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Sensitivity of the CAM to Small Ice Crystals As Measured by the FSSP

Sensitivity of the CAM to Small Ice Crystals As Measured by the FSSP David L. Mitchell Desert Research Institute, Reno, Nevada Philip Rasch NCAR, Boulder, Colorado. Small mode enhancement with decreasing T for tropical anvil cirrus;

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Sensitivity of the CAM to Small Ice Crystals As Measured by the FSSP

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  1. Sensitivity of the CAM to Small Ice Crystals As Measured by the FSSP David L. Mitchell Desert Research Institute, Reno, Nevada Philip Rasch NCAR, Boulder, Colorado

  2. Small mode enhancement with decreasing T for tropical anvil cirrus; Opposite for mid-latitude cirrus.

  3. Albedo and emissivity differences between mid-latitude and tropical anvil cirrus clouds having the same ice water path (IWP). Also, note the contribution of the small crystal mode to the albedo and emissivity (dashed curves are for the large mode only).

  4. A = fraction of the “original” concentration to total concentration of ice particles. From Field et al. 2003, J. Atmos. Ocean. Tech., 20, 249-261.

  5. Corrected for Ice Particle Shattering Effects

  6. CAM RESULTS: Tropical vs. mid-latitude size distributions ML PSD Tropical LW cloud forcing SW cloud forcing

  7. Tropical minus mid-latitude PSD simulations: Cloud forcing Tropical – ML SWCF differences Tropical – ML LWCF differences

  8. PSD median mass flux fall velocities

  9. Impact of ice particle fall velocities IWP zonal means for in-cloud conditions, tropical & mid-latitude PSD schemes M L PSD Tropical % difference in high-level cloud coverage, tropical – ML PSD scheme

  10. Heating rates and tropical cold bias

  11. Cloud Parcel Model Formulation dSw dXl dXi —— = A1w – A2 —— – A3 —— dt dt dt Sw = supersaturation with respect to water w = updraft dXl/dt = condensation rate of liquid phase dXi/dt = condensation rate of ice phase A terms = f(T,P,supersaturation ratio) Fick’s 1st Law if Diffusion for ice crystal growth Δm Δρv —— = A Dv —— Δt Δx

  12. Nucleation clue: Small mode crystals are quasi-spherical CPI Images from SPEC

  13. Ice Crystal Nucleation Assumptions Small Mode: Inside-out contact nucleation (Shaw and Durrant 2005, J. Phys. Chem. B, Letters). Cloud droplets evaporate via Bergeron-Findeisen process and some percent freeze as ice nuclei within contacts the droplet interface. We assume this occurs for D < 2 µm about. Large Mode: Based on Huffman (1973); NIN = C SK. C modified to give typical N as measured by the 2DC probe (i.e. 10-20 l-1). Frozen droplet

  14. Criteria for testing ice crystal nucleation process: Small crystal mode, mid-latitude ice clouds: Mean length: 27 ± 3 μm (for cirrus) Typical N: 500 – 5000 l-1 IWC: 10 – 40% of total Large crystal mode, mid-latitude ice clouds: Mean length: > 100 μm, up to 1-2 mm in frontal clouds Typical N: 10-20 l-1 IWC: 60-90% of total

  15. Different growth rates produce bimodal spectra, but small mode IWC >> large mode IWC. Simultaneous nucleation.

  16. Propose 2-stage ice crystal nucleation process to satisfy observations of ice cloud bimodal size spectra + Large mode crystals form from ice nuclei and grow at expense of cloud droplets Vapor pressure drops below water saturation as droplet surface area decreases. Fraction of smallest evaporating droplets freeze (2%) to form small crystal mode.

  17. 2-stage nucleation process in an updraft

  18. 2-stage nucleation process with negligible vertical motion. Initial droplet diameter = 10 μm, LWC = 0.10 g m-3.

  19. 2-stage process for droplet diameter = 5 μm, LWC = 0.10 g m-3

  20. Conclusions – Parcel Model • Predicted size spectra in ice clouds were bimodal due to different crystal shapes • and their corresponding growth rates. • To approximate the observed properties of ice cloud bimodal size spectra, a 2-stage • process of ice crystal production appears necessary. • This 2-stage process may account for the stable observed properties of the small • crystal mode and observed ice supersaturations (Korolev and Isaac 2005). • This 2-stage process indicates that the small crystal mode in ice clouds should be • affected by changes in aerosol concentration similar to liquid water clouds. Since the • small mode may potentially dominate cloud radiative properties and heating rates • for polluted atmospheres, aerosol particles may play a critical role in determining the • radiative properties of mid-latitude cirrus clouds. • An aerosol-cirrus IOP should relate aerosol concentrations to the small crystal • mode of the cirrus size distribution.

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