Application of Ice Microphysics to CAM

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Application of Ice Microphysics to CAM

• Xiaohong Liu, S. J. Ghan
• (Pacific Northwest National Laboratory)
• M. Wang, J. E. Penner
• (University of Michigan)

National Science Foundation
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Motivation

• Anthropogenic aerosol effects on cirrus cloud?
• Homogeneous ice nucleation of SO4
• Heterogeneous ice nucleation of IN (soot dust)
• Contact freezing of cloud droplets by IN
• Processes in cirrus and mixed-phase clouds depend
  on ice number
• Vapor deposition, Bergeron-Findeison process
• Gravitational settling of crystals depends on
  sizes
• Radiation depends on effective radius

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Prognostic ice crystal number in CAM

• R(Ni) advection, turbulence, and convective
  transport (detainment at the top of convective
  cloud)
• Jnuc nucleation of ice crystals
• Jsec secondary production of ice crystals
• Qagg aggregation of ice crystals to form snow
• Qsaci accretion of ice crystals by snow
• Qmlt melting of ice crystals

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• Detrainment from convective clouds (Tlt-35 C)
• Homogeneous nucleation of sulfate and
  heterogeneous immersion nucleation on soot in
  cirrus clouds with Tlt-35C (Liu Penner, 2005)
• ice number depends on temperature, updraft
  velocity, sulfate and soot number, considering
  the competition between the two mechanisms.

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• Contact freezing of cloud droplets in mixed-phase
  clouds (-35 to 0 C) based on Young (1974), and
  assume contact IN to be mineral dust (Brownian
  coagulation)
• Deposition/condensation ice nucleation in
  mixed-phase clouds (-35 to 0 C) based on Meyers
  et al. (1992)
• Secondary ice production between -3 and -8 C
  (Jsecb) based on Cotton et al. (1986) for
  Hallet-Mossop mutiplication
• Ice crystal sublimation based on homogeneous
  mixing ice crystals sublimate completely only
  when the cloud dissipates

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Ice supersaturation

• Hybrid RH in standard CAM RHw (T gt 0 C) RHi (T
  lt -20 C) RHw RHi (-20 lt T lt 0 C)
• Condensation and evaporation (C-E) scheme removes
  ice supersaturation (assume RHi100 in ice cloud)

Compare with MOZAIC data (year 1997) statistics
at every three hours in cloud-free CAM grid cells
(2.5x2) with flight tracks
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CAM Modified
CAM Standard

• C-E used only for liquid water, diagnosed from
  RHw and liquid cloud fraction (based on RHw) get
  rid of fice(T)
• Dv2i vapor deposition on ice crystals in grid
  cells (Rotstayn et al., 2000), in proportion to
  (Si-1)
• Cloud fraction based on RHi is used in radiation
• reff of ice crystals diagnosed from mass
  number number effects on radiation and ice
  gravitational settling

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January
Ice number
Ice mass
RHw
T
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Ice number balance (January)
Nucleation (hf/immersion/deposition)
Detrainment
Contact freezing
Secondary production
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Precipitation
Evaporation
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Effective radius of ice crystals (January)
Modified CAM
Standard CAM
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RHi from Modified CAM Compared with MOZAIC Data
Compare with MOZAIC data (year 1997) statistics
at every three hours in cloud-free CAM grid cells
(2.5x2) with flight tracks
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Annual Mean Ice Water Content
Modified CAM
Standard CAM
Pressure (hPa)
Aura MLS
Pressure (hPa)
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Ice Water Content at 316 hPa (January)
Modified CAM
Standard CAM
Aura MLS
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Zonal Mean Shortwave Cloud Forcing
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Zonal Mean Longwave Cloud Forcing
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Global Annual Means
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Summary

• CAM modified to allow supersaturation add water
  vapor deposition on ice to replace C-E for ice
  clouds get rid of f(T)
• Water vapor increased significantly in the upper
  troposphere. However, still not too much at
  200-500 hPa compared to MOZAIC data
• Ice water content improved comparing with Aura
  MLS data
• Ice number concentration is predicted with the
  dominant sources being detainment from convection
  and in situ ice nucleation balanced by
  precipitation and evaporation losses
• Work needed reduce LWP and IWP to improve SWCF.

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Zonal Mean Water Vapor (January)
Relative difference () between modified and
standard CAM
Standard CAM