AEROSOL INFLUENCE ON CLOUD 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, Particles, 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 (Aerosols influence cloud properties) More droplets -- Brighter clouds (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) 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 57˚-60˚N 38˚-43˚N 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 ) 25˚-30˚W, 50˚-55˚N, 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. #RTKN0976% C D EO UT O9O %JCPPGN4GHNGEVCPEG %JCPPGN.QECN5VCP CT &GXKCVKQP %JCPPGN4C KCVKPI6GORGTCVWTG - %JCPPGN.QECN/GCP4C KCVKPI 6GORGTCVWTG - DETERMINING CLOUD OPTICAL THICKNESS AND EFFECTIVE DROP RADIUS FROM SATELLITE RADIANCE MEASUREMENTS IN LOOK-UP TABLE OUT Visible and IR radiance Cloud optical thickness Based on Radiative (0.63 µm) Channel 1 Transfer Calculations τ (0.58-0.68 µm) c f (,,)θθφ Channel 3 0 Effective radius (3.6-3.9 µm) re = µ3/µ2 Nakajima and Nakajima, J. Atmos. Sci. (1995) 76% 76% #RTKN #RTKN O µ 76% 76% #RTKN #RTKN C D E 76% 76% 76% #RTKN #RTKN #RTKN 'HHGEVKXG&TQRNGV4CKWU 76% #RTKN G H I J %NQW1RVKECN&GRVJ SULFATE COLUMN BURDEN AND CLOUD DROP EFFECTIVE RADIUS 25˚-30˚W, 50˚-55˚N, 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 25˚-30˚W, 50˚-55˚N, 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 Liquid water path W = wce 3 where re is mean drop radius in cloud, taken as (5/6)re, and τ ().1−+g 0 097 Cloud spherical albedo α ≈ c sph τ −+ c().1143g 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 25˚-30˚W, 50˚-55˚N, 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 25˚-30˚W, 50˚-5˚N 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 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 25˚-30˚W, 50˚-55˚N 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) 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, -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 25˚-30˚W, 50˚-55˚N, April 2-8, 1987 Sulfate, µmol m-2 120 80 40 0 Eff.