Effects of Aerosols on Cirrus Clouds

1. Effects of Aerosols on Cirrus Clouds Joyce E. Penner Department of Atmospheric, Oceanic and Space Sciences University of Michigan Jet Propulsion Laboratory July 8, 2008 Thanks to: Yang Chen2, Minghuai Wang1,Li Xu1, Xiaohong Liu3, 2Jet Propulsion Laboratory 3Pacific Northwest National Laboratory

2. Outline • Microphysical mechanisms determining ice crystal concentrations • Calculation of radiative forcing due to anthropogenic aerosols (offline model) • Climate - aerosol interactions • Effects of aerosols on stratospheric water vapor • Summary

3. Homogeneous freezing Contact freezing Immersion freezing Deposition nucleation Condensation freezing Cirrus cloud indirect forcing • Ice crystal nucleation mechanisms √ √ √

4. What types of aerosols are found in ice crystals? C: Carbonaceous as inferred by absence of elemental signature DM: dust and metallic (some oxides and some not) TEM Data from P. DeMott

5. Measurements show that dust freezes at RHi =140% << sulfates Data courtesy of Paul De Mott

6. Laboratory studies indicate that “soot” particles act as ice nuclei • DeMott (1990): small fractions of acetylene soot active by immersion freezing below -24oC • Diehl and Mitra (1998): kerosene soot active by immersion freezing, all drops freeze by -28oC, but high soot content in drops • Gorbunov et al. (2001): two lab soots active in small numbers as warm as -10oC, role of soot oxidation, IN mechanisms not defined and possibly misinterpreted as representing contact freezing in some modeling studies. • DeMott et al. (1998), highly selective conditions for ice nucleation at cirrus temps by commercial BC • Moehler et al. (2005, 2006): combustion soot OC content deleterious to IN activity, graphite spark soot highly active, less so when coated with sulfuric acid • Many don’t… (e.g., Dymarska et al. 2007)

7. Recent measurements indicate that aircraft, biomass, and hydrophobic soot are not better IN than are sulfates Hydrophobic Soot Biomass Burning Homogeneous sulfate Aircraft Engine Soot Graphitized soot from natural gas Arizona Test Dust Data from P. DeMott

8. Observed distribution of RHi indicates that cirrus clouds form at lower threshold RHi in NH (Haag and Kaercher, 2003)

9. Ni w=0.04 m/s Ni w=0.5 m/s • Ice number from nucleation of soot Ni,s depends on updraft and role of sulfate homogeneous nucleation 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)

10. Outline • Microphysical mechanisms determining ice crystal concentrations • Calculation of radiative forcing due to anthropogenic aerosols • Climate - aerosol interactions • Summary

11. Simulation method: non-interactive climate/aerosol-ice 2 Ice nucleation parameterizations: LP (Liu and Penner, 2005) and KL (Kaercher et al., 2006) Coupled GCM and CTM Aerosol concentration Ice crystal number concentration (Ni) Ice crystal effective radius (re) Meteorological field SW RTM LW RTM TOA SW radiative flux TOA LW radiative flux • Emission Scenarios • Present day emissions (PD) • PD - anthropogenic sulfate • PD - anthropogenic soot from surface sources • PD - soot from aircraft sources • Pre-industrial emissions (PI) Nucleation mode Aerosol Homogeneous Sulfate Immersion Soot Deposition Dust

12. cloud process condensation SO4 (nuclei) SO4 (accumulation) SO2+H2O2/O3 (aq) nucleation SO2+OH (gas) H2SO4 coagulation condensation coagulation cloud process coagulation Non-sulfate aerosol (OC/BC/dust/sea salt) Two versions of 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 for sulfate aerosol

13. 3-mode model Mass-only model Soot: Dust: Sulfate: Mass-only model has much higher number concentrations than 3-mode model

14. Cirrus cloud indirect forcing • Ni and re (pre-industrial aerosols) Heterogeous Homogenous Scenario 1: Natural aerosols (NAT)

15. Cirrus cloud indirect forcing • Ni and re (add anthropogenic sulfate) Add anthrop. SO4 aerosols Homogenous Scenario 1: Natural aerosols (NAT) Scenario 2: NAT + anthSO4

16. Cirrus cloud indirect forcing • Ni and re (add anthropogenic soot from surface) Heterogeneous Add surface soot aerosols Homogenous Scenario 1: Natural aerosols (NAT) Scenario 3: NAT + anthSO4 + surface soot Scenario 2: NAT + anthSO4

17. Cirrus clouds and contrails cool (by reflecting solar radiation) and warm (by trapping infrared radiation): Long wave warming Net effect is a warming if ice number increases, but a cooling if ice number decreases Solar radiative cooling

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

19. Ice number concentrations • 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

20. Ice number concentrations • 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

21. Ice number concentrations • 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 decreases Ni at higher altitudes and in Southern hemisphere

22. Radiative forcing (W/m2) • Shortwave forcing is positive in tropics where Ni decreases and negative where Ni increases at lower altitudes • Longwave forcing is opposite in sign to shortwave forcing • Net forcing is dominated by longwave forcing

23. TOA net forcing: 3 mode model, KL param. • TOA net forcing: mass-only model, KL param.

24. 3-mode model: Total forcing depends on soot modeling assumptions but could be as large as -0.6 W/m2 with aircraft providing up to -0.16 W/m2 : Negative forcing unexpected: Not included in IPCC estimates of aerosol effects

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

26. Cirrus impacts may actually be negative: Penner et al., 2008 -160

27. Outline • Microphysical mechanisms determining ice crystal concentrations • Calculation of radiative forcing due to anthropogenic aerosols • Climate - aerosol interactions: changes in cloud fraction • Effects of aerosols on stratospheric water vapor • Summary

28. Inclusion of Ice Nucleation in NCAR CAM3(Liu et al., 2007) • Implement a prognostic equation for ice number concentration • Couple to IMPACT mass-only model • Nucleation of ice crystals: • homo. freezing & heter. immersion freezing (T<-35 C) (Liu & Penner, 2005) • Contact freezing of cloud droplets (-35 to 0 C), assuming dust as IN (Young 1974) • Deposition/condensation ice nucleation (-35 to 0 C) (Meyers et al., 1992) • secondary production of ice crystals • C-E used only for liquid water; allow ice supersaturation • Dv2i : vapor deposition on ice crystals in grid cells (Rotstayn et al., 2000); get rid of fice(T) • reff for ice crystals diagnosed from mass & number: number effects on radiation and ice gravitational settling

29. Annual Mean Ice Water Content Modified CAM Standard CAM Pressure (hPa) Aura MLS Pressure (hPa)

30. Comparison of CAM with Aura MLS IWC (annual mean at 215 hPa) Standard CAM CAM-ICE MLS ice concentration (mg/m3) Ice still underestimated in CAM-ICE: effects of aerosols may be under- estimated!

31. Comparison with no-feedback case

32. Outline • Microphysical mechanisms determining ice crystal concentrations • Calculation of radiative forcing due to anthropogenic aerosols (offline model) • Climate - aerosol interactions • Effects of aerosols on stratospheric water vapor • Summary

33. H2O has long term trends in the stratosphere “Forcing” = 0.12 to 0.2 W/m2/decade (Smith et al., 2001) Jan 1992 to Apr 1999 trend; or 1979 - 1997 from radiosonde

34. Decreases observed during 2001 - 2003: Randel et al., 2004

35. Long term trend from Boulder radiosonde and HALOE: Randel et al., 2004

36. Corrected data from Boulder radiosonde and HALOE: Scherer et al. (2008) Randel et al., 2004

37. Randel et al., 2004

38. Lagged H2O at 82 hPa is correlated with 100 hPa temperature:

39. Cause of change in H2O: changes in Ttrop CTM with met fields Nudged to ECMWF Lelieveld et al., 2007 (resolution near tropopause= 600 m)

40. Radiative effects of clouds: Manus Fueglistaeler and Fu, 2005

41. Radiative effects of clouds Net heating at 100 hPa

42. Radiative effects of clouds Net cooling by observed clouds at 100 hPa Annual average heating rate (2 O3 profiles) and difference between all-sky and clear sky

43. Radiative effects of clouds Heating at 100 - 108 hPa if thin cirrus inserted above observed clouds

44. Anthropogenic aerosol effect on water flux Anthropogenic aerosol Increase in ice nuclei Change in ice particle number and radius Change in supersaturation Change in the settling velocity of ice crystals Change in radiative Heating change in ice amount and water vapor amount Change of the vertical flux of ice crystals Change in vertical velocity and temperature in TTL Change in the water flux into the stratosphere

45. Change in stratospheric water vapor from the sensitivity test with perturbed settling velocity (decrease Re) for 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

46. 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)

47. Stratospheric water vapor in HALOE and CAM

48. Water vapor anomalies 10S - 10N

49. Temperature at the cold point

50. Summary and Conclusions • Forcing by anthropogenic aerosols acting in cirrus clouds has been estimated to range between 0.16 to -0.67 Wm-2 • An ice nucleation parameterization has been included in CAM3. The modified CAM3 version improves the IWC in the UT/LS and temperature in the tropical tropopause. • If surface & aircraft BC are efficient IN, then the total off-line forcing is < 0 for soot (without cloud feedbacks), but can be > 0 in the LP parameterization • If feedbacks are included with the LP parameterization, there are large effects on ice number in cirrus clouds (>50%). IWC increases by 5-10% in some regions of upper troposphere, global high cloud cover by 2.5%, and a positive net cloud forcing of up to +0.5 W m-2.