean loss of solar irradiance, W m-2

90

-1.0 -1.0 -2.0 -1.0 -4.0 -1.0 -2.0 60 -2.0 -2.0

-4.0 -2.0 -6.0 -2.0 -1.0 -4.0

-4.0 -2.0 -1.0 30 -2.0

0.0 -2.0 -1.0 -2.0 -2.0 -1.0 -1.0 -1.0 -1.0 0.0 0 -1.0 0.0 -2.0 -2.0

0.0 0.0 -1.0 -1.0 -30 -1.0 -1.0

0.0 0.0 0.0 -60 0.0 -1.0 -2.0 0.0 0.0 0.0 0.0 0.0 -4.0 -6.0 -90 -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 LMD model; Boucher and Lohmann, 1995 SHORTWAVE FORCING, ANNUAL AVERAGE

GHG's + O3 + Sulfate (Direct and Indirect) Two Formulations of Cloud Droplet Concentration

Kiehl et al., JGR, 2000 CLIMATE FORCING COMPONENTS OVER THE INDUSTRIAL PERIOD

Committee on the Science of Climate Change National Research Council June 6, 2001 INTERHEMISPHERIC COMPARISON OF ALBEDO COMPONENTS Data from Nimbus 4

Schwartz (1989) LONGITUDE DEPENDENCE OF CLOUD ALBEDO Test for Anthropogenic Influence in Northern Hemisphere vs. Southern Hemisphere as Control

Kim and Cess, JGR, 1994 LONGITUDE DEPENDENCE OF CLOUD ALBEDO Test for Anthropogenic Influence in North Atlantic 57û-60ûN 38û-43ûN

April Albedo SST Albedo SST

Chlorophyll Chlorophyll

Falkowski, Kim, Kolber, Wilson, Wirick and Cess, Science (1992) LATITUDE DEPENDENCE OF CLOUD DROP RADIUS Test for Anthropogenic Influence in Northern Hemisphere vs. Southern Hemisphere as Control

Han , Rossow, and Lacis , J. Climate, 1994 MICROPHYSICAL AND RADIATIVE PROPERTIES OF SHIP TRACKS IN CLOUDS

Radke, Coakley & King, Science, 1989 CLOUD MICROPHYSICAL PROPERTIES AND SATELLITE VISIBLE RADIANCE ASTEX, Northeast Atlantic, June, 1992

Albrecht et al., BAMS, 1995 THIS STUDY

¥ Uses a chemical transport and transformation model to identify a situation where a strong aerosol influence on cloud albedo might be expected. ¥ Examines for this signal in satellite data. ¥ Finds a strong signal in cloud drop radius. ¥ Does not find an immediate strong signal in cloud albedo: Why? ¥ Offers an explanation and extracts the signal of a strong perturbation in cloud albedo. MODELED SULFATE COLUMN BURDEN 2− ∫[]SO4 dz April 2-8, 1987 April 2 April 3 April 4 April 5

April 6 April 7 April 8

Sulfate Column Burden

510 20 50 100 200 500 µmol m-2 MODELED SULFATE CONCENTRATION PROFILE 2− AND COLUMN BURDEN ( ∫[]SO4 dz ) 25û-30ûW, 50û-55ûN, April 2-8, 1987 10

8 Date April 1987

4 6 6

8

2

Altitude (km) 4

3

7 4 2

5

6

0 0 5 10 15 20 25 30 Sulfate Concentration (nmol m-3)

Sulfate, µmol m -2 120 80

40

0 2 3 4 5 6 7 8 9 Date, April, 1987 SYNOPTIC METEOROLOGY Height of surfaces of constant pressure

Benkovitz, Miller, Schwartz & Kwon, G3, in press, 2001 ¥ Note cut-off low-pressure system -- vertically aligned low pressure. ¥ Persistent low-pressure system transports material from northern European continent and Britain to northwest North Atlantic.

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'HHGEVKXG&TQRNGV4CKWU  76%  #RTKN G H I J          %NQW1RVKECN&GRVJ SULFATE COLUMN BURDEN AND CLOUD DROP EFFECTIVE RADIUS 25û-30ûW, 50û-55ûN, April 2-8, 1987

Sulfate, µmol m -2 120 80

40

0 Eff. Rad., µm 20 15

10

5 2 3 4 5 6 7 8 9 Date, April, 1987 SULFATE COLUMN BURDEN, CLOUD DROP EFFECTIVE RADIUS, AND CLOUD OPTICAL DEPTH 25û-30ûW, 50û-55ûN, April 2-8, 1987

Sulfate, µmol m -2 120 80

40

0 Eff. Rad., µm 20 15

10

5 Optical Depth 60

40

20

2 3 4 5 6 7 8 9 Date, April, 1987 DERIVED MICROPHYSICAL QUANTITIES

Primary measured quantities:

Effective radius at top of cloud re τ Cloud optical depth c

Derived quantities: 2ρτr path W = wce 3 where re is mean drop radius in cloud, taken as (5/6)re, and

().1−+g 0 097 Cloud spherical albedo α ≈τ c sph ().1143−+g τc where g is asymmetry parameter ranging from 0.834 for re = 6 µm to 0.872 for re = 19 µm. CLOUD PROPERTIES AND SULFATE COLUMN BURDEN 25û-30ûW, 50û-55ûN, April 2-8, 1987

Sulfate, µmol m -2 120 80

40

0 Eff. Rad., µm 20 15

10

5 Optical Depth 60

40

20

0.9 Sph. Albedo 0.8 0.7 0.6

500 LW Path, g m-2 300

100

2 3 4 5 6 7 8 9 Date, April, 1987 CLOUD OPTICAL DEPTH Dependence on Liquid Water Path 25û-30ûW, 50û-55ûN April 2-8, 1987

re = 4 8 12 16 µm re = 4 8 12 16 µm re = 4 8 12 16 µm re = 4 8 12 16 µm 60

c 40 τ

20 1628 UTC 1617 UTC 1606 UTC 1555 UTC 2 April 3 April 4 April 5 April 0 re = 4 8 12 16 µm re = 4 8 12 16 µm re = 4 8 12 16 µm re = 4 8 12 16 µm 60

Cloud Optical Depth Cloud Optical 40

20 1544 UTC 1534 UTC 1524 UTC 1705 UTC 6 April 7 April 8 April 8 April 0 0 200 400 600 0 200 400 600 0 200 400 600 0 200 400 600 Liquid Water Path, g m-2 CLOUD-TOP ALBEDO Dependence on Liquid Water Path 25û-30ûW, 50û-55ûN April 2, 5 and 7,1987

0.9

0.8

0.7

0.6 April 5 April 2 0.5 April 7 re = 0.4 4 8 16 µm Spherical Albedo (0.25 - 1.19 µm) Spherical Albedo (0.25 - 1.19

0.3 2 3 4 5 6 7 8 2 3 4 5 6 7 8 10 100 1000 Liquid Water Path, g m-2 CLOUD-TOP ALBEDO DIFFERENCE April 5 relative to April 2, 1987 Evaluated for LWP of April 5 Data

0.3 sph ∆ α 0.2

0.1

0.0

Spherical Albedo, Spherical -0.1

∆ 2 3 4 5 6 7 8 9 2 3 4 5 6 7 10 100 -2 Liquid Water Path, g m CLOUD PROPERTIES AND SULFATE COLUMN BURDEN 25û-30ûW, 50û-55ûN, April 2-8, 1987 Sulfate, µmol m -2 120 80

40

0 Eff. Rad., µm 20 15

10

5 Optical Depth 60

40

20

0.9 Sph. Albedo 0.8 0.7 0.6

500 LW Path, g m-2 300

100 0.2 ∆ Sph. Albedo 0.1

0.0 2 3 4 5 6 7 8 9 Date, April, 1987 CONCLUSIONS ¥ Demonstrated ability to relate satellite-derived cloud microphysical properties to modeled aerosol loading. ¥ Cloud-drop effective radius is strongly correlated with modeled sulfate loading. ¥ Lack of evident correlation of cloud optical thickness and spherical albedo with modeled sulfate is attributed to variation in liquid water path. ¥ Plotting optical thickness or spherical albedo vs. LWP allows dependence on LWP to be accounted for. ¥ When LWP variation is accounted for, clouds on high sulfate days exhibit marked enhancement in spherical albedo. ¥ This approach takes advantage of variable LWP and aerosol loading to identify and quantify the enhancement in spherical albedo due to aerosols. ¥ This approach may be broadly applicable to examining aerosol influences on cloud microphysical properties and radiative forcing.