Developing Ice Nucleation Parameterization for Application in CAM

1. Developing Ice Nucleation Parameterization for Application in CAM Xiaohong Liu University of Michigan

2. 1. Motivation • Cirrus Clouds • cover 30% of the earth’s surface • important effects on global shortwave and longwave radiation • stratospsheric water vapor • heterogeneous ozone chemistry

3. Anthropogenic aerosol effects on cirrus cloud? • significant surface sources of upper tropospheric sulfate (Dibb et al., 1998) • aircraft contrail, aircraft emitted soot • change in aerosol number and properties ice number concentration and size radiative effects of cirrus cloud/cloud cover • Predicting ice number concentration in the global climate model • Developing ice nucleation parameterization link ice number to aerosol number, properties, and air dynamics in the ice nucleation

4. 2. Ice nucleation mechanisms • Homogeneous freezing of solution droplets • Spontaneous freezing of, e.g., sulfate aerosol • At low temperature < -37 C • High supersaturation RHi > 140% (Koop et al., 1998) • Dominated for ice clouds with high updraft velocity (>0.5 m/s), e.g., wave-clouds (Heymsfield & Miloshecich, 1995)

5. Heterogeneous nucleation mechanisms • Deposition nucleation (deposition nuclei) • Freezing nucleation H2O contact immersion condensation/freezing

6. Two important mechanisms for upper tropospheric cirrus: immersion and deposition nucleation • Require lower supersaturation for heterogeneous freezing of efficient ice nuclei (Pruppacher and Klett, 1997) • Potential effective ice nuclei: • soot particles (e.g., DeMott et al., 1999; Ström and Ohlsson, 1998; Chen et al., 1998) • mineral dust (e.g., Chen et al., 1998; DeMott et al., 2003; Zuberi et al., 2002)

7. 3. Developing ice nucleation parameterization • Former parameterizations of ice number • Fletcher: temperature-dependence • Meyers et al.: supersaturation-dependence • Cutton et al.: a combination of above • No links to the aerosol and air dynamics • Karcher and Lohmann (2002a,b): homogeneous sulfate freezing. no explicitly related to sulfate number. Karcher and Lohmann (2003): heterogeneous immersion nucleation. No specific process described, prescribed freezing threshold.

8. A parameterization in global models • Multiple aerosol types (soot, mineral dust, pure sulfate) • Homogeneous and heterogeneous (immersion and deposition) nucleation modes • pure sulfate as homogeneous • soot as immersion nuclei • mineral dust as deposition & immersion nuclei • The transition from heterogeneous to homogeneous dominated regimes

9. Adiabatic parcel model ice • Include nucleation and initial growth of ice crystals in a constant updraft • Thermodynamics of sulfate aerosol: Kohler equation • Parcel cooling rate: • Deposition growth rate of ice crystal: Pruppacher and Klett (1997) w so4 soot

10. Homogeneous freezing of sulfate particles • Homogeneous freezing rate of H2SO4 aerosol (Jhaze) using effective freezing temperature (Teff) approach (Sassen and Dodd, 1988) such that ΔTm is equilibrium melting point depression, λ=2.0 for H2SO4/H2O solutions [Chen et al., 2000; Koop et al., 1998]

11. Heterogeneous ice nucleation • Immersion nucleation of soot particles based on classical theory (Pruppacher and Klett, 1997). The ice nucleation rate per particle is

12. Deposition nucleation (representing on giant mineral dust particles) based on Meyers et al. (1992): Nid=exp{a+b[100(RHi-1)]}, Nid (l-1) is number of ice crystals, RHirelative humidity with respect to ice, a = -0.639, and b = 0.1296.

13. Fig.1. Ice crystal number in the parcel as a function of height at starting T=-60˚C. The soot concentration was 0.1 cm-3 while sulfate concentration was 200 cm-3. w=0.04 m/s w=1 m/s

14. Parameterization of homogeneous ice nucleation • Threshold RHw (%) for homogeneous nucleation as a function of T and w RHw = A T 2 + B T + C where A = 6×10-4 ln w + 6.6×10-3, B = 6×10-2 ln w + 1.052, and C = 1.68 ln w + 129.35. Figure 2. Critical RHi as a function of temperature (w effect very small)

15. Ice number Ni from homogeneous freezing as a function of T, w, and aerosol number Na • fast growth regime (higher T and lower w) • slow growth regime (lower T and higher w) • condition of fast growth regime T ≥ 6.07 ln w - 55.0, or w ≤ exp[(T + 55.0)/6.07]

16. Fig.3. Ice crystal number concentrations Ni as a function of total aerosol number Na at different T and w w=0.04 m/s w=1.0 m/s

17. Parameterization of heterogeneous deposition & immersion ice nucleation on soot • Threshold RHw (%) for heterogeneous nucleation as a function of T (Fig.2) RHw = 0.0073 T2 + 1.477 T + 131.74 • Threshold soot number Ns,c for heterogeneous ice nucleation only (Fig.4) Fig.4. w=0.1,0.2,0.5,1.0 m/s

18. Ice number from immersion nucleation of soot Ni,s in the heterogeneous nucleation-only regime (Fig.5) w=0.04 m/s w=0.5 m/s Fig.5

19. Transition from the heterogeneous-dominated to the homogeneous-dominated regime occurs over a range of soot concentrations varying over one order of magnitude maximum supersaturation (Simax) in the air parcel Simax(%) = A×T2 + B×T + C Ice number from deposition nucleation using Meyers’ formula with above Simax

20. 4. Initial applications • Calculate the initial global ice number concentration using • the above ice nucleation parameterization • aerosol (sulfate, soot from surface, soot from aircraft) simulated with the IMPACT model • NASA DAO met data T, RH, • in-cloud • aerosol size distributions in the upper troposphere • SO4 (Jensen et al., 1994) • soot from surface sources (Pueschel et al., 1992) • soot from aircraft (Petzold & Schrolder, 1998)

21. aerosol mass from IMPACT model so4 from surface soot from surface soot from aircraft

22. aerosol number derived so4 from surface soot from surface soot from aircraft

23. ice number calculated Ice from surface so4 Ice from surface so4 & surface soot Ice from surface so4, surface soot & aircraft soot

24. 5. Next plan • Couple the IMPACT model with CAM • Predicted ice number concentration with CAM coupled with IMPACT • Study anthropogenic aerosol effects on cirrus clouds (radiation, cloud coverage) and climate.