Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Sensitivity of the CAM to Small Ice CrystalsAs Measured by the FSSP

David L. Mitchell

Desert Research Institute, Reno, Nevada

Philip Rasch

NCAR, Boulder, Colorado

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Albedo and emissivity differences between mid-latitude and tropical anvilcirrus 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).

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Corrected for Ice Particle Shattering Effects

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Tropical

ML PSD

CAM RESULTS: Tropical vs. mid-latitude size

distributions

SW cloud forcing LW cloud forcing

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Tropical – ML SWCF differences

Tropical – ML LWCF differences

Tropical minus mid-latitude PSD simulations: Cloud forcing

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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PSD median mass flux fall velocities

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Impact of ice particle fall velocities

IWP zonal means for in-cloud conditions,

tropical & mid-latitude

PSD schemes

% difference in high-level cloud coverage,tropical – ML PSD scheme

Tropical

M L PSD

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Heating rates and tropical cold bias

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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dSw dXl dXi —— = A1w – A2 —— – A3 —— dt dt dt

Sw = supersaturation with respect to waterw = updraftdXl/dt = condensation rate of liquid phasedXi/dt = condensation rate of ice phaseA terms = f(T,P,supersaturation ratio)

Fick’s 1st Law if Diffusion for ice crystal growth

Δm Δρv—— = A Dv —— Δt Δx

Cloud Parcel Model Formulation

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Nucleation clue: Small mode crystals are quasi-spherical

CPI Imagesfrom SPEC

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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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 percentfreeze 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 givetypical N as measured by the 2DC probe (i.e. 10-20 l-1).

Frozen droplet

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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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 cloudsTypical N: 10-20 l-1

IWC: 60-90% of total

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Propose 2-stage ice crystal nucleation process to satisfy observations of ice cloud bimodal size spectra

Large mode crystalsform from ice nucleiand grow at expenseof cloud droplets

Vapor pressure drops below water saturationas droplet surface area decreases. Fraction of smallest evaporating droplets freeze (2%) to form small crystal mode.

+

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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2-stage nucleation process in an updraft

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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

Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Particle Size, Water Path, and Photon Tunneling in Water and Ice Clouds ARM STM Albuquerque 22-26 Mar

2004

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Conclusions – Parcel Model

1. Predicted size spectra in ice clouds were bimodal due to different crystal shapesand their corresponding growth rates.

2. To approximate the observed properties of ice cloud bimodal size spectra, a 2-stageprocess of ice crystal production appears necessary.

3. This 2-stage process may account for the stable observed properties of the smallcrystal mode and observed ice supersaturations (Korolev and Isaac 2005).

4. This 2-stage process indicates that the small crystal mode in ice clouds should beaffected by changes in aerosol concentration similar to liquid water clouds. Since thesmall mode may potentially dominate cloud radiative properties and heating ratesfor polluted atmospheres, aerosol particles may play a critical role in determining theradiative properties of mid-latitude cirrus clouds.

5. An aerosol-cirrus IOP should relate aerosol concentrations to the small crystalmode of the cirrus size distribution.