Effects of Aerosols on Cirrus Clouds

Effects of Aerosols on Cirrus Clouds

Jet Propulsion LaboratoryJet Propulsion LaboratoryJuly 8, 2008July 8, 2008

Thanks to: Yang ChenThanks to: Yang Chen22, Minghuai Wang, Minghuai Wang11,, Li XuLi Xu11, , Xiaohong LiuXiaohong Liu33,,

2Jet Propulsion Laboratory33Pacific Northwest National Laboratory

Joyce E. PennerJoyce E. Penner

Department of Atmospheric, Oceanic and Space SciencesDepartment of Atmospheric, Oceanic and Space Sciences

University of MichiganUniversity of Michigan

OutlineOutline

•Microphysical mechanisms determining Microphysical mechanisms determining ice crystal concentrationsice crystal concentrations

•Calculation of radiative forcing due Calculation of radiative forcing due to anthropogenic aerosols (offline to anthropogenic aerosols (offline model)model)

•Climate - aerosol interactionsClimate - aerosol interactions•Effects of aerosols on stratospheric Effects of aerosols on stratospheric water vaporwater vapor

•SummarySummary

Cirrus cloud indirect forcingCirrus cloud indirect forcing

• Ice crystal nucleation mechanismsIce crystal nucleation mechanisms

Homogeneous freezing

Contact freezing

Immersion freezing

Deposition nucleation

Condensation freezing

√

√

√

What types of aerosols What types of aerosols are found in ice are found in ice crystals?crystals?

C: Carbonaceous as inferred by absence of elemental signature

DM: dust and metallic DM: dust and metallic (some oxides and some (some oxides and some not)not)

TEM Data from P. DeMott

Measurements show that dust Measurements show that dust freezes at freezes at RHRHi i =140% << sulfates=140% << sulfates

Data courtesy of Paul De Mott

Laboratory studies indicate that “soot” Laboratory studies indicate that “soot” particles act as ice nucleiparticles act as ice nuclei• DeMott (1990): small fractions of acetylene soot DeMott (1990): small fractions of acetylene soot active by immersion freezing below -24active by immersion freezing below -24ooCC

• Diehl and Mitra (1998): kerosene soot active by Diehl and Mitra (1998): kerosene soot active by immersion freezing, all drops freeze by -28immersion freezing, all drops freeze by -28ooC, but high C, but high soot content in dropssoot content in drops

• Gorbunov et al. (2001): two lab soots active in small Gorbunov et al. (2001): two lab soots active in small numbers as warm as -10numbers as warm as -10ooC, role of soot oxidation, IN C, role of soot oxidation, IN mechanisms not defined and possibly misinterpreted as mechanisms not defined and possibly misinterpreted as representing contact freezing in some modeling representing contact freezing in some modeling studies.studies.

• DeMott et al. (1998), highly selective conditions for DeMott et al. (1998), highly selective conditions for ice nucleation at cirrus temps by commercial BCice nucleation at cirrus temps by commercial BC

• Moehler et al. (2005, 2006): combustion soot OC Moehler et al. (2005, 2006): combustion soot OC content deleterious to IN activity, graphite spark content deleterious to IN activity, graphite spark soot highly active, less so when coated with sulfuric soot highly active, less so when coated with sulfuric acidacid

• Many don’t… (e.g., Dymarska et al. 2007)Many don’t… (e.g., Dymarska et al. 2007)

Recent measurements indicate Recent measurements indicate that aircraft, biomass, and that aircraft, biomass, and hydrophobic soot are hydrophobic soot are notnot better better IN than are sulfatesIN than are sulfates

Arizona Test Dust

Aircraft Engine Soot

Biomass Burning

Hydrophobic Soot

Graphitized soot from natural gas

Data from P. DeMott

Homogeneous sulfate

Observed distribution of RHi Observed distribution of RHi indicates that cirrus clouds indicates that cirrus clouds form at lower threshold RHi in form at lower threshold RHi in NHNH

(Haag and Kaercher,2003)

• Ice number from nucleation of soot Ice number from nucleation of soot NNi,si,s depends on updraft and role of sulfate depends on updraft and role of sulfate homogeneous nucleationhomogeneous nucleation

0.001

0.01

0.1

1

10

100

0.001 0.01 0.1 1 10Total soot concentration (cm-3)

Ice numer concentration (cm

-3) 230.8K

221.1K

211.4K

201.5K

0.001

0.01

0.1

1

10

100

0.001 0.01 0.1 1 10Total soot concentration (cm-3)

Ice numer concentration (cm

-3)

230.8K

221.1K

211.4K

201.5K

w=0.04 m/s

w=0.5 m/s

Ni

Ns Ns

Parameterization has threshold RHi (%) for immersion freezing on soot = 120-130% using wettability parameter mis = 0.5. (DeMott et al., 1990; Mohler et al., 2005)

Ni

OutlineOutline

•Microphysical mechanisms Microphysical mechanisms determining ice crystal determining ice crystal concentrationsconcentrations

•Calculation of radiative forcing Calculation of radiative forcing due to anthropogenic aerosolsdue to anthropogenic aerosols

•Climate - aerosol interactionsClimate - aerosol interactions

•SummarySummary

• Simulation method: non-interactive Simulation method: non-interactive climate/aerosol-iceclimate/aerosol-ice

Meteorological field

Aerosol concentration Ice crystal number concentration (Ni)

Ice crystal effective radius (re)

TOA SW radiative flux

TOA LW radiative flux

Coupled GCM and CTM2 Ice nucleation parameterizations: LP (Liu and Penner, 2005) and KL (Kaercher

et al., 2006)

SW RTM LW RTM

Emission Scenarios

1. Present day emissions (PD)

2. PD - anthropogenic sulfate

3. PD - anthropogenic soot from surface sources

4. PD - soot from aircraft sources

5. Pre-industrial emissions (PI)

Sulfate

Homogeneous

Soot Immersion

Dust Deposition

Aerosol

Nucleation mode

H2SO4SO2+OH (gas)

(OC/BC/dust/sea salt)

SO4 (nuclei) SO4 (accumulation)

Non-sulfate aerosol

coagulation

condensation

nucleation

coagulation

condensation

SO2+H2O2/O3 (aq)

cloud process

cloud process

coagulation

Two versions of aerosol model: Two versions of aerosol model: 3-mode sulfate aerosol model:3-mode sulfate aerosol model:

Treatment of nucleation:1) Binary homogeneous nucleation (acts mainly in free troposphere)

Mass-only model assumes size distribution forsulfate aerosol

Sulfate:Mass-onlymodel hasmuch highernumberconcentrationsthan 3-modemodel

Soot:

Dust:

3-mode model Mass-only model


• NNii and r and ree (pre-industrial aerosols) (pre-industrial aerosols)

Scenario 1: Natural aerosols (NAT)

Heterogeous

Homogenous


• NNii and r and ree (add anthropogenic sulfate) (add anthropogenic sulfate)

Scenario 1: Natural aerosols (NAT) Scenario 2: NAT + anthSO4

Add anthrop.SO4 aerosols

Homogenous


• NNii and r and ree (add anthropogenic soot from surface) (add anthropogenic soot from surface)

Scenario 1: Natural aerosols (NAT) Scenario 2: NAT + anthSO4

Scenario 3: NAT + anthSO4 + surface soot

Add surfacesoot aerosols

Homogenous

Heterogeneous

Cirrus clouds and contrails Cirrus clouds and contrails coolcool (by (by reflecting solar radiation) and reflecting solar radiation) and warmwarm (by (by trapping infrared radiation):trapping infrared radiation):

Long wave warming

Solar radiative cooling

Net effect is a warming if ice number increases, but a cooling if ice number decreases

Ice number Ice number concentrationsconcentrations

• Ice number concentrations decrease at high altitudes: Homogeneous nucleation dominates in PI calculation



• An increase in sulfate aerosol causes almost no change in ice number concentrations




• Soot from surface and aircraft sources increase ice concentrations at lower altitudes




• Soot from surface and aircraft sources increase ice concentrations at lower altitudes

• Soot decreases Ni at higher altitudes and in Southern hemisphere

Radiative forcing Radiative forcing (W/m(W/m22)• Shortwave forcing is Shortwave forcing is positive in tropics where positive in tropics where Ni decreases and negative Ni decreases and negative where Ni increases at where Ni increases at lower altitudeslower altitudes

• Longwave forcing is Longwave forcing is opposite in sign to opposite in sign to shortwave forcingshortwave forcing

• Net forcing is dominated Net forcing is dominated by longwave forcingby longwave forcing

• TOA net forcing: 3 mode model, KL param.TOA net forcing: 3 mode model, KL param.

• TOA net forcing: mass-only model, KL param.TOA net forcing: mass-only model, KL param.

3-mode model: Total forcing depends on 3-mode model: Total forcing depends on soot modeling assumptions but could be soot modeling assumptions but could be as large as -0.6 W/mas large as -0.6 W/m2 2 with aircraft with aircraft providing up to -0.16 W/mproviding up to -0.16 W/m22 : :

Total Forcing (LP to KL)

Short wave

Longwave

Total forcing: all anthr o. aerosols

-0.52 to -0.68 0.6 3 to 0. 91 -1.15 to -1.58

Anthropogenic sul fat e ae rosols

0.0 4 to -0.01 -0.04 to 0.01

0.0 8 to -0.01

Anthropogenic sur face soo t aerosols

-0.26 to -0.40

0.1 3 to 0. 37

-0.39 to -0.77

Aircraft soot

-0.12 to -0.16

0.1 8 to 0. 26

-0.30 to -0.42

Negative forcing unexpected: Not included in IPCC estimates of aerosol effects

Mass-only model: Total forcing is more Mass-only model: Total forcing is more positive than 3-mode model because positive than 3-mode model because sulfate number concentration is much sulfate number concentration is much larger, making the effects of soot larger, making the effects of soot small:small:

Total Forcing

Mass -only (LP to KL)

Total Forcing 3-mode

(LP to KL) Total forcing: all anthr o. aerosols

0.1 6 to -0.15 -0.52 to -0.67

Anthropogenic sul fat e ae rosols

0.1 3 to 0. 17 0.0 2 to 0.04

Anthropogenic sur face soo t aerosols

0.0 1 to -0.64

-0.26 to -0.40

Aircraft soot

0.0 2 to -0.08

-0.12 to -0.16

Cirrus impacts may actually be negative:Cirrus impacts may actually be negative:

Penner et al., 2008

-160

OutlineOutline

• Microphysical mechanisms determining Microphysical mechanisms determining ice crystal concentrationsice crystal concentrations

• Calculation of radiative forcing due Calculation of radiative forcing due to anthropogenic aerosolsto anthropogenic aerosols

• Climate - aerosol interactions: Climate - aerosol interactions: changes in cloud fractionchanges in cloud fraction

• Effects of aerosols on stratospheric Effects of aerosols on stratospheric water vaporwater vapor

• SummarySummary

Inclusion of Ice Nucleation in NCAR Inclusion of Ice Nucleation in NCAR CAM3CAM3

(Liu et al., 2007)(Liu et al., 2007)• Implement a prognostic equation for ice number Implement a prognostic equation for ice number

concentrationconcentration• Couple to IMPACT mass-only modelCouple to IMPACT mass-only model• Nucleation of ice crystals:Nucleation of ice crystals:

homo. freezing & heter. immersion freezing (T<-35 homo. freezing & heter. immersion freezing (T<-35 C) (Liu & Penner, 2005)C) (Liu & Penner, 2005)

Contact freezing of cloud droplets (-35 to 0 C), Contact freezing of cloud droplets (-35 to 0 C), assuming dust as IN (Young 1974)assuming dust as IN (Young 1974)

Deposition/condensation ice nucleation (-35 to 0 Deposition/condensation ice nucleation (-35 to 0 C) (Meyers et al., 1992)C) (Meyers et al., 1992)

secondary production of ice crystalssecondary production of ice crystals• C-E used only for liquid water; allow ice C-E used only for liquid water; allow ice

supersaturationsupersaturation

• DDv2iv2i : vapor deposition on ice crystals in grid cells : vapor deposition on ice crystals in grid cells (Rotstayn et al., 2000); get rid of (Rotstayn et al., 2000); get rid of fficeice(T)(T)

• rreffeff for ice crystals diagnosed from mass & number: for ice crystals diagnosed from mass & number: number effects on radiation and ice gravitational number effects on radiation and ice gravitational settlingsettling

Annual Mean Ice Water Content

Modified Modified CAMCAM

Standard CAMStandard CAM

Aura MLSAura MLS

Pre

ssur

e (h

Pa)

Pre

ssur

e (h

Pa)

Comparison of CAM with Aura MLS IWC (annual mean at 215 hPa)

Ice still underestimated in CAM-ICE:effects of aerosols may be under-estimated!

MLS ice concentration (mg/m3)

CAM-ICE Standard CAM

Comparison with no-feedback Comparison with no-feedback casecase

Anthr. Anthr. Sfc. BCSfc. BC

Aircraft Aircraft BCBC

Anthr. SO4Anthr. SO4 Total Total Anthr. Anthr. aerosolaerosol

ΔΔLWP, g mLWP, g m-2-2 +2.9+2.9 0.60.6 1.21.2 +2.9 ±1.1+2.9 ±1.1

ΔΔIWP, g mIWP, g m-2-2 +0.06+0.06 0.00.0 +0.05+0.05 +0.1 ±0.1+0.1 ±0.1

ΔΔSWCF, W mSWCF, W m-2-2 -1.1-1.1(-0.38)(-0.38)

-0.23-0.23(-0.04)(-0.04)

-0.07-0.07(-0.09)(-0.09)

-1.1 ±0.4-1.1 ±0.4(-0.49)(-0.49)

ΔΔLWCF, W mLWCF, W m-2-2 +1.4+1.4(+0.39)(+0.39)

+0.12+0.12(0.07)(0.07)

+0.08+0.08(0.22)(0.22)

+1.7 ±0.1+1.7 ±0.1(0.66)(0.66)

ΔΔnetCF, W mnetCF, W m-2-2 +0.22±0.24+0.22±0.24(+0.01)(+0.01)

-0.11±0.27-0.11±0.27(+0.02)(+0.02)

+0.01±0.26+0.01±0.26(0.13)(0.13)

+0.53±0.4+0.53±0.4(0.16)(0.16)

ΔΔCLDTOT, %CLDTOT, % +1.5+1.5 0.00.0 0.130.13 +1.7 ±0.1+1.7 ±0.1

ΔΔCLDHGH, %CLDHGH, % +2.0+2.0 +0.0+0.0 +0.06+0.06 +2.5 ±0.2+2.5 ±0.2

OutlineOutline

•Microphysical mechanisms determining Microphysical mechanisms determining ice crystal concentrationsice crystal concentrations

•Calculation of radiative forcing due Calculation of radiative forcing due to anthropogenic aerosols (offline to anthropogenic aerosols (offline model)model)

•Climate - aerosol interactionsClimate - aerosol interactions•Effects of aerosols on stratospheric Effects of aerosols on stratospheric water vaporwater vapor

•SummarySummary

Jan 1992 to Apr 1999 trend; or 1979 - 1997 from radiosonde

“Forcing” = 0.12 to 0.2 W/m2/decade

(Smith et al., 2001)

H2O has long term trends in the H2O has long term trends in the stratospherestratosphere

Randel et al., 2004

Decreases observed during 2001 - Decreases observed during 2001 - 2003:2003:

Randel et al., 2004

Long term trend from Boulder radiosonde and HALOE:

Randel et al., 2004

Scherer et al. (2008)

Corrected data from Boulder radiosonde and HALOE:

Randel et al., 2004

Lagged HLagged H22O at 82 hPa is O at 82 hPa is correlated with 100 hPa correlated with 100 hPa temperature:temperature:

Cause of change in H2O: changes in Ttrop

CTM with met fieldsNudged to ECMWF

Lelieveld et al., 2007

(resolution neartropopause= 600 m)

Fueglistaeler and Fu,2005

Radiative effects of clouds: Manus

Net heating at100 hPa

Radiative effects of clouds

Net coolingby observedclouds at100 hPa

(a) Annual average heating rate (2 O3 profiles) and (b) difference between all-sky and clear sky



Heating at100 - 108 hPa ifthin cirrusinserted above observedclouds

Anthropogenic aerosol effect on water Anthropogenic aerosol effect on water fluxflux

Anthropogenicaerosol

Change insupersaturation

Change in ice particle number and radius

Increase in ice nuclei

Change in radiative Heating

Change in the settling velocity of ice crystals

change in ice amount and water vapor amount

Change of the vertical flux of ice crystalsChange in vertical velocity

and temperature in TTL

Change in the water flux into the stratosphere

Change in stratospheric water vapor Change in stratospheric water vapor from from the sensitivity test with perturbed the sensitivity test with perturbed settling velocity (decrease Rsettling velocity (decrease Ree) for for ice (30N - 30S)ice (30N - 30S)

0.5-1.0 ppmv increase vs. 0.17 ppmv increase from vertical flux of ice crystals

1-2 K increase

Re -> qi -> T

Annual zonal mean latitude versus pressure cross sections of (a) ice number absolute difference (# g-1), (b) cloud ice water mixing ratio difference (mg kg-1), (c) specific humidity relative difference (%), and temperature absolute difference (K) between the present-day and pre-industrial day simulations. Soot acts as efficient IN (Mohler et al., 2005) with RH threshold of 120-130% (Liu and Penner, 2005)

Stratospheric water vapor in HALOE Stratospheric water vapor in HALOE and CAMand CAM

Water vapor anomalies 10S - 10NWater vapor anomalies 10S - 10N

Temperature Temperature at the cold at the cold pointpoint

Summary and ConclusionsSummary and Conclusions

• Forcing by anthropogenic aerosols acting in cirrus Forcing by anthropogenic aerosols acting in cirrus clouds has been estimated to range between 0.16 to clouds has been estimated to range between 0.16 to -0.67 Wm-0.67 Wm-2-2

• An ice nucleation parameterization has been An ice nucleation parameterization has been included in CAM3. The modified CAM3 version included in CAM3. The modified CAM3 version improves the IWC in the UT/LS and temperature in improves the IWC in the UT/LS and temperature in the tropical tropopause.the tropical tropopause.

• If surface & aircraft BC are efficient IN, then the If surface & aircraft BC are efficient IN, then the total off-line forcing is < 0 for soot (without total off-line forcing is < 0 for soot (without cloud feedbacks), but can be > 0 in the LP cloud feedbacks), but can be > 0 in the LP parameterizationparameterization

• If feedbacks are included with the LP If feedbacks are included with the LP parameterization, there are large effects on ice parameterization, there are large effects on ice number in cirrus clouds (>50%). IWC increases by 5-number in cirrus clouds (>50%). IWC increases by 5-10% in some regions of upper troposphere, global 10% in some regions of upper troposphere, global high cloud cover by 2.5%, and a positive net cloud high cloud cover by 2.5%, and a positive net cloud forcing of up to +0.5 W mforcing of up to +0.5 W m-2-2.

Summary and ConclusionsSummary and Conclusions

• The predicted decrease in Ni and increase in cloud fraction heats the tropical upper troposphere causing increases in water vapor in the stratosphere

• Validation of such effects requires better information on RHi (supersaturation) and Ni (crystal number) in the upper troposphere.

• Work under way: improve subgrid-RHWork under way: improve subgrid-RHii, include , include higher resolution near tropopause and in higher resolution near tropopause and in stratospherestratosphere

Documents

Effects of Aerosols on Cirrus Clouds