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 earths 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 lt -37 C
• High supersaturation RHi gt 140 (Koop et al.,
  1998)
• Dominated for ice clouds with high updraft
  velocity (gt0.5 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

• 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)

ice
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
• ?Tm is equilibrium melting point depression,
• ?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)
• Nidexpab100(RHi-1),
• Nid (l-1) is number of ice crystals, RHi
  relative 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-60C. The soot
concentration was 0.1 cm-3 while sulfate
concentration was 200 cm-3.
w0.04 m/s
w1 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 610-4 ln w 6.610-3, B 610-2 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
w0.04 m/s
w1.0 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 T 2 1.477 T 131.74
• Threshold soot number Ns,c for heterogeneous ice
  nucleation only (Fig.4)

Fig.4. w0.1,0.2,0.5,1.0 m/s
18

• Ice number from immersion nucleation of soot Ni,s
  in the heterogeneous nucleation-only regime
  (Fig.5)

w0.04 m/s
w0.5 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() AT2 BT 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.